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@xxxxxxx --------------------------------------------------------------------- The official User-To-User support forum of the Apache HTTP Server Project. See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx
|