all, we have two servers with exactly the same hardware running apache service. but always, there are difference between apache processes on the two boxes, in "shared memory" of the output result of cmd- "top".
in detail the difference lies in the file of /SYSV00000000, as is displayed below(this is the output of a smem.pl tool, which analyze smap msg): _________________________________________________ SHARED MAPPINGS vmsize rss clean rss dirty file 30720 kb 19856 kb 2676 kb /SYSV00000000 30720 kb 9292 kb 2540 kb /SYSV00000000 what's more, when 'cat /proc/PID/maps', we saw: __________________________________________________ 8275/maps:2ab550d26000-2ab552b26000 rw-s 00000000 00:09 13139968 /SYSV00000000 (deleted) 8275/maps:2ab552b26000-2ab554926000 rw-s 00000000 00:09 13172737 /SYSV00000000 (deleted) 8275/maps:2ab554926000-2ab556726000 rw-s 00000000 00:09 13205506 /SYSV00000000 (deleted) 8275/maps:2ab556726000-2ab558526000 rw-s 00000000 00:09 13238275 /SYSV00000000 (deleted) and output of cmd 'pmap': __________________________________________________ 00002ab550d26000 30720K rw-s- [ shmid=0xc88000 ] 00002ab552b26000 30720K rw-s- [ shmid=0xc90001 ] 00002ab554926000 30720K rw-s- [ shmid=0xc98002 ] 00002ab556726000 30720K rw-s- [ shmid=0xca0003 ] and finally output of cmd 'ipcs' __________________________________________________ # ipcs -m ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00000000 13139968 root 622 31457280 44 dest 0x00000000 13172737 root 622 31457280 44 dest 0x00000000 13205506 root 622 31457280 44 dest 0x00000000 13238275 root 622 31457280 44 dest platform: gentoo 2.6.18 x86_64 intel 5130, apache 2.0.61, php5, 8G RAM+8G swap, a lot of free memory. it seems like, apache is simply utilizing shared memory. but my question are: 1) is it necessary to use shm? b'coz apache runs happily as well on another box without quoting the SYSVxx file. 2) if it is necessary, what's that for?