N.J. Thomas wrote:
We have a website with moderately high traffic, load balanced among 3
webservers.
During peak traffic times however (when the volume is higher than
normal), the load shoots up to over a 100, and the site crawls to its
knees.
We set up a script to take snapshots of top every 20 seconds. Here is
what it looks like when everthing is normal:
127
last pid: 12003; load averages: 0.93, 1.36, 1.35 up 41+04:22:14
14:00:23
243 processes: 12 running, 230 sleeping, 1 zombie
Mem: 222M Active, 74M Inact, 186M Wired, 16M Cache, 111M Buf, 503M Free
Swap: 2048M Total, 16M Used, 2032M Free
[...]
When the load shoots up, the number of http clients hits Apache's
MaxClients setting, here is what top shows:
last pid: 12407; load averages: 87.84, 51.91, 27.52 up 41+04:40:51
14:19:00
268 processes: 2 running, 266 sleeping
Mem: 715M Active, 68M Inact, 187M Wired, 29M Cache, 111M Buf, 2100K Free
Swap: 2048M Total, 272M Used, 1776M Free, 13% Inuse
[...]
The state of all the httpd prcesses are "nfsrcv". Does this mean the
bottleneck is at the NFS server that hosts the htdocs (and PHP scripts)
or just that the server is low on memory?
Hi Thomas,
What's your MaxClients set to?
Please define your values for "lot of traffic".
What CPU? RAM (512MB seems a bit low nowadays)? Disks?
I dont think i can give much advice on the NFS side of things but in the
meantime I would :
- increase # of MaxClients (the default is RIDICULOUSLY small,
specially in 1.3. You will probably have to recompile with a new max.
- Look at what the PHP scripts do : i.e., is there anything under your
control that can be improved?
- Install a PHP accelerator (like IonCube's, or Zend's). Or some
reverse proxies if you feel like playing with the whole design of your
architecture.
- if you use mod_gzip, make sure you dont' compress on the fly - it
takes a huge hit on the CPU. (Hint: You can pre-compress the files )
- You RAM seems OK ... you may want to tweak some sysctl or memory
settings in Apache (I seem to remember in 1.3 some to do with MMap, but
i could be wrong) ... or just add more RAM. Check vmstat (or systat -vm
1) to see how much swapping is going on.
- Make sure you have tweaked your network sysctl settings (like send +
receive buffers, depending on the kind of traffic you get), reduce your
close timeouts,etc.
- make use of the kernel modules accf_http.ko and accf_data.ko.
- disable from Apache ANYTHING that is NOT needed. Enable stuff only
as needed on a per virtual host basis.
Hope this helps.
Beto
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