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语法填空名校模拟真题强化练24养成良好的答题习惯，是决定高考英语成败的决定性因素之一。
做题前，要认真阅读题目要求、题干和选项，并对答案内容作出合理预测;答题时，切忌跟着感觉走，最好按照题目序号来做，不会的或存在疑问的，要做好标记，要善于发现，找到题目的题眼所在，规范答题，书写工整;答题完毕时，要认真检查，查漏补缺，纠正错误。
（2023上·江西鹰潭·高三贵溪市实验中学校考阶段练习）阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。
Decorative stove artwork attracts a growing number of 1 (admire), who enjoy the memories it evokes, Yang Feiyue reports. The distinctive art form in Jiaxing 2 (list) as a national intangible cultural heritage in 2011.An old wooden stove, 3 (bear) rich vivid images, took center stage at a village Spring Festival gala in Jiaxing, Zhejiang province, in early February. The images feature historical and legendary figures, fairy tales, animals and flowers, all of 4 are depicted in bright colors and have a way of claiming attention from viewers.It was once common for farmhouses, especially those along the lower reaches of the Yangtze River, 5 (have) a zaotou that was painted with floral (绘有花的) figures. Locals 6 (burn) firewood and straw in it to cook before gas burners or microwave ovens were available. “Historical evidence shows the art has 7 history of at least 300 years,” says Zhao from Haiyan’s Tongyuan town.Most of those engaged 8 zaotou painting were bricklayers (砌砖工) in rural areas where they were often hired by villagers to build a mud stove in 9 (they) new home. Tools used to produce such art included a selection of brushes, rulers, and paint. As people’s aesthetic appreciation grew, watercolors were applied, mostly red, yellow and blue. “But no matter what material is used, the stove painting is always 10 (symbol) and harbors good wishes for the owners,” Zhao says.（2024上·湖南邵阳·高三校联考阶段练习）阅读下面短文, 在空白处填入1个适当的单词或括号内单词的正确形式。
学术英语理工教师手册Unit 1 Choosing a TopicI Teaching ObjectivesIn this unit , you will learn how to:1.choose a particular topic for your research2.formulate a research question3.write a working title for your research essay4.enhance your language skills related with reading and listening materials presented in this unit II. Teaching Procedures1.Deciding on a topicTask 1Answers may vary.Task 21 No, because they all seem like a subject rather than a topic, a subject which cannot be addressed even by a whole book, let alone by a1500-wordessay.2Each of them can be broken down into various and more specific aspects. For example, cancer can be classified into breast cancer, lung cancer, liver cancer and so on. Breast cancer can have such specific topics for research as causes for breast cancer, effects of breast cancer and prevention or diagnosis of breast cancer.3 Actually the topics of each field are endless. Take breast cancer for example, we can have the topics like:Why Women Suffer from Breast Cancer More Than Men?A New Way to Find Breast TumorsSome Risks of Getting Breast Cancer in Daily LifeBreast Cancer and Its Direct Biological ImpactBreast Cancer—the Symptoms & DiagnosisBreastfeeding and Breast CancerTask 31 Text 1 illustrates how hackers or unauthorized users use one way or another to get inside a computer, while Text2 describes the various electronic threats a computer may face.2 Both focus on the vulnerability of a computer.3 Text 1 analyzes the ways of computer hackers, while Text 2 describes security problems of a computer.4 Text 1: The way hackers “get inside” a computerText 2: Electronic threats a computer facesYes, I think they are interesting, important, manageable and adequate.Task 41Lecture1:Ten Commandments of Computer EthicsLecture 2:How to Deal with Computer HackersLecture 3:How I Begin to Develop Computer Applications2Answersmay vary.Task 5Answers may vary.2 Formulating a research questionTask 1Text 3Research question 1: How many types of cloud services are there and what are they? Research question 2: What is green computing?Research question 3: What are advantages of the cloud computing?Text 4Research question 1: What is the Web 3.0?Research question 2: What are advantages and disadvantages of the cloud computing? Research question 3: What security benefits can the cloud computing provide?Task 22 Topic2: Threats of Artificial IntelligenceResearch questions:1) What are the threats of artificial intelligence?2) How can human beings control those threats?3) What are the difficulties to control those threats?3 Topic3: The Potentials of NanotechnologyResearch questions:1) What are its potentials in medicine?2) What are its potentials in space exploration?3) What are its potentials in communications?4 Topic4: Global Warming and Its EffectsResearch questions:1) How does it affect the pattern of climates?2) How does it affect economic activities?3) How does it affect human behavior?Task 3Answers may vary.3 Writing a working titleTask 1Answers may vary.Task 21 Lecture 4 is about the security problems of cloud computing, while Lecture 5 is about the definition and nature of cloud computing, hence it is more elementary than Lecture 4.2 The four all focus on cloud computing. Although Lecture 4 and Text 4 address the same topic, the former is less optimistic while the latter has more confidence in the security of cloud computing. Text3 illustrates the various advantages of cloud computing.3 Lecture 4: Cloud Computing SecurityLecture 5: What Is Cloud Computing?Task 3Answers may vary.4 Enhancing your academic languageReading: Text 11.Match the words with their definitions.1g 2a 3e 4b 5c 6d 7j 8f 9h 10i2. Complete the following expressions or sentences by using the target words listed below with the help of the Chinese in brackets. Change the form if necessary.1 symbolic 2distributed 3site 4complex 5identify6fairly 7straightforward 8capability 9target 10attempt11process 12parameter 13interpretation 14technical15range 16exploit 17networking 18involve19 instance 20specification 21accompany 22predictable 23profile3. Read the sentences in the box. Pay attention to the parts in bold.Now complete the paragraph by translating the Chinese in brackets. You may refer to the expressions and the sentence patterns listed above.ranging from（从……到）arise from some misunderstandings（来自于对……误解）leaves a lot of problems unsolved（留下很多问题没有得到解决）opens a path for（打开了通道）requires a different frame of mind（需要有新的思想）4.Translate the following sentences from Text 1 into Chinese.1) 有些人声称黑客是那些超越知识疆界而不造成危害的好人（或即使造成危害，但并非故意而为），而“骇客”才是真正的坏人。
奇葩实验英语作文Title: The Peculiar Experiment: Unraveling the Unknown。
In the realm of scientific inquiry, curiosity often leads researchers down unconventional paths, prompting them to embark on peculiar experiments that challenge the boundaries of what is known. Such endeavors can yield unexpected insights and push the frontiers of knowledge. Today, we delve into the intriguing world of peculiar experiments, exploring their significance and the lessons they offer.One such experiment involved investigating the effectsof prolonged exposure to complete darkness on humancognition and sensory perception. Subjects were isolated in a light-sealed chamber for an extended period, devoid ofany external light sources. As the days stretched into weeks, researchers observed intriguing changes in the participants' behavior and mental faculties.Initially, subjects reported feelings of disorientation and unease as they grappled with the absence of light cues. Without the cyclical patterns of day and night, their circadian rhythms became disrupted, leading to irregular sleep patterns and fluctuations in mood. However, as time passed, a remarkable adaptation occurred.In the absence of visual stimuli, other senses sharpened in response to the heightened demand for sensory input. Participants reported an acute awareness of sounds, textures, and even subtle shifts in temperature withintheir environment. Some described experiencing vivid auditory hallucinations, attributing them to the brain's attempt to compensate for the lack of visual input.Cognitive functions also underwent significant changes under prolonged darkness. With limited external distractions, subjects found themselves immersed in deep introspection, contemplating existential questions and grappling with profound insights. Freed from theconstraints of conventional perception, they tapped into dormant reservoirs of creativity and imagination.Yet, the experiment also posed ethical dilemmas and raised questions about the limits of scientific inquiry. Is it ethical to subject individuals to extreme conditions in the pursuit of knowledge? What are the long-term psychological effects of such experiments on participants? These are complex issues that warrant careful consideration and ethical oversight.Another peculiar experiment delved into the realm of synesthesia, a phenomenon where stimulation of one sensory or cognitive pathway leads to involuntary experiences in another. Researchers sought to induce synesthetic experiences in non-synesthetes through a combination of sensory stimuli and cognitive conditioning.Subjects underwent a series of sensory deprivation sessions while listening to specific auditory sequences and viewing corresponding visual patterns. Over time, many reported experiencing synesthetic perceptions, such as seeing colors in response to sound or tasting flavors when exposed to certain shapes.The implications of this experiment extend beyond the realm of sensory perception, offering insights into the plasticity of the human brain and its capacity for cross-modal integration. By understanding the mechanisms underlying synesthesia, researchers hope to unlock new avenues for enhancing cognitive function and treating neurological disorders.However, the ethical considerations surrounding this experiment are paramount. Manipulating individuals' sensory experiences raises concerns about consent, autonomy, and potential harm. As we venture into the uncharted territory of the mind, we must tread carefully and prioritize the well-being of participants above all else.In conclusion, peculiar experiments serve as catalysts for scientific discovery, challenging our preconceptions and expanding the boundaries of human knowledge. From probing the effects of darkness on cognition to unraveling the mysteries of synesthesia, these endeavors offer valuable insights into the complexities of the humanexperience.Yet, they also underscore the importance of ethical conduct and responsible research practices. As we venture into uncharted territory, we must remain vigilant in safeguarding the rights and well-being of participants. Only through a balanced approach can we navigate the murky waters of peculiar experiments and unlock the secrets they hold.。
心理作用英文作文英文：The psychological effects of certain situations or experiences can have a profound impact on an individual's mental well-being. These effects can manifest in a variety of ways, including changes in mood, behavior, and cognitive function. As a psychology student, I have had the opportunity to learn about and observe the various ways in which psychological factors can influence a person's thoughts and actions.For example, I recently conducted a study on theeffects of stress on college students. I found that high levels of stress can lead to increased anxiety, difficulty concentrating, and changes in sleep patterns. These psychological effects can have a significant impact on a student's academic performance and overall well-being. In my research, I also discovered that certain coping mechanisms, such as mindfulness meditation and exercise,can help mitigate the negative psychological effects of stress.In addition to stress, other psychological factors such as trauma, grief, and social isolation can also have profound effects on an individual's mental health. For instance, I have worked with individuals who have experienced traumatic events, and I have seen firsthand the lasting psychological effects that trauma can have. These individuals often experience symptoms of post-traumatic stress disorder, such as flashbacks, nightmares, and hypervigilance. It is important to recognize and address these psychological effects in order to help individuals heal and move forward.中文：某些情况或经历的心理作用可能对个人的心理健康产生深远影响。
Scientists studying the effects of various phenomena play a crucial role in expanding our understanding of the natural world, human behavior, and the complex interplay between different variables. This process involves systematic investigation, data collection, analysis, and interpretation to draw meaningful conclusions. Let's explore the broader concept of scientists studying the effects of different factors and delve into the methodologies, significance, and challenges associated with such studies.### **Introduction:**Scientists engaging in the study of effects often aim to uncover causal relationships, identify patterns, or understand the impact of certain factors on a given system. This exploration encompasses a wide range of disciplines, including physics, chemistry, biology, psychology, environmental science, and social sciences. The effects being studied can be diverse, ranging from the microscopic level of particles to the macroscopic level of ecosystems or human societies.### **Methodologies in Studying Effects:**1. **Experimental Design:**- **Controlled Experiments:** Scientists often use controlled experiments to isolate specific variables and observe their effects systematically. This involves manipulating one variable while keeping others constant.2. **Observational Studies:**-**Longitudinal Studies:** Researchers track subjects over an extended period to observe changes and identify potential causative factors.- **Cross-Sectional Studies:** Examining a diverse group at a single point in time to uncover correlations and associations.3. **Field Studies:**-**Ecological Studies:** Scientists study effects within natural environments, observing interactions between organisms and their surroundings.-**Social Science Field Studies:** Researchers may conduct surveys or interviews to understand the effects of social, economic, or cultural factors on individuals or communities.4. **Computer Modeling:**- **Simulation Studies:** Scientists use computer models to simulate real-world scenarios, allowing them to predict and analyze potential effects without real-world experimentation.### **Significance of Studying Effects:**1. **Scientific Advancement:**- **New Discoveries:** Research on the effects of various factors often leads to the discovery of new phenomena, principles, or relationships.-**Advancement of Knowledge:** Building on existing knowledge, scientists contribute to the continuous advancement of their respective fields.2. **Problem Solving:**- **Environmental Solutions:** Studying the effects of human activities on the environment aids in developing strategies for sustainable resource use and conservation.- **Medical Breakthroughs:** Understanding the effects of drugs, diseases, and lifestyle on health contributes to medical advancements and improved healthcare.3. **Policy Formulation:**-**Informed Decision-Making:** Governments and organizations use scientific studies to formulate policies addressing societal issues, such as public health, education, and environmental protection.-**Risk Assessment:** Studying the effects of potential hazards helps in assessing and mitigating risks to human health and safety.4. **Technological Innovation:**- **Materials Science:** Studying the effects of different materials on each other contributes to the development of new materials with enhanced properties.-**Engineering Advancements:** Understanding the effects of forces, temperature, and other factors on structures and systems informs engineering practices and innovations.### **Challenges in Studying Effects:**1. **Complexity of Systems:**-**Interconnected Variables:** Natural systems are often complex, with numerous interconnected variables. Isolating the effect of one variable while keeping others constant can be challenging.2. **Ethical Considerations:**- **Human Subjects:** In social and medical studies, ethical considerations, such as informed consent and the potential for harm, must be carefully addressed.-**Environmental Impact:** Researchers studying ecological effects must consider the potential impact of their studies on the environment.3. **Resource Limitations:**-**Financial Constraints:** Conducting comprehensive studies requires financial resources for equipment, personnel, and data analysis.- **Time Constraints:** Longitudinal studies, in particular, can be time-consuming, requiring sustained funding and commitment.4. **Data Interpretation:**-**Statistical Challenges:** Interpreting data and drawing meaningful conclusions require statistical expertise to avoid misinterpretation or bias.- **Correlation vs. Causation:** Distinguishing between correlation and causation is critical to avoid drawing incorrect causal relationships.### **Case Study: Studying the Effects of Climate Change:**Consider a case study where scientists are studying the effects of climate change:1. **Methodology:**- **Observational Studies:** Scientists analyze long-term climate data, including temperature records, sea-level measurements, and ice core samples.-**Computer Modeling:** Climate scientists use sophisticated models to simulate future climate scenarios based on different emission scenarios.2. **Significance:**- **Policy Impact:** Findings contribute to global efforts to mitigate climate change, shaping international agreements and policy decisions.-**Environmental Awareness:** Studying the effects raises public awareness of climate change impacts, fostering environmentally conscious behaviors.3. **Challenges:**- **Data Uncertainty:** Climate systems are intricate, and uncertainties in data interpretation can pose challenges in predicting future scenarios.- **Global Collaboration:** Studying a phenomenon as pervasive as climate change requires international collaboration and coordination.### **Conclusion:**In conclusion, scientists studying the effects of various factors contribute significantly to human knowledge, technological innovation, and policy formulation across diverse disciplines. The methodologies employed, the significance of their findings, and the challenges they face vary depending on the field of study. Despite challenges, the pursuit of understanding the effects of different variables remains integral to scientific progress and addressing global challenges.。
关于天气的介绍英语The Enigma of Weather.Weather, a phenomenon that has fascinated and perplexed mankind for centuries, is a dynamic display of atmospheric conditions that constantly change and evolve. It is the combination of various meteorological elements such as temperature, humidity, wind, precipitation, and atmospheric pressure that determines the state of the weather at any given time and place. The study of weather and its patterns is known as meteorology, a branch of Earth science that employs a range of tools and technologies to observe, predict, and understand weather systems.The atmosphere, the layer of gases surrounding the Earth, is the medium through which weather occurs. It is composed primarily of nitrogen and oxygen, with trace amounts of other gases such as water vapor, carbon dioxide, and ozone. The interaction of these gases with solar radiation, the Earth's surface, and each other gives riseto the complex array of weather phenomena we experience.Temperature, a key player in weather systems, is influenced by the amount of solar radiation reaching the Earth's surface, the reflectivity of the surface, and the presence of water vapor and clouds. The distribution of temperature across the globe creates pressure gradientsthat drive wind systems, causing air to move from regionsof high pressure to low pressure.Humidity, another crucial factor, is determined by the amount of water vapor present in the air. High humidity can lead to the formation of clouds, fog, and dew, while low humidity can result in dry, clear weather. The water cycle, which involves the evaporation of water from the Earth's surface, its condensation into clouds, and itsprecipitation back to the surface in the form of rain, snow, or hail, is a crucial process that maintains the humidity balance in the atmosphere.Wind, generated by the pressure gradients created by temperature and humidity differences, plays a pivotal rolein weather systems. It can carry moisture, heat, and pollutants across vast distances, affecting weather patterns far away from their source. Winds can alsointeract with the Earth's terrain, causing localized weather effects such as mountain waves and sea breezes.Precipitation, the fall of water from clouds to the Earth's surface, is a critical component of weather. It can take various forms, including rain, snow, hail, and sleet, depending on the temperature and humidity conditions. Precipitation is essential for life on Earth, providing water for plants, animals, and humans alike.Atmospheric pressure, which is determined by the weight of the air column above a given point on the Earth's surface, also plays a role in weather systems. High-pressure systems are typically associated with clear, dry weather, while low-pressure systems often bring clouds, rain, and windy conditions.The complexity of weather systems is further compounded by the presence of large-scale atmospheric circulationpatterns, such as the Hadley cell and the Coriolis effect. These patterns influence the movement of air and moisture across the globe, shaping weather patterns and climates on regional and global scales.In addition to these fundamental elements, weather can be influenced by a range of natural and anthropogenic factors. Natural factors include volcanic eruptions, which can inject ash and gases into the atmosphere, affecting temperature and precipitation patterns. Anthropogenic factors, such as climate change, can also have significant impacts on weather systems, altering temperature and precipitation patterns on a global scale.The study of weather is not just about understanding the present state of the atmosphere; it is also about predicting future weather conditions. Meteorologists employ a range of tools and techniques, including weather radar, satellites, and numerical weather prediction models, to forecast weather systems and warn the public of potential hazards such as severe storms, tornadoes, hurricanes, and blizzards.In conclusion, weather is a dynamic and complex phenomenon that is influenced by a range of factors, including temperature, humidity, wind, precipitation, and atmospheric pressure. The study of weather and its prediction is crucial for understanding our environment, planning daily activities, and protecting life and property from the potential hazards associated with severe weather events. As we continue to explore the mysteries of the atmosphere and the impact of human activities on weather systems, the field of meteorology remains a vibrant and essential area of scientific inquiry.。
词义用英语怎么说词义：包括词的本义、引申义、比喻义。
一个词的最初的含义称作本义。
以本义为出发点，根据它所反映的事物或现象的各个特点，词在它的发展过程中又会产生若干个与本义相关但并不相同的意义，这就是词的引申义。
那么你知道词义用英语怎么说吗?下面来学习一下吧。
词义英语说法1：acceptation词义英语说法2：word meaning词义英语例句：嫉妒和羡慕词义相隔太多。
Are jealousy and admiring acceptation be apart too much 有些辞典的编纂者用引文作例证以解释词义。
Some dictionary writer use citation to show what word mean.语境是词义的唯一决定者，没有语境就没有词义。
Context is the sole determiner of meaning without which meaning does not exist.小孩子曲解词义是很正常的事。
It is normal that children understand the acceptation wrongly.词义引伸是有规律的。
There are regularities in the meaning transference.小孩子曲解词义是很正常的事。
It is nromal for children misconstrue words.小孩子曲解词义是很正常的事。
It is normal for the kids to misinterpre the acceptation.他详述两个词词义的细微差别。
He dwelt on the subtle distinction between the two words.根据上下文猜词义。
Guess general meanings of words from the context.美洲原住民的语言为英语增加了许多词汇，但词义往往有所变化。
做实验有什么启示英语作文The Significance of Conducting Experiments。
Experiments are an essential part of scientific research. They provide scientists with a way to test their hypotheses and theories. Through experiments, scientists can observe and measure the effects of different variables on a particular phenomenon. The results obtained from experiments are used to draw conclusions and make predictions about the natural world. In this essay, we will discuss the significance of conducting experiments and the lessons we can learn from them.Firstly, experiments allow us to test our ideas and theories. In the absence of experiments, we would have to rely on guesswork and speculation. For example, if a doctor wants to find out which treatment is more effective for a particular disease, he or she would have to conduct experiments on patients to test the different treatments. Without experiments, the doctor would have to rely onanecdotal evidence or intuition, which may not be reliable.Secondly, experiments help us to understand the natural world. Through experiments, we can observe and measure the effects of different variables on a particular phenomenon. This allows us to identify patterns and relationships that may not be apparent otherwise. For example, scientists use experiments to study the behavior of subatomic particles, which are too small to observe directly. Through experiments, they can observe the behavior of these particles and make predictions about their properties.Thirdly, experiments can lead to new discoveries and innovations. Many of the most significant discoveries in science and technology have come from experiments. For example, the discovery of penicillin, the first antibiotic, was the result of an experiment conducted by Alexander Fleming. Similarly, the discovery of the Higgs boson, a subatomic particle that gives other particles mass, was the result of experiments conducted at the Large Hadron Collider.Finally, experiments teach us important lessons about the scientific process. Experiments are often designed to test a specific hypothesis or theory. However, the results of experiments are not always what we expect. Sometimes, experiments can lead to unexpected results or even contradict our ideas and theories. When this happens, scientists must revise their hypotheses and theories to account for the new information. This process of revising and refining our ideas is an essential part of the scientific process.In conclusion, experiments are an essential part of scientific research. They allow us to test our ideas and theories, understand the natural world, make new discoveries and innovations, and learn important lessons about the scientific process. As we continue to conduct experiments and push the boundaries of our knowledge, we will undoubtedly make new discoveries and innovations that will benefit humanity.。
全球气候变暖影响英文版Effects of Global Climate ChangeGlobal climate change refers to the long-term shifts in temperature and weather patterns on Earth caused by human activities, primarily the emission of greenhouse gases. These changes have far-reaching impacts on various aspects of our planet, including the environment, ecosystems, and human societies. In this essay, we will explore some of the major effects of global climate change on a global scale.One of the most visible impacts of climate change is therise in global temperatures. Over the past century, the average temperature of the Earth has increased by about 1 degree Celsius due to the accumulation of greenhouse gases in the atmosphere. This slight increase in temperature may not seem significant, but it has profound effects on our planet. Higher temperatures lead to the melting of ice caps and glaciers, resulting inrising sea levels. As a consequence, low-lying coastal areas are at risk of flooding, displacing millions of people and causing substantial economic losses.Another consequence of global warming is the alteration of weather patterns and an increase in extreme weather events. Higher temperatures lead to more evaporation, resulting in increased rainfall in some areas. This can lead to more frequent and intense storms, hurricanes, and floods. On the other hand,other regions may experience decreased rainfall and more frequent droughts, leading to water scarcity and agricultural losses. These changes in weather patterns can disrupt ecosystems and have a significant impact on agriculture, affecting food production and availability.Furthermore, climate change has profound impacts on ecosystems and wildlife. As temperatures rise, many species are forced to migrate to cooler regions or higher elevations to survive. However, some species, particularly endemic ones that are restricted to specific areas, may not be able to adapt or find suitable habitats, leading to their extinction. These changes in the distribution of species can disrupt ecosystems and have cascading effects on other species that rely on themfor food or other ecological interactions.In conclusion, global climate change has extensive and multifaceted impacts on our planet. Rising temperatures, changing weather patterns, ocean warming, ecosystem disruptions, and societal implications are just some of the effects that have been observed. Given the seriousness of these consequences, itis crucial for global efforts to mitigate greenhouse gas emissions and adapt to the changes already taking place to ensure a sustainable future for our planet and its inhabitants.。
Effects of User Request Patterns on a Multimedia Delivery SystemChristopher B.Mayer K.Selc¸uk CandanV enkatesh SangamComputer Science and Engineering DepartmentArizona State Universitye-mail:chris.mayer,candan,venkatesh.sangam@AbstractWe recently introduced a novel method for creating replication systems where the replicated objects’sizes and/or per-object service times are large[10].Such replication systems are well-suited to delivering multimediaobjects on the Internet.Assuming that user request patterns to the system are known,the method creates repli-cation systems that distribute read load fairly to servers so that the likelihood of servers overloading is reduced.Thus,the systems produced are highly available and responsive to user requests.In this paper,we report on resultsthat reveal(i)how server loads are affected and(ii)the impact of two system design parameters(indicators of asystem’s load distribution qualities)have on server load when user request patterns differ from that for which asystem was designed.1IntroductionReplication is an accepted method for improving availability and response times of Internet content.The main idea behind replication is that storing copies of an object(file,database,web page,etc.)on servers throughout a network provides single points of failure.Since the object is available at many servers,high demand loads can be met and the failure of an individual server does not make the object inaccessible.Multimedia content can be delivered over the Internet and can benefit from replication.In this paper,we focus specifically on systems for delivering(making available for download)multimedia content.When designing a multimedia delivery system the following considerations and constraints apply.Read Load:In a multimedia delivery system,an object is written by its author and remains accessible for some period of time before it is removed.Since an object is supposed to be afinished product,it is rarely updated once in publication.Therefore,the load on a system’s servers is due mainly to handling users’read requests and the load for writing can be ignored.Since we are concerned with only the read request load(read load),we will use the term “load”and“read load”interchangeably.Content Size:Multimedia objects such as videofiles tend to be large(tens or hundreds of megabytes)and therefore require special consideration when being replicated.Large objects can not be rapidly replicated in response tofluctuating demand.Therefore,it is sensible to pre-position multimedia objects.While pre-positioning is good,over-provisioning can be bad.Creating too many copies of an object having relatively low demand is wasteful.Replication costs for an object should be relative to demand for the object. Service Times:Even if users connect to a server over broadband connections,delivering a multimedia object to a user requires the server’s attention for a long period of time.These long service times occur because the content is either large or streamed,or both.Server Behavior:A common behavior of servers is that they can support multiple simultaneous requests while maintaining acceptable quality of service.Once a server’s load capacity is exceeded,the server’s service quality rapidly declines,resulting in stalled requests and disappointed users.In combination with the service time property above,this behavior suggests that the best way to maintain the availability of a multimedia object and keep users happy is to ensure that servers operate below their load capacities.This research funded by NSF grant998404-0010819000.Techniques for easing the load on,or improve the performance of,multimedia servers(mainly video and streamed media servers)include:caching[1,6,11,14,15],protocols for stream and download sharing[5,8,11], and customized server designs[4,5,7].While all these approaches are beneficial,they all depend on access to the content’s source.Thus,they are not a panacea for availability and responsiveness;abundant access to the source content,as replication provides,is required.The profusion of special techniques for video and streamed media delivery are indirect signs that multimedia should be stored and delivered separately from other types of Internet content.Towards this end,several video-only delivery schemes have been proposed([3,12,13]for instance).Common weaknesses of these schemes are that content is assumed to come from a single source(i.e.,only one entity is using the system)and the delivery network is a tree(which networks are commonly not).In a previous work[10],we introduced a replication architecture and an accompanying design method well-suited for delivery of multimedia content,or any other Internet content where service times or object sizes are large. In our approach,servers are organized into write-sets and read-sets and requests are fulfilled using a specific read protocol.Our system structure is more general than that of the tree-based video distribution systems and freely handles multiple content providers.In our system,a server’s share of system load is proportional to the server’s contribution to total system read load capacity.In other words,load is distributed fairly to the servers.Fairly distributing load minimizes the odds that a server will exceed its load capacity.Since servers operate under their load limit,the system is responsive and content is highly available.In[10],we rigorously examined the implications of our server organization and read protocol and showed that load fairness is a complex non-linear condition that can not be solved easily.Therefore,in[10]we derived two key parameters:(describing good request distribution strategies)and(describing good server organization)that reflect a system’s load ing and,we developed and tested a design method for quickly designing replication systems with nearly optimal load fairness.In this paper,we further the work begun in[10]with a study of(i)server loads and(ii)the importance of and to server loads when the pattern of read requests entering a system differs from that for which the system was designed.In Section2we review our replication approach.We then we explain why divergent request patterns require further investigation in Section3.Next,we describe how we conducted experiments to evaluate the impact of and(Section4).Section5contains the results of our study and Section6summarizes.2A Replication System Suitable for Multimedia Content DeliveryIn this section we describe the features and mechanics of our method for creating a replication system.A detailed accounting of the material summarized here can be found in[10].2.1System StructureA replication system has a set of servers,,which it uses to replicate objects.In our approach, we organize these servers into intersecting subsets called write-sets and read-sets.To write an object,a write-set is chosen and the object is replicated on each server in the write-set.To read an object,a read-set is chosen and the object is delivered to the requesting user from a server in the selected read-set.It is allowable for an object to be written to multiple write-sets.We ensure that every written object can be accessed from any read-set by requiring that each read-set and each write-set have at least one server in common.Although there are many ways to construct write-sets and read-sets while maintaining this requirement,we limit ourselves,for simplicity reasons,to the special case where the write-sets and read-sets are determined by arranging servers in a grid.In a grid fully populated with one server per grid cell,rows correspond to write-sets and columns to read-sets.As Fig.1a indicates,each read-set intersects every write-set with at least one server.Some readers may notice that this grid-based structure resembles grid-based quorum systems([2]and[9]for example).This is intentional since quorum systems feature decentralized operation and have the potential load-balanced server operation.1Due to the unpredictable nature of Internet routing and to generalize for any request routing scheme,we do not specify how requests areRead Protocol1.A user generates a request for an object(an initial-request)which is directedto one of the system’s servers(a proxy).The distribution of a particular user’srequests to the proxies is called the user request pattern.12.The proxy selects a read-set using a preset,probabilistic proxy strategy.3.The proxy identifies the server(s)with the most up-to-date copy(in case theobject has been updated)of the object in this read-set.If more than one serverhas the most up-to-date object,one of the servers is picked equiprobably toserve the data.4.The proxy redirects the user to the server it has selected.5.The user receives the object from the appropriate server,thus inducing a readload on the server.(a)(b)Figure1:A grid-based replication system where rows are write-sets and columns are read-sets is shown in(a).The read protocol for the replication system is shown in(b).2.2Read ProtocolAn important part of our replication system is the read protocol shown in1b.Note that locating the appropriate server within a read-set adds to the delay in responding to requests.However,this delay is extremely small compared to the time required to serve a multimedia object and is unlikely to detract much from the user’s experience.Further, this delay can be minimized by using a directory service,especially since object locations rarely change.Based on the structure of write-sets and read-sets and the read protocol,we now the issue of load fairness.2.3FairnessTo minimize the likelihood of a server overloading,a server should experience a load proportional to its contribution to the system’s total capacity.In other words,system load should be distributed fairly to servers.This will ensure that no server is pushed beyond its capacity unless the whole system is.This can be accomplished by identifying a good assignment of servers to write-/read-sets(thus defining the system’s structure)andselecting effective proxy strategies for directing initial-requests to read-sets.As afirst step towards fairness,we further refine the system’s structure by splitting each server into one or more virtual servers.Each server in the system,,is represented by its read load capacity,.Denoting a base capacity as,we split each server into virtual servers,.Populating the cells of a grid with virtual servers(Fig.1),instead of regular servers, results in nearly equal amounts of capacity at each grid cell.If the system read load can be distributed equally to each grid cell,then we have achieved the goal of fairness.Unfortunately,the load on the virtual server in each grid cell,and in turn on the servers,depends on(i)the arrangement of virtual servers in the grid(the grid mapping), (ii)the policy for deciding to which write-sets an item should be written(write-policy),(iii)initial-request loads at the proxies determined by the user strategies,and(iv)the proxy strategies.Hence,load fairness requires more than placing virtual servers in the grid.Given a grid of write-sets(rows)and read-sets(columns)populated with virtual servers,a total system read load,,the read load of server,,is:(1)In the above equation,denotes the probability that write-set contains a server with the requested content;denotes the probability that read-set is chosen by a proxy given that write-set contains the content; directed to servers.Instead,we rely on the observation that request routing is somewhat predictable and can be expressed probabilistically.and denotes the probability that server is selected for serving the request given that write-set contains the requested content and read-set is chosen.To obtain load fairness,we need to ensure that the grid mapping and proxy strategies distribute the total system read load onto individual servers in proportion to each server’s contribution to total system capacity:(2)Note that this fairness condition is a complex non-linear equation,and solving it directly is expensive.Also,it is not straightforward tofind a mapping and determine proxy strategies using(1)and(2)directly.Therefore,we use(1)and(2)to identify parameters,and,that can be used to construct highly fair replication systems.2.4Deriving andTo begin the derivation of and,we assume that the write-policy distributes objects to write-sets such that the request load for each write-set is equal.That is,of the system load is directed to the virtual servers in each write-set.With this assumption(1)becomes(3) Using this equation,we can rewrite the fairness condition,(2),as(4)Note that,if a request can be served from write-set and a proxy chooses read-set,then one of the servers having a virtual server in the intersection of and will be selected to serve the content.If there is more than one server in the intersection,then each server has an equal chance of being chosen.Consequently,if we let be the set of servers that have virtual servers in write-set,;i.e.,;be the set of servers that have virtual servers in read-set,;i.e.,;andbe the set of virtual servers in read-set that have a corresponding server in write-set;i.e.,,then,.Hence,(4)becomes(5)Notice that(5)has two terms that can be manipulated:and.Thefirst term is a function of the proxy strategies and the latter depends on the grid mapping.By isolating these two terms,we gain a measure of insight into how to construct an optimally fair system.2.4.1Isolating Proxy Strategies:ParameterIn order to extract the term related to proxy strategies,we isolate the term in(5)related to read-set selection by proxies to get(6) which says that the fraction of requests directed to read-set should be inversely proportional to the number of read-sets,.In other words,initial requests should be directed in equal amounts to each column.This implies thatFigure2:Replication using andthe combined effect of all the proxy strategies should ensure an equal distribution of initial request to each column. We denote the fraction of initial read requests directed to read-set as,or read-set-value.The ideal-value for a read-set is.If is greater(less)than the ideal-value,then,and the virtual servers in,will receive more(less)than their fair share of system load.Likewise,since a server’s load is the sum of the load on its virtual servers,the higher the s of the read-sets in which a server has a virtual server(average server),the more load the server will receive.2.4.2Isolating the Grid Mapping:ParameterAssuming that all read-sets have the ideal-value of,we can reduce(5)to(7) This equation can be satisfied by ensuring that(8) holds.The left-hand side of(7)captures the degree of content overlap of server with other servers that share both read-sets and write-sets with.Isolating this overlap we get(9),or server-value,indicates’s vulnerability to being selected for serving a read request.The ideal value for is,the number of virtual servers has.If is too high()or too low(),then will be selected too often or not often enough and will not receive its fair share of load.As an example of how to calculate an-value,consider server and the second write-set and second read-set highlighted in Fig.1.Server has three virtual servers,thus and.Three virtual servers in the second read-set have a server in the second write-set,so(i.e.,if read-set2is chosen by a proxy and the requested content is on a server in write-set2,then three servers can be selected for download).Since and only have in common..Thus,(i.e.,given that a request is for content contained in write-set2and read-set2was chosen,server has a1-in-3chance of serving the request).Repeating these calculations for server for all rows and columns and summing the results shows that has a perfect-value:.2.5Replication Based on andTo produce the fairest system possible,-and-values must be as close to their ideal values as possible.While and indicate how fair a system is,they are derivatives of the complex,non-linear fairness condition and,therefore, can not be used directly to construct a replication system either.However,they do provide insight as to how a fair replication system should look.In[10],we exploited and to develop a two-step heuristic approach for creating a replication system(Fig.2).In thefirst step,we set the system’s structure by mapping virtual servers to grid cells so that each server has good-values.This structure is then used to formulate proxy strategies that result in the best possible-values for each grid column.Given a set of servers and a grid,1.Put the servers into groups such that serversin each group have the same number of vir-tual servers.2.Try tofill the grid with the given clusters3.If such afilling is not possible,break someof them into smaller clusters tofit them intothe grid.The result is a server-to-grid mapping.Given a server set,a grid,and server-to-grid mapping,and user strategies,1.Identify the linear constraints for(a)-optimality,(b)write-policy dependence(c)fairness,and(d)column selection restrictions on the proxies.2.Solve while minimizing error terms in the constraints.3.Extract proxy strategies from the solution.The proxy strategies result in optimal-values.(a)(b)Figure3:Pseudocode for(a)the cluster-based mapping algorithm and(b)producing-optimizing proxy strategies.Pseudocode for mapping a grid is shown in Fig.3a.The algorithm clusters virtual servers and then maps the clusters to the grid.Clustering limits the interaction between servers that gives imperfect-values.If all virtual servers can be mapped,while maintaining cluster integrity,then all servers will have perfect-values.Note that clusters must sometimes be split into smaller pieces in order to facilitate placement.This can create imperfections in-values.However,as we showed in[10],the negative effects of splitting are minimal.Once the system’s structure is set,we use the structure to formulate a linear program(LP)and solve the LP tofind proxy strategies for the system.In addition to the system’s structure,the LP considers other fairness-related factors such as the system’s write-policy and user request patterns.Figure3b shows pseudocode for the LP’s construction and extraction of proxy strategies.The specifics of the LP can be found in[10].The output of the LP are the proxy strategies that produce the best possible(closest to ideal)-values for each read-set.For each proxy,its particular proxy strategy gives the frequency at which it should select a read-set when handling initial-requests from users.In[10]we studied the impact of and on server load fairness.We showed that grid-based replication systems constructed using our two-step approach are highly fair when operating conditions are exactly that for which the system was designed.In this paper,we investigate what happens when user request patterns no longer match the patterns for which the system was designed.Specifically,we examine(i)the importance of and to load fairness and(ii)how server loads are affected user request patterns deviate from expectations.3Divergent Initial-Request LoadsSince proxies redirect client initial-requests to servers,the performance of the replication system depends on the expected distribution of the users’initial-requests to proxies.As explained earlier as part of the read protocol,the probability that a given user’s initial-requests arrive at a certain proxy,is given as a distribution function called the user request er request patterns(or at least their cumulative effect on the proxies)are an input to the linear program(LP)that is solved to get the proxy strategies for selecting read-sets.Thus,a replication system is tailored for the user request patterns input into the LP.Since user request patterns are will change over time,using afixed user request pattern to construct a replication system is a potential weakness of our approach.If request patterns change too much,server load fairness could be lost and the system would perform poorly.In the remainder of this paper,we present two kinds of results,obtained experimentally,about our proposed replication system.We show the importance of and to load distribution when user request patterns deviate from those for which the system was designed.We show that systems built using our two-step construction approach(see previous section)are resilient to changes in user request loads.4Experimental SetupIn order to test the performance of replication systems that use our write-/read-set structure and read protocol,we have constructed a testbed system that uses real web servers.The use of real servers adds a degree of realism thatordinary simulation does not provide.Because of space constraints we can not go into the details of the testbed system in this paper.However,we do describe the conditions for conducting experiments.To prepare for an experiment,servers are arranged into their write-sets and read-sets using the grid structure and given their proxy strategies(calculated in advance based on expected loads).A different object is written to each write-set(a row of the grid).All data items have the same size and hence the same download times.Having each object be the same size and having each write-set contain a single object captures the effects of a perfectly tuned write-policy.The download time of an object is simulated by having servers execute a sleep operation of5seconds. The running time for an experiment is40times the sleep time,or200seconds.This is the minimum time needed for an experiment to show long-term loading behavior.Once the setup stage is complete,the experiment can begin.Performing an experiment consists of generating user requests for objects and the handling of those requests by the er requests are regularly-spaced over a second to meet a specified request rate.For example,if the request rate is10requests/sec,then a new request is generated every tenth of a second.Uniform request generation, while simple,is adequate since object sizes,and hence download times,are relatively large compared to request inter-arrival times.For each request,an object is selected uniformly at random.The proxy server that will receive a newly generated initial-request is selected at random using a probability distribution that models the effects of the user request patterns.Experiments were performed using20sets of servers.A server set is the servers available for use by a replication system.Server sets were generated so that the number of virtual servers in each set equalled64and wouldfill an 8x8grid.The number of virtual servers per server was randomly generated according to the following distribution: of the servers have1,have2,have3,have4,and have5virtual servers.In order to observe the effect of as request patterns change,we map a server set to a grid using two different mapping strategies:Random.This strategy randomly maps virtual servers to a grid.This results in server-values that differ greatly,both up and down,from their ideals.Cluster.Grids are mapped using an algorithm based on the clustering pseudocode of Fig.3a.Clustering results in ideal or nearly ideal-values for all servers in a grid.To observe the effect of,we used two methods for formulating proxy-strategies that result in favorable and unfavorable-values.Not-optimized.Each proxy redirects initial-requests equiprobably to the read-sets(grid columns)in which it has virtual servers.As such,read-set s can vary greatly,being highly influenced by the system’s structure.-optimized.Here a linear program is formulated and solved to obtain proxy-strategies that produce optimal s.As with the above non--optimized strategy,proxies can only redirect initial-requests to read-sets in which the proxy has virtual servers.Even with this restriction,resulting s are close to ideal regardless of mapping strategy.Mixing mapping and proxy strategies results in four replication systems(mapping/proxy systems or MP-systems) for each server set.The mix of good and bad-and-values in the four systems allows us to observe the influence of and on server load as initial-request loads at the proxies vary in response to changing user request patterns. We refer to an MP-system by the mapping method used and the presence of-optimization as shown in Fig.4and listed below.R ANDOM:not-or-optimized R ANDOM-:not-optimized,but-optimizedC LUSTER:-optimized,but not-optimized C LUSTER-:-and-optimized.For the tests,a baseline load of6.4requests per second is the arrival rate of initial-requests to each proxy(256 requests per proxy divided by the40second experiment length).Thus,the-optimized systems were constructed for user request patterns whose cumulative effect is that each proxy is equally loaded with initial-requests.In order to systematically explore the effects of varying user request patterns,we randomly selected subsets of proxies in each server set and subjected them to increased initial-request loads.2We refer to the combination 2Note that we only increase initial-request loads at proxies.Since Internet demand only grows over time and unevenly,this is a reasonable thing to do.Figure4:Naming concept for the four types of replication systems created from a server set.of proxies selected to receive extra load and the extra load they are given as an initial-request-combination(IR-combo).For a given server set,there are seven IR-combos which form an initial-request-set(IR-set).An IR-set is built as follows.Thefirst IR-combo in the set is each proxy receiving the baseline load In this combination, 0%of the proxies receive0%extra load.We call this the0%-0%IR-combo or the baseline system.Next,three proxy subsets of sizes,,and of the number of proxies in the server set are formed,with the larger subsets reusing servers from the smaller ones.The proxies in these subsets will receive25%and then50% extra initial-request load above the baseline bining the proxy subset sizes and extra request percentages produces the remaining six IR-combos in an IR-set:10%-25%,10%-50%,20%-25%,20%-50%,40%-25%,and 40%-50%.Example4.1We now illustrate how to create an initial-request-set.Consider a server set with thirty servers num-bered1through30and a baseline initial-request load of10requests per second(req/sec).1.The0%-0%combination is all proxies receiving10req/sec.2.For the10%proxy subset we pick three proxies,say5,9,and21.For the10%-25%combination,we increasethe number of requests to these three proxies by25%;they will each receive12.5req/sec.Proxies not in the subset still get only10req/sec.To create the10%-50%combination,initial-requests are increased by50%to 15req/sec at the three proxies.3.To build the20%subset,2,5,9,11,21,29,the10%subset is augmented by three more servers:2,11,andbinations20%-25%and20%-50%are created by increasing the initial loads at these proxies by25%and50%,respectively.4.The40%subset is the20%subset plus servers1,15,17,23,24,and28.Increasing the initial-requests by25%and50%at the selected proxies gives combinations40%-25%and40%-50%.By running a server set through each of its four MP-systems and each of its seven IP-combos(each server set is run28times),we can detect trends in server read loads and compare the influence of and on server load as user request patterns vary.5Results and ObservationsIn this section,we answer six questions about the effects of and when initial-request loads to proxies diverge from their expected values.To do this,we observe the extra load experienced by a server when operating as part of the four MP-systems created from the server set of which the server is a member.Extra load is the difference in a server’s read load in an experiment where user request patterns have changed and in an experiment where user request patterns are exactly what the system was designed for(the0%-0%IR-combo or baseline system).Server read load is the average number of reads(downloads)experienced by a server in each second of an experiment.For example,if server had a load of5read requests per second in a baseline system experiment and then had a load of 7requests per second in a20%-50%combination then’s extra load is2,an increase of40%.The evidence supporting answers to Questions One through Four involve twenty different server sets.Questions Five and Six are answered using results from four rounds of repeated experiments on the second of the twenty server sets.In the four rounds,the initial-request-sets were not changed.Since space is limited,we present evidence representing behavioral trends seen throughout all the experiments.。
