Chapter 7. Identifying Your Performance Problems
You have been assigned to improve the performance of your company's web service. The hardest thing is to get started. How should you tackle this task? And how do you sort out the insignificant issues and identify those that will make a difference once resolved?
In this chapter, we look at this problem from different angles. Only after you understand the problem should you start looking for solutions. Don't search for a solution before the problem has been precisely identified, or you'll end up wasting a lot of time concentrating on trivial issues. Instead, try to identify where you can make the biggest difference in performance.
Note that in this book, we use the term "web service" to mean the whole aggregate that provides the service: the machine, the network, and the software. Don't confuse this with web services such as SOAP and XML-RPC.
7.1. Looking at the Big Picture
To make the user's web-browsing experience as painless as possible, every effort must be made to wring the last drop of performance from the server. Many factors affect web site usability, but one of the most important is speed. (This applies to any web server, not just Apache.)
How do we measure the speed of a server? Since the user (and not the computer) is the one that interacts with the web site, one good speed measurement is the time that elapses between the moment the user clicks on a link or presses a Submit button, and the time when the resulting page is fully rendered in his browser.
The requests and resulting responses are broken into packets. Each packet has to make its own way from one machine to another, perhaps passing through many interconnection nodes. We must measure the time starting from when the request's first packet leaves our user's machine to when the reply's last packet arrives back there.
A request may be made up of several packets, and a response may contain a few hundred (typical for a GET request). Remember that the Internet standard for Maximum Transmission Unit (MTU), which is the size of a TCP/IP packet, is 576 bytes. While the packet size can be 1,500 bytes or more, if it crosses a network where the MTU is 576, it will be broken into smaller packets.
It is also possible that a request will be made up of many more packets than its response (typical for a POST request where an uploaded file is followed by a short confirmation response). Therefore, it is important to optimize the handling of both the input and the output.
A web server is only one of the entities the packets see on their journey. If we follow them from browser to server and back again, they may travel via different routes through many different entities. For example, here is the route the packets may go through to reach perl.apache.org from our machine:
% /usr/sbin/traceroute -n perl.apache.org traceroute to perl.apache.org (188.8.131.52), 30 hops max, 38 byte packets 1 10.0.0.1 0.847 ms 1.827 ms 0.817 ms 2 184.108.40.206 7.628 ms 11.271 ms 12.646 ms 3 220.127.116.11 8.613 ms 7.882 ms 12.479 ms 4 18.104.22.168 10.131 ms 8.686 ms 12.163 ms 5 22.214.171.124 9.033 ms 7.281 ms 9.930 ms 6 126.96.36.199 225.319 ms 231.167 ms 234.747 ms 7 188.8.131.52 252.473 ms * 252.602 ms 8 184.108.40.206 250.532 ms 251.693 ms 226.962 ms 9 220.127.116.11 232.632 ms 231.504 ms 232.019 ms 10 18.104.22.168 225.417 ms 224.801 ms 252.480 ms 11 22.214.171.124 254.443 ms 225.056 ms 259.674 ms 12 126.96.36.199 227.754 ms 226.362 ms 253.664 ms 13 188.8.131.52 252.921 ms 252.573 ms 258.014 ms 14 184.108.40.206 237.191 ms 234.256 ms * 15 220.127.116.11 254.539 ms 252.895 ms 253.895 ms
As you can see, the packets travel through 14 gateways before they reach perl.apache.org. Each of the hops between these gateways may slow down the packet.
Before they are processed by the server, the packets may have to go through proxy servers, and if the request contains more than one packet, packets might arrive at the server by different routes and at different times. It is possible that some packets may arrive out of order, causing some that arrive earlier to have to wait for other packets before they can be reassembled into a chunk of the request message that can then be read by the server. The whole process is then repeated in the opposite direction as response packets travel back to the browser.
Even if you work hard to fine-tune your web server's performance, a slow Network Interface Card (NIC) or a slow network connection from your server might defeat it all. That is why it is important to think about the big picture and to be aware of possible bottlenecks between your server and the Web.
Of course, there is little you can do if the user has a slow connection. You might tune your scripts and web server to process incoming requests ultra quickly, so you will need only a small number of working servers, but even then you may find that the server processes are all busy waiting for slow clients to accept their responses.
There are techniques to cope with this. For example, you can compress the response before delivery. If you are delivering a pure text response, gzip compression will reduce the size of the sent text by two to five times.
You should analyze all the components involved when you try to create the best service for your users, not just the web server or the code that the web server executes.
_ _ _ _ _ | A web service is like a car, if one of the parts or mechanisms is broken the car may ~ not ~ run smoothly; it can even stop dead if pushed too far without first fixing it. \_ _ _/ \_ _ _/
If you want to have success in the web service business, you should start worrying about the client's browsing experience, not only how good your code benchmarks are.
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