Jeremy Chadwick a écrit :
On Tue, Oct 28, 2008 at 10:44:49AM -0400, 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.

I'm not sure you fully understand the concept of swapping (the term can
be used for a multitude of things).  :-)  Some processes which sit
idle/unused will have portions of their memory "swapped out" (to
swap/disk) to allow for actively running processes to utilise physical
memory.  This is something to keep in mind.

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.

156M x 450 process = way more RAM than what we have (same for RES). Concretely, how must I interpret these results ?

It's as I expected -- you don't understand the difference between
SIZE (SZ) and RES (RSS).  The simple version:

SIZE == amount of memory that's shared across all processes on the
machine, e.g. shared libraries.  It doesn't mean "156MB is being taken
up per process".

RES == amount of memory that's specifically allocated to that individual
process.  The three httpd processes above are taking up a total of
~38MBytes of memory (13108K + 12684K + 12280K).

As I said, even with RES the numbers dont seems to have any sense.

Let's say 12500K x 450 = ~5500MBytes. Considering there's a lot of process other than Apache running on the server...there's something wrong. Is there something shared in RES too ?

Right ! I would really appreciate few explanation on this. Do the shared pages counts as active or inactive RAM ? How can i calculate how much physical RAM an apache process is taking ? How the VM works in this regard ? ;)

Others will have to explain the shared memory/pages aspect, as it's
beyond my understanding.  But recent versions of 7.0 and 7.1-PRERELEASE
contain a tool called procstat(1) which can help you break down the
memory usage within a process.

Our next server will be in 7.0 for sure.

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