PHP & Caching an in Depth Review.

Sounds simple enough, right?

Use a cache to serve pages faster, well yes that is true but people often do not realize the fundamentals of caching and how if not done properly it can lead to a detriment in performance.

The first thing you need to realize that by caching your content is no longer dynamic, … (short pause while we wait for the outrage in the back to die down).

The whole point behind your cache is that it will be used instead of processing all your code, why this is beneficial?

You have to remember that PHP is an interpreted language, meaning it takes the following I/O flow:

Apache -> mod_php -> Script -> Interpreter -> Bytecode -> Execution -> Output Buffer

Now there are two types of caching to consider, the first is completion output caching, this also yields the best performance, the second is opcode caching, this caches the byte code generated by the interpreter thus removing that step from the chain of execution.

With me so far? Ok take a deep breath because here we go …

Output caching

This option often yields the best performance, but at the cost of removing the dynamic element from your web app. But this can be summed up in a single line: What good is dynamic content if you can serve all of 5% of your audience at a given time?

Another turn of phrase is “The slashdot effect”, there are many options for output caching, and you should ideally provide gziped and plain cache files to your end user, for instance on this blog I use WP Super Cache, and can high recommend it, as new content is posted the relevant caches are regenerated, if you are writing your own WebApp check for the “Accept-Encoding:gzip” header being sent via the users browser.

For end user transparency couple this with some mod_rewrite voodoo

RewriteCond %{HTTP:Accept-Encoding} gzip
RewriteCond %{DOCUMENT_ROOT}/cache/%{HTTP_HOST}/%{REQUEST_FILENAME}.gz -f
RewriteRule ^(.*) "/cache/%{HTTP_HOST}/%{REQUEST_FILENAME}.gz" [L]

1: If gzip is supported 2: and the cache file exists 3: Redirect visitor to compressed cached file

You “chain of execution” is now

Apache -> readfile

To serve non gziped content:

RewriteCond %{HTTP:Accept-Encoding} !gzip
RewriteCond %{DOCUMENT_ROOT}/cache/%{HTTP_HOST}/%{REQUEST_FILENAME} -f
RewriteRule ^(.*) "/cache/%{HTTP_HOST}/%{REQUEST_FILENAME}" [L]

Now to clarify a point you should not be caching images,css,js etc, we’re only covering dynamic content here, and the above are only examples to get you started, you should write rules to exclude certain content specific to your needs.

And before going of at any more of a tangent, here are some figures for you!

ab -c 100 -n 500 -g ./saiweb-nocache-nogzip.bpl http://www.saiweb.co.uk/

  • No caching
  • No Gzip

Server Hostname: www.saiweb.co.uk Server Port: 80

Document Path: / Document Length: 109086 bytes

Concurrency Level: 100 Time taken for tests: 123.304 seconds Complete requests: 500 Failed requests: 0 Write errors: 0 Total transferred: 54831652 bytes HTML transferred: 54692607 bytes Requests per second: 4.06 [#/sec] (mean) Time per request: 24660.828 [ms] (mean) Time per request: 246.608 [ms] (mean, across all concurrent requests) Transfer rate: 434.26 [Kbytes/sec] received

Connection Times (ms) min mean[+/-sd] median max Connect: 57 423 225.5 374 1837 Processing: 2331 20460 16701.2 17232 115192 Waiting: 270 1835 4155.8 576 38549 Total: 2656 20882 16648.1 17692 115421

Percentage of the requests served within a certain time (ms) 50% 17692 66% 20700 75% 24063 80% 25770 90% 35157 95% 53328 98% 82957 99% 101497 100% 115421 (longest request)

As can be seen as the number of requests grew the response time began to increase sharply and the overall performace of the site degrade, bare in mind these benchmarks are being made on my home DSL for the time being.

ab -c 100 -n 500 -g ./saiweb-cached.bpl http://www.saiweb.co.uk/

Server Hostname: www.saiweb.co.uk Server Port: 80

Document Path: / Document Length: 109086 bytes

Concurrency Level: 100 Time taken for tests: 79.212 seconds Complete requests: 500 Failed requests: 0 Write errors: 0 Total transferred: 54889292 bytes HTML transferred: 54705058 bytes Requests per second: 6.31 [#/sec] (mean) Time per request: 15842.342 [ms] (mean) Time per request: 158.423 [ms] (mean, across all concurrent requests) Transfer rate: 676.70 [Kbytes/sec] received

Connection Times (ms)

          min  mean[+/-sd] median   max

Connect: 56 314 112.5 322 1341 Processing: 2545 14721 5116.7 14296 36677 Waiting: 216 1283 2228.2 351 13776 Total: 2647 15035 5108.9 14624 36897

Percentage of the requests served within a certain time (ms) 50% 14624 66% 16675 75% 18058 80% 19093 90% 21608 95% 23489 98% 27684 99% 29972 100% 36897 (longest request)

A much more consistent line here, however as you can clearly see response times are roughly equal this is due to my DSL connection, so lets run these tests from somewhere with a little more bandwidth say the webserver itself using a loop back connection.

ab -c 100 -n 500 -g ./saiweb-cached.bpl http://www.saiweb.co.uk/

Server Hostname: www.saiweb.co.uk Server Port: 80

Document Path: / Document Length: 109086 bytes

Concurrency Level: 100 Time taken for tests: 0.262199 seconds Complete requests: 500 Failed requests: 0 Write errors: 0 Total transferred: 54945406 bytes HTML transferred: 54761172 bytes Requests per second: 1906.95 [#/sec] (mean) Time per request: 52.440 [ms] (mean) Time per request: 0.524 [ms] (mean, across all concurrent requests) Transfer rate: 204642.27 [Kbytes/sec] received

Connection Times (ms)

          min  mean[+/-sd] median   max

Connect: 0 1 2.6 0 9 Processing: 4 45 10.3 49 58 Waiting: 1 38 9.9 41 50 Total: 9 47 9.5 50 64

Percentage of the requests served within a certain time (ms) 50% 50 66% 51 75% 52 80% 52 90% 54 95% 56 98% 59 99% 61 100% 64 (longest request)

In this case the response times rise and then plateau, no after which no further degradation occurs.

ab -c 100 -n 500 -g ./saiweb-nocache.bpl http://www.saiweb.co.uk/

Server Hostname: www.saiweb.co.uk Server Port: 80

Document Path: / Document Length: 109086 bytes

Concurrency Level: 100 Time taken for tests: 8.919565 seconds Complete requests: 500 Failed requests: 0 Write errors: 0 Total transferred: 54680788 bytes HTML transferred: 54543000 bytes Requests per second: 56.06 [#/sec] (mean) Time per request: 1783.913 [ms] (mean) Time per request: 17.839 [ms] (mean, across all concurrent requests) Transfer rate: 5986.73 [Kbytes/sec] received

Connection Times (ms)

          min  mean[+/-sd] median   max

Connect: 0 14 30.7 0 85 Processing: 246 1556 714.3 1365 6735 Waiting: 241 1539 707.8 1360 6731 Total: 250 1571 708.0 1368 6735

Percentage of the requests served within a certain time (ms) 50% 1368 66% 1451 75% 1550 80% 1700 90% 2658 95% 3121 98% 3491 99% 3638 100% 6735 (longest request)

Oh dear of dear lets cut to the hard facts shall we?

We’ve gone from serving 1906.95 requests a second to 56.06

  • a 97.1% decrease in performance when removing caching
  • or a 3401.1% increase in performance when implementing caching

We’ve gone from a response time of ~50ms to ~2000ms

  • a 97.5% decrease in performance when removing caching
  • or a 4000% increase in performance when caching is on

Then there is the CPU an memory overheads to consider, in this case a more prolonged test is required to gain the relevant sar data, now let me tell you that intentionally trying to get a test like this to run over a 10 minute period with the correct caching on is a lot harder than it sounds, the tests infact were completing far too quickly …

The problem I face is to make ab perform a long enough timed duration of results cached, I know for a fact uncached the server will fail under the load, so I have no way at present of grabbing this reliably,

what I can tell you is that this command: ab -c 300 -n 1000000 -g ./saiweb-cached.bpl http://www.saiweb.co.uk/

caused a load average of 2.96, 1.9,0.93 cache, and got as high as 21 before I killed it uncached.

Now I am going to bring this post to an end as it is getting quiet long, I plan to cover the following in a 2nd part.

  1. Opcode caching
  2. CPU & Memory usage, Cached vs. UNcached

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