semaphores

mmcds · 01-02-2008, 02:08 PM

I am using SuSE Linux and have reached my array limit for semaphores. I did the ipcs -u to see what I am using and ipcs -l to show my limits. I tried the ipcs -s to see what all arrays are being used and it is showing this

------ Semaphore Arrays --------
key semid owner perms nsems
0x00000000 360448 nobody 600 1
0x00000000 3997697 nobody 600 1
0x00000000 4030466 nobody 600 1

My question is how do I know what semid is belonging to what process that is running on my machine that I can safely destroy to get some of my arrays back?

btmiller · 01-02-2008, 02:34 PM

Hmmm ... I'm playing around with ipcs and I'm not sure if there's a way to do what you want because IIRC semaphores don't really "belong" to any given process. They have a process that creates tghem, but then any process with the appropriate permissions can then use them. Do you have a process that is creating semaphores but not removing them? If so that's an application bug and you should try to get it fixed. Otherwise you can adjust the per user and per process limits with the kernel.sem sysctl variable. There might be some other way of determining what processes hold a semaphore but I'm not familiar with it.

colucix · 01-02-2008, 02:41 PM

Code:

$ ipcs -p

------ Shared Memory Creator/Last-op --------
shmid      owner      cpid       lpid
32768      root       3288       3474
65537      root       3288       3348
2424834    colucix    13252      13263
2457603    colucix    13252      3288
2654212    colucix    13252      3288
2588678    colucix    13252      13252

where cpid is the process ID of the creator (the information I think you're looking for) and lpid is the process ID of the last process that accessed that particular segment of memory.

Edit: notes by btmiller are correct: killing a creator process will free shared memory and the related semaphores, but bugs may prevent the normal behavior.

btmiller · 01-02-2008, 03:27 PM

Aha ... you can look for the shared memory segment that corresponds with a particular semaphore in order to see the creator and who accessed it last (the shmid of the shared memory segment will be equal to the semaphore key). I didn't realize that but it would make sense that semaphores are implemented as part of the shared memory segments, which I guess is going on.

mmcds · 01-03-2008, 08:16 AM

Thanks for both of your replies. I am sure it is an application problem, but I just wanted to see if there was a way to relate an semid to an actual process to pinpoint it down further to go back to the developer to say ok this is what is causing this.