英文翻译本文出自《ComputerNetwork》第四版Andrew

1、英文翻译：本文出自 Computer Network 第四版 Andrew S.Tanenbaum 著Network Performance MeasurementWhen a network performs poorly, its users often complain to the folks running it, demanding improvements. To improve the performance, the operators must first determine exactly what is going on. To find out what is rea

2、lly happening, the operators must make measurements. In this section we will look at network performance measurements. The discussion below is based on the work of Mogul (1993). 矚慫润厲钐瘗睞枥庑赖賃軔。矚慫润厲钐瘗睞枥庑赖賃。The basic loop used to improve network performance contains the following steps: 聞創沟燴鐺險爱氇 谴净祸測。聞創

3、沟燴鐺險爱氇谴净祸。Measure the relevant network parameters and performance. 残骛楼諍锩瀨濟溆塹籟婭骒。 残骛楼諍锩瀨濟溆 塹籟婭。Try to understand what is going on.Change one parameter.These steps are repeated until the performance is good enough or it is clear that the last drop ofimp roveme nt has bee n squeezed out酽锕极額閉镇桧猪訣锥顧荭。酽锕极

4、額閉镇桧猪訣锥顧。Measurements can be made in many ways and at many locations (both physically and in the protocol stack). The most basic kind of measurement is to start a timer when beginning some activity and see how long that activity takes. For example, knowing how long it takes for a TPDU to be acknowle

5、dged is a key measurement. Other measurements are made with counters that record how often some event has happened (e.g., number of lost TPDUS). Finally, one is often interested in knowing the amount of something, such as the number of bytes processed in a certain time interval. 彈贸摄尔霁毙攬砖卤庑诒尔。彈贸摄尔霁毙攬

6、砖卤庑诒。Measuring network performance and parameters has many potential pitfalls. Below we list a few of them. Any systematic attempt to measure network performance should be careful to avoid these. 謀荞抟箧飆鐸怼类蒋薔點鉍。謀荞抟箧飆鐸怼类蒋薔點。Make Sure That the Sample Size Is Large EnoughDo not measure the time to send o

7、ne TPDU, but repeat the measurement, say, one million times and take the average. Having a large sample will reduce the uncertainty in the measured mean and standard deviation. This uncertainty can be computed using standard statistical formulas. 厦礴恳蹒骈 時盡继價骚卺癩。厦礴恳蹒骈時盡继價骚卺。Make Sure That the Samples

8、Are RepresentativeIdeally, the whole sequence of one million measurements should be repeated at different times of the day and the week to see the effect of different system loads on the measured quantity. Measurements of congestion, for example, are of little use if they are made at a moment when t

9、here is no congestion. Sometimes the results may be counterintuitive at first, such as heavy congestion at 10, 11, 1, and 2 oclock, but no congestion at noon (when all the users are away at lun ch).茕桢广鳓鯡选块网羈泪镀齐。茕桢广鳓鯡选块网羈泪镀。Be Careful When Using a Coarse-Grained ClockComputer clocks work by increment

10、ing some counter at regular intervals. For example, a millisecond timer adds 1 to a counter every 1 msec. Using such a timer to measure an event that takes less than 1 msec is possible, but requires some care. (Some computers have more accurate clocks, of course.) 鹅娅尽損鹌惨歷茏鴛賴縈诘。鹅娅尽損鹌惨歷茏鴛賴縈。To measure

11、 the time to send a TPDU, for example, the system clock (say, in milliseconds) should be read out when the transport layer code is entered and again when it is exited. If the true TPDUsend time is 300 sec, the differenee between the two readings will be either 0 or 1, both wrong. However, if the mea

12、surement is repeated one million times and the total of all measurements added up and divided by one millio n, the mean time will be accurate to better tha n 1sec.籟丛妈羥为贍偾蛏练淨槠挞。籟丛妈羥为贍偾蛏练淨槠。Be Sure That Nothing Unexpected Is Going On during Your Tests 預頌圣鉉儐歲龈讶骅籴買闥。預頌圣鉉 儐歲龈讶骅籴買。Making measurements on a

13、 university system the day some major lab project has to be turned in may give different results than if made the next day. Likewise, if some researcher has decided to run a video conference over your network during your tests, you may get a biased result. It is best to run tests on an idle system a

14、nd create the entire workload yourself. Even this approach has pitfalls though. While you might think nobody will be using the network at 3 A.M., that might be precisely when the automatic backup program begins copying all the disks to tape. Furthermore, there might be heavy traffic for your wonderf

15、ul World Wide Web pages from distant time zones. 渗 釤呛俨匀谔鱉调硯錦鋇絨。渗釤呛俨匀谔鱉调硯錦鋇。Caching Can Wreak Havoc with MeasurementsThe obvious way to measure file transfer times is to open a large file, read the whole thing, close it, and see how long it takes. Then repeat the measurement many more times to get a

16、good average. The trouble is, the system may cache the file, so only the first measurement actually involves network traffic. The rest are just reads from the local cache. The results from such a measurement are essentially worthless (unless you want to measure cache performance). 铙誅卧泻噦圣骋贶頂廡缝勵。 铙誅卧泻

17、噦圣骋贶頂廡缝。Often you can get around caching by simply overflowing the cache. For example, if the cache is 10 MB, the test loop could open, read, and close two 10-MB files on each pass, in an attempt to force the cache hit rate to 0. Still, caution is advised unless you are absolutely sure you understan

18、d the caching algorithm. 擁締凤袜备訊顎轮烂蔷報赢。擁締凤袜备訊顎轮烂蔷報。Buffering can have a similar effect. One popular TCP/IP performance utility program has been known to report that UDP can achieve a performance substantially higher than the physical line allows. How does this occur? A call to UDP normally returns co

19、ntrol as soon as the message has been accepted by the kernel and added to the transmission queue. If there is sufficient buffer space, timing 1000 UDP calls does not mean that all the data have been sent. Most of them may still be in the kernel, but the performance utility thinks they have all been

20、transmitted. 贓熱俣阃歲匱阊邺镓騷鯛 汉。贓熱俣阃歲匱阊邺镓騷鯛。Understand What You Are MeasuringWhen you measure the time to read a remote file, your measurements depend on the network, the operating systems on both the client and server, the particular hardware interface boards used, their drivers, and other factors. If t

21、he measurements are done carefully, you will ultimately discover the file transfer time for the configuration you are using. If your goal is to tune this particular configuration, these measurements are fine. 坛摶乡囂忏蒌鍥铃氈淚跻馱。坛摶乡囂忏蒌鍥铃氈淚 跻。However, if you are making similar measurements on three differen

22、t systems in order to choose which network interface board to buy, your results could be thrown off completely by the fact that one of the network drivers is truly awful and is only getting 10 percent of the performance of the board.蜡變黲癟報伥铉锚鈰赘籜葦。蜡變黲癟報伥铉锚鈰赘籜。网络性能的测量当一个网络的运行效果很差的时候， 它的用户通常会向网络运行商抱怨并要求

23、提高网络的质 量。为了改善网络的性能， 网络操作人员首先必须确定发生了什么问题。 为了找出真正的问 题所在，操作人员必须进行测量工作。在这一小节中，我们来看一看网络性能的测量问题。 下面的讨论以 Mogul（1993） 的工作为基础。 買鲷鴯譖昙膚遙闫撷凄届嬌。買鲷鴯譖昙膚遙闫撷凄届。 用来改善网络性能的基本循环过程包括以下步骤： 测量有关的网络参数和性能。试图理解当前的网络状况。 改变一个参数。这些步骤不断重复， 直到网络的性能已经足够好， 或者改善性能的全部空间都已经被发掘出 来了。测量工作可以有许多做法， 也可以在许多地点或场所进行 （既指物理位置， 也指协议栈中的 位置）。最基本的一种

24、测量手段是：在开始某一个动作的时候启动一个定时器，然后确定该 需要多长时间。例如，知道一个 TPDU 需要多长时间才被确认是一个很关键的测量指标。 其他有一些测量指标可以通过计数器来完成，即记录某种事件发生的次数，比如丢失的 TPDU 的数量。最后，人们通常对于某些事物的数量比较感兴趣，比如在特定的时间间隔内 所处理的字节数。 綾镝鯛駕櫬鹕踪韦辚糴飙钪。綾镝鯛駕櫬鹕踪韦辚糴飙。 测量网络的性能和参数有许多潜在的陷阱。 以下我们列出其中一部分。 任何一种系统化的网 络性能测量手段都应该小心地避免这些陷阱。 驅踬髏彦浃绥譎饴憂锦諑琼。驅踬髏彦浃绥譎饴憂锦諑。 确保样本空间足够大 不要测量发送一个

25、TPDU 的时间，而是重复也测量。比如说测量 1 百万次，然后再取平均。 采用大量的样本将可以减小所测量的均值和标准方差中的不确定性。这种不确定性可以利用标准的统计公式来计算。 猫虿驢绘燈鮒诛髅貺庑献鵬。猫虿驢绘燈鮒诛髅貺庑献。确保样本具有代表性 理想情况下， 这 1 百万次测量的完整序列应该在一天或者一周的不同时刻进行重复，从而可以看到不同的系统负载对于所测量指标的影响。 例如， 对于拥塞的测量， 如果仅仅在没有拥 塞的那一时刻来测量拥塞， 则这样的测量和结果并没有用。 有时候测量结果初看起来可能不 符合直觉，比如在 10，11，1 和 2 点钟网络严重拥塞，但是中午时候没有拥塞（所用的用户

26、 都去吃午饭了） 。 锹籁饗迳琐筆襖鸥娅薔嗚訝。锹籁饗迳琐筆襖鸥娅薔嗚。当使用粗粒度时钟的时候一定要谨慎 计算机时钟的工作原理是， 每隔固定的时间间隔就递增某一个计数器， 例如， 一个毫秒定时 器每隔 1ms 就让一个计数器加 1。使用这样的定时器来测量一个持续时间小于1ms 的事件是有可能的，但要非常小心。 （当然，有些计算机还有更加精确的时钟。 ） 構氽頑黉碩饨荠龈话骛 門戲。構氽頑黉碩饨荠龈话骛門。然而，如果重复测量Q还要精确得多。例如，为了测量出发送一个 TPDU 所需要的时间，当进入传输层代码时以及离开传输层代 码时，应该将系统时钟（比如说以毫秒为单位） 读出来。如果TPDU真正的发

27、送时间是 300阴, 则两次读取的时间之差要么是0,要么是1,这两个结果都是错误的。1百万次，则所有测量的总和累加起来，再除以1百万，则平均时间比1 輒峄陽檉簖疖網儂號泶蛴镧。輒峄陽檉簖疖網儂號泶蛴。确保在测试过程中不会发生不可预知的事情 在一个大学的网络系统进行测量有可能发生这样的情况： 有一天, 当一个大型的实验项目在 运行的时候你测量的结果跟第二天测量出来的结果可能会有所不同。同样地, 如果有的研究人员决定在你们的网络上运行一个视频会议, 而在这个时候你正好在测量, 那么你得到的结 果可能会偏差。 你最好在一个空闲的系统上运行测试过程, 并且根据需要自己来创建所有的 工作负载。 不过这种做法也有缺陷。 你可能认为在凌晨 3 点钟的时候不会有人使用网络, 但是，当自动备份程序这时候开始将所有的磁盘数据复制到磁带上的时候， 你的想法就不再正 确了。而且，此时其他时区的用户可能会访问你精美的 WWW, 从而也导致繁重的流量。 尧侧 閆繭絳闕绚勵蜆贅瀝纰。尧侧閆繭絳闕绚勵蜆贅瀝。因此，这样的测量结果本质。识饒鎂錕缢灩筧嚌俨淒侬减。识饒鎂例如，如果缓存空间的大小 文件