collecting pv entries -- suggest increasing PMAP_SHPGPERPROC

[Mel]

On Tuesday 28 October 2008 15:44:49 Francis Dubé wrote:
> Jeremy Chadwick a écrit :
> > On Mon, Oct 27, 2008 at 12:56:30PM -0700, Chuck Swiger wrote:
> >> On Oct 27, 2008, at 12:38 PM, FreeBSD wrote:
> >>>> You need to keep your MaxClients setting limited to what your system
> >>>> can run under high load; generally the amount of system memory is the
> >>>> governing factor. [1]  If you set your MaxClients higher than that,
> >>>> your system will start swapping under the load and once you start
> >>>> hitting VM, it's game over: your throughput will plummet and clients
> >>>> will start getting lots of broken connections, just as you describe.
> >>>
> >>> According to top, we have about 2G of Inactive RAM with 1,5G Active
> >>> (4G total RAM with amd64). Swapping is not a problem in this case.
> >>
> >> With 4GB of RAM, you're less likely to run into issues, but the most
> >> relevant numbers would be the Swap: line in top under high load, or the
> >> output of "vmstat 1" / "vmstat -s".
>
> We're monitoring our swap with cacti, and we've never been swapping even
> during high load because we dont let apache spawn enough process to do so.
>
> >> It would also be helpful to know what your httpd's are looking like in
> >> terms of size, and what your content is like.  For Apache serving mostly
> >> static content and not including mod_perl, mod_php, etc, you tend to
> >> have 5-10MB processes and much of that is shared, so you might well be
> >> able to run 400+ httpd children.  On the other hand, as soon as you pull
> >> in the dynamic language modules like perl or PHP, you end up with much
> >> larger process sizes (20 - 40 MB) and much more of their memory usage is
> >> per-process rather than shared, so even with 4GB you probably won't be
> >> able to run more than 100-150 children before swapping.
>
> Here's an example of top's output regarding our httpd process :
> 54326 apache        1  96    0   156M 13108K select 1   0:00  0.15% httpd
> 54952 apache        1  96    0   156M 12684K select 1   0:00  0.10% httpd
> 52343 apache        1   4    0   155M 12280K select 0   0:01  0.10% httpd
>
> Most of our page are in HTML with a LOT of images. Few PHP pages, very
> light PHP processing.

Then your best bet is to properly set up mod_expires for the images. More then 
anything else that will reduce the load on your server.

http://httpd.apache.org/docs/2.2/mod/mod_expires.html#expiresbytype

The better solution involves recoding your site to serve images from a 
different webserver (can be the same machine, simply use a very light jail). 
This installation only loads mod_expires and mod_headers and serves images 
only.

I would do this regardless, for two reasons:
1) You will almost certainly get rid of PMAP_SHPGPERPROC on the document 
server
2) You will more easily detect bottlenecks in scripts, because the problem is 
not aggrevated/masked by the image serving.
-- 
Mel

Problem with today's modular software: they start with the modules
    and never get to the software part.