sigprocmask and fork

Alexandr Matveev Wed, 26 Oct 2011 06:14:25 -0700

Hi,

We are using FreeBSD 8.2 on our servers for high load projects.

When I was preparing system for production I saw strange (as I think)behavior,

that leads to increased load on servers.

If I made truss on httpd (apache22) process, I saw too much sigprocmasksyscalls:


24822: sigprocmask(SIG_BLOCK,0x0,0x0)            = 0 (0x0)
24822: sigprocmask(SIG_BLOCK,0x0,0x0)            = 0 (0x0)
24822: sigprocmask(SIG_BLOCK,0x0,0x0)            = 0 (0x0)
24822: sigprocmask(SIG_BLOCK,0x0,0x0)            = 0 (0x0)
24822: sigprocmask(SIG_BLOCK,0x0,0x0)            = 0 (0x0)
... too many lines ...

and

24822:

IGWINCH|SIGINFO|SIGUSR1|SIGUSR2,0x0) = 0 (0x0)
24822: sigprocmask(SIG_SETMASK,0x0,0x0)          = 0 (0x0)
24822:

IGWINCH|SIGINFO|SIGUSR1|SIGUSR2,0x0) = 0 (0x0)
24822: sigprocmask(SIG_SETMASK,0x0,0x0)          = 0 (0x0)
24822:

IGWINCH|SIGINFO|SIGUSR1|SIGUSR2,0x0) = 0 (0x0)
24822: sigprocmask(SIG_SETMASK,0x0,0x0)          = 0 (0x0)
... too many lines ...


but apache, and modules loaded from it do not call this directly.
I was trying to use DTRACE for getting information about syscalls, and I
got same result.

I wrote a tiny sample

code:
$ cat sigproc_test.c
#include<unistd.h>

main()
{
        fork();
}
I ran it on FreeBSD with different compilers:
$ cc sigproc_test.c -o sigproc_test
$ truss ./sigproc_test 2>&1 | grep sigprocmask | wc -l
       8
$ g++ sigproc_test.c -o sigproc_test
$ truss ./sigproc_test 2>&1 | grep sigprocmask | wc -l
      20

Is it normal to make so many sigprocmask syscalls for such simple program?

For example, there is no sigprocmask syscalls when I run it on DebianLinux.

Another sample. Here we have sigprocmask syscalls on Linux too, butFreeBSD makes this syscall significantly more often:

$ cat test.c
#include<unistd.h>

main()
{
    int i;
    sleep(2);
    for (i = 0; i<3; i++) {
        int pid = fork();
        if (!pid) {
            sleep(0.5);
            return 0;
        }
        sleep(2);
    }
}

FreeBSD 8.2:
# truss -f ./test
... SKIPPED ...

48666: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48666: __sysctl(0xbfbfe6a4,0x2,0x28192700,0xbfbfe6ac,0x0,0x0) = 0 (0x0)

48666: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48666: nanosleep({2.000000000 }) = 0 (0x0)
48666: fork() = 48667 (0xbe1b)
48667: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)

48667: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48667: process exit, rval = 0
48666: nanosleep({2.000000000 }) = 0 (0x0)
48666: fork() = 48669 (0xbe1d)

48669: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48669: process exit, rval = 0
48666: nanosleep({2.000000000 }) = 0 (0x0)
48666: fork() = 48674 (0xbe22)
48674: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)

48674: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48674: process exit, rval = 0
48666: nanosleep({2.000000000 }) = 0 (0x0)

48666: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)

48666: sigprocmask(SIG_SETMASK,0x0,0x0) = 0 (0x0)
48666: process exit, rval = 0

Linux:
# strace ./test
... SKIPPED ...
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({2, 0}, 0x7fff0e6b7050) = 0

clone(child_stack=0,flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7fb553b0d9d0) = 24310

rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
nanosleep({2, 0}, 0x7fff0e6b7050) = 0

clone(child_stack=0,flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7fb553b0d9d0) = 24311

rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
nanosleep({2, 0}, 0x7fff0e6b7050) = 0

clone(child_stack=0,flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7fb553b0d9d0) = 24312

rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
nanosleep({2, 0}, 0x7fff0e6b7050) = 0
exit_group(0) = ?

--
Alexander Matveev
.masterhost


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sigprocmask and fork

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