It depends on how big your processes are, and whether or not you have
kernel safety features turned on.
For example, if I have a 500M program that tries to run a program that
fits in 30K of space, I still have to have 500M of free memory to run it.
why? The way UNIX creates processes is by forking before execing.
Forking creates two IDENTICAL copies of a process, with the only
difference between the ttwo being the return value of fork(). That means
it has to copy the ENTIRE process in order to fork, so you have to have
that much memory available (it doesn't physically copy it - it's got lots
of nifty optimizations including Copy-On-Write, but it has to at least
reserve the space first). So, if you have a large process trying to run
external commands, that could easily bbe causing your problems.
There's some overcommit setting that you can turn off, but I forget what
this is.
Jon
On Thu, 2 Oct 2003, Tom Stevelt wrote:
>
> We are having issues with fork() failing sporadically -- but we can't
> figure what resource is short supply.
>
> This shows we still have 45 meg of ram available.
>
> rhd </home/tms/src/systest>$ free -m
> total used free shared buffers cached
> Mem: 123 113 9 0 4 30
> -/+ buffers/cache: 78 45
> Swap: 250 19 231
>
> This is the strace output from a test program. Can anybody suggest how
> to tell which kernel parameter is in short supply?
>
> rhd </home/tms/src/systest>$ strace -f -ores forktest 3
> fork failed, errno 12
> rhd </home/tms/src/systest>$
> rhd </home/tms/src/systest>$ cat res
> 4042 execve("./forktest", ["forktest", "3"], [/* 26 vars */]) = 0
> 4042 uname({sys="Linux", node="rhd", ...}) = 0
> 4042 brk(0) = 0x80497dc
> 4042 open("/etc/ld.so.preload", O_RDONLY) = -1 ENOENT (No such file or directory)
> 4042 open("/etc/ld.so.cache", O_RDONLY) = 3
> 4042 fstat64(3, {st_mode=S_IFREG|0644, st_size=48658, ...}) = 0
> 4042 old_mmap(NULL, 48658, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40014000
> 4042 close(3) = 0
> 4042 open("/lib/i686/libc.so.6", O_RDONLY) = 3
> 4042 read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\260Y\1"..., 512) = 512
> 4042 fstat64(3, {st_mode=S_IFREG|0755, st_size=1452984, ...}) = 0
> 4042 old_mmap(0x42000000, 1290052, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x42000000
> 4042 old_mmap(0x42134000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x134000) = 0x42134000
> 4042 old_mmap(0x42139000, 8004, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x42139000
> 4042 close(3) = 0
> 4042 old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40020000
> 4042 munmap(0x40014000, 48658) = 0
> 4042 mmap2(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40021000
> 4042 fork() = -1 ENOMEM (Cannot allocate memory)
> 4042 fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
> 4042 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40014000
> 4042 write(1, "fork failed, errno 12\n", 22) = 22
> 4042 munmap(0x40014000, 4096) = 0
> 4042 _exit(22) = ?
> rhd </home/tms/src/systest>$
>
> Thanks - Tom
>
> -----Tom Stevelt--------------------Victory Wholesale Grocers-----------
> tms@xxxxxxx 400 Victory Drive
> 513-746-1742 ext 1224 Springboro, OH 45066
> ------------------------------------------------------------------------
>
>
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