Debian – Performance Tuning a High-Load Apache Server

amazon ec2apache-2.2debianperformance-tuningvmstat

I am looking to understand some server performance problems I am seeing with a (for us) heavily loaded web server. The environment is as follows:

  • Debian Lenny (all stable packages + patched to security updates)
  • Apache 2.2.9
  • PHP 5.2.6
  • Amazon EC2 large instance

The behavior we're seeing is that the web typically feels responsive, but with a slight delay to begin handling a request — sometimes a fraction of a second, sometimes 2-3 seconds in our peak usage times. The actual load on the server is being reported as very high — often 10.xx or 20.xx as reported by top. Further, running other things on the server during these times (even vi) is very slow, so the load is definitely up there. Oddly enough Apache remains very responsive, other than that initial delay.

We have Apache configured as follows, using prefork:

StartServers          5
MinSpareServers       5
MaxSpareServers      10
MaxClients          150
MaxRequestsPerChild   0

And KeepAlive as:

KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 5

Looking at the server-status page, even at these times of heavy load we are rarely hitting the client cap, usually serving between 80-100 requests and many of those in the keepalive state. That tells me to rule out the initial request slowness as "waiting for a handler" but I may be wrong.

Amazon's CloudWatch monitoring tells me that even when our OS is reporting a load of > 15, our instance CPU utilization is between 75-80%.

Example output from top:

top - 15:47:06 up 31 days,  1:38,  8 users,  load average: 11.46, 7.10, 6.56
Tasks: 221 total,  28 running, 193 sleeping,   0 stopped,   0 zombie
Cpu(s): 66.9%us, 22.1%sy,  0.0%ni,  2.6%id,  3.1%wa,  0.0%hi,  0.7%si,  4.5%st
Mem:   7871900k total,  7850624k used,    21276k free,    68728k buffers
Swap:        0k total,        0k used,        0k free,  3750664k cached

The majority of the processes look like:

24720 www-data  15   0  202m  26m 4412 S    9  0.3   0:02.97 apache2                                                                       
24530 www-data  15   0  212m  35m 4544 S    7  0.5   0:03.05 apache2                                                                       
24846 www-data  15   0  209m  33m 4420 S    7  0.4   0:01.03 apache2                                                                       
24083 www-data  15   0  211m  35m 4484 S    7  0.5   0:07.14 apache2                                                                       
24615 www-data  15   0  212m  35m 4404 S    7  0.5   0:02.89 apache2

Example output from vmstat at the same time as the above:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 8  0      0 215084  68908 3774864    0    0   154   228    5    7 32 12 42  9
 6 21      0 198948  68936 3775740    0    0   676  2363 4022 1047 56 16  9 15
23  0      0 169460  68936 3776356    0    0   432  1372 3762  835 76 21  0  0
23  1      0 140412  68936 3776648    0    0   280     0 3157  827 70 25  0  0
20  1      0 115892  68936 3776792    0    0   188     8 2802  532 68 24  0  0
 6  1      0 133368  68936 3777780    0    0   752    71 3501  878 67 29  0  1
 0  1      0 146656  68944 3778064    0    0   308  2052 3312  850 38 17 19 24
 2  0      0 202104  68952 3778140    0    0    28    90 2617  700 44 13 33  5
 9  0      0 188960  68956 3778200    0    0     8     0 2226  475 59 17  6  2
 3  0      0 166364  68956 3778252    0    0     0    21 2288  386 65 19  1  0

And finally, output from Apache's server-status:

Server uptime: 31 days 2 hours 18 minutes 31 seconds
Total accesses: 60102946 - Total Traffic: 974.5 GB
CPU Usage: u209.62 s75.19 cu0 cs0 - .0106% CPU load
22.4 requests/sec - 380.3 kB/second - 17.0 kB/request
107 requests currently being processed, 6 idle workers

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K.C.K..WK_K..KKW_CK.WK..W.KKKWKCKCKW.W_KKKKK.KKWKKKW._KKK.CKK...
KK_KWKKKWKCKCWKK.KKKCK..........................................
................................................................

From my limited experience I draw the following conclusions/questions:

  • We may be allowing far too many KeepAlive requests

  • I do see some time spent waiting for IO in the vmstat although not consistently and not a lot (I think?) so I am not sure this is a big concern or not, I am less experienced with vmstat

  • Also in vmstat, I see in some iterations a number of processes waiting to be served, which is what I am attributing the initial page load delay on our web server to, possibly erroneously

  • We serve a mixture of static content (75% or higher) and script content, and the script content is often fairly processor intensive, so finding the right balance between the two is important; long term we want to move statics elsewhere to optimize both servers but our software is not ready for that today

I am happy to provide additional information if anybody has any ideas, the other note is that this is a high-availability production installation so I am wary of making tweak after tweak, and is why I haven't played with things like the KeepAlive value myself yet.

Best Answer

I'll start by admitting that I don't much about running stuff in clouds - but based on my experience elsewhere, I'd say that this webserver config reflects a fairly low volume of traffic. That the runqueue is so large suggests that there just isn't enough CPU available to deal with it. What else is in the runqueue?

We may be allowing far too many KeepAlive requests

No - keeplive still improves performance, modern browsers are very smart about knowing when to pipeline and when to run requests in parallel, although a timeout of 5 seconds is still rather high, and you've got a LOT of servers waiting - unless you've got HUGE latency problems I'd recommend cranking this down to 2-3. This should shorten the runqueue a little.

If you've not already got mod_deflate installed on the webserver - then I'd recommend you do so - and add the ob_gzhandler() to your PHP scripts. You can do this as an auto-prepend:

if(!ob_start("ob_gzhandler")) ob_start();

(yes, copression uses more CPU - but you should save CPU overall by getting servers out of the runqueue faster / handling fewer TCP packets - and as a bonus, your site is also faster).

I'd recommend setting an upper limit on MaxRequestsPerChild - say something like 500. This just allows some turnover on processes in case you've got a memory leak somewhere. Your httpd processes look to be HUGE - make sure you've removed any apache modules you don't need and make sure you're serving up static content with good caching information.

If you're still seeing problems, then the problem is probably within the PHP code (if you switch to using fastCGI, this should be evident without any major performance penalty).

update

If the static content doesn't vary much across pages, then it might also be worth experimenting with:

if (count($_COOKIE)) {
    header('Connection: close');
}

on the PHP scripts too.

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