Hi, as I have learned from the "Debugging memory allocation" thread Apache::SizeLimit can be used to kill a child if it's unshared part grows to big. To measure this unshared part on linux /proc/PID/statm is used. I was in doubt that the simple difference between the total number of pages and the shared pages really reflect the unshared part. So, I tried out some things:
First, in httpd.conf I allocate a noticeable amount of memory: <Perl> $My::Klaus="x"x10000000; </Perl> This memory is shared among all children due to linux' copy on write. Now /proc/PID/statm for the master and a child say: master: 5254 4190 2089 71 0 3094 0 child: 5272 4208 2107 71 0 3094 0 When I write to $My::Klaus from within a child I expect the total size to remain the same and the shared portion to drop considerably (actually ~2500 pages which is not possible since it is only 2107). My first try was to assign simply a new string: $My::Klaus="y"x10000000; Now /proc/PID/statm shows for this child: 7749 6718 2107 71 0 5571 0 Not what I have expected. The shared portion remains but the total size grows by 2477 pages. Well, perl allocates simply a new string. Hence the total process size grows. My second try was to modify the string in place: $My::Klaus=~tr/x/y/; Now /proc/PID/statm shows: 5307 4276 2107 71 0 3129 0 Nothing has changed much. Particularly the number of shared pages remains exactly the same. To check that $My::Klaus is really copied on write I have checked the total used and free space shown in /proc/meminfo. The figures show exactly what is to be expected. The used memory grows by the amount needed to hold a second copy of $My::Klaus. Hence, killing an apache child when the unshared part grows beyond a certain limit do on linux NOT account the copy on write pages that get unshared. As I have heard libgtop also only provides the information from /proc. That means do also not account the copy on write effect. Is that information right? Is there a way on linux to get the amount of pages shared among processes by copy on write? Torsten
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