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From Torsten Foertsch <torsten.foert...@gmx.net>
Subject Re: [users@httpd] How memory are actually used?
Date Wed, 17 Sep 2008 18:03:24 GMT
On Wed 17 Sep 2008, howard chen wrote:
> I am running Apache HTTPD 1.3.37 with mod_php, when using the top
> command, I see:
>
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
> 12584 web       15   0  142m  12m 9176 S    8  0.3   2:07.01 httpd
> 12586 web       15   0  142m  12m 9168 S    8  0.3   2:08.85 httpd
> 12589 web       15   0  141m  10m 7376 S    8  0.3   2:06.83 httpd
> 12591 web       15   0  142m  11m 7420 S    8  0.3   1:59.64 httpd
> 12594 web       15   0  142m  11m 7444 S    8  0.3   2:06.25 httpd
> 12585 web       15   0  141m  12m 9200 S    6  0.3   2:03.83 httpd
>
> Assume all the httpd process is show above...
>
> 1. Is the total memory currently used is : 12+12+10+11+11+12 = 68M ?
> 2. Is the max. memory even allocated by is: 142+142+141+142+142+141 =
> 850M

No and no.

How to interprete these figures depend a bit on your operating system. 
But in general UNIX systems try to share memory between processes. 
There are several ways how that can be achieved. In one way a program 
module (executable binary, shared lib) is divided into sections. Each 
section is marked by at compile time if it can be shared between 
processes or not. Code sections are usually read-only during program 
execution hence they can be shared. If your programs use a shared libc 
for example all the code in this lib is shared between all processes 
that use the lib. But a program module does not have to be loaded 
completely into memory to do some work. Code or data pages that are not 
needed are not loaded.

A second way to achieve memory sharing is called copy-on-write. When a 
process forks both processes at first share all their memory. Now one 
of them starts writing to a specific area of its memory. Only at this 
time the operating system allocates a personal copy of that piece of 
memory for that process. It also does not copy the whole process memory 
but only a small chunk (page). So the more both processes write to 
their memory the more memory they consume together. But they probably 
never consume together twice the amount of memory one of them has 
consumed while it has run alone.

So what you see as VIRT is the amount of mem that process would consume 
if it runs completely alone and if all of its segments are loaded into 
memory. RES is the part of VIRT that is actually allocated. Both say 
nothing about sharing. SHR says how much of RES is shared between at 
least 2 processes by the first method (not copy-on-write). But it does 
not say among how much or which processes it is shared.

Hence you can't simply add up these numbers.

A better way to judge memory consumption on Linux is /proc/$PID/smaps 
and tools that use it. /proc/$PID/clear_refs on recent kernels is also 
interesting in that regard. To get a general impression how much memory 
my apache needs you can start vmstat with a small parameter, stop the 
apache and look how it affects the memory related columns.

Torsten

--
Need professional mod_perl support?
Just hire me: torsten.foertsch@gmx.net

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