Re: [PATCH 03/21] Remove use of signalfd in block-raw-posix.c

[Avi Kivity <avi@redhat.com>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>>>
>>> Oh okay.  But signal delivery is slow; for example the FPU needs to 
>>> be reset.
>>
>> Is it really justified to add all of this extra code (including 
>> signalfd emulation) for something that probably isn't even measurable?
>
> We don't have to add signalfd emulation; we can simply use signal+pipe 
> in that case.
>
> We won't know if it's measurable or not until we measure it (or not).
>
>>
>> I like using wiz-bang features of Linux as much as the next guy, but 
>> I think we're stretching to justify it here :-)
>>
>
> I think it's worth it in this case.  It will become more important in 
> time, too.
>

Out of curiosity, I measured this:

[avi@balrog test]$ ./signal
 2777 ns/signal (pipe)
  844 ns/signal (signalfd)


At 10000 signals/sec, this will save about 2% cpu time.  It's definitely 
worthwhile for the handful of lines it takes.

test program:

#include <signal.h>
#include <unistd.h>
#include <sys/signalfd.h>
#include <sys/time.h>
#include <stdio.h>

static int wfd;

static void handler(int signum)
{
    char b;

    write(wfd, &b, 1);
}

static int create_pipe(void)
{
    int fd[2];
    sigset_t s;

    pipe(fd);
    wfd = fd[1];
    signal(SIGUSR1, handler);
    sigemptyset(&s);
    sigaddset(&s, SIGUSR1);
    sigprocmask(SIG_UNBLOCK, &s, NULL);
    return fd[0];
}

static int create_signalfd(void)
{
    sigset_t s;

    sigemptyset(&s);
    sigaddset(&s, SIGUSR1);
    sigprocmask(SIG_BLOCK, &s, NULL);
    return signalfd(-1, &s, 0);
}

static uint64_t time_usec(void)
{
    struct timeval tv;

    gettimeofday(&tv, NULL);
    return (uint64_t)tv.tv_sec * 1000000 + tv.tv_usec;
}

#define N 10000000

static void test(const char *name, int fd, int len)
{
    int i;
    uint64_t t1, t2;
    char buf[128];

    t1 = time_usec();
    for (i = 0; i < N; ++i) {
        raise(SIGUSR1);
        read(fd, buf, len);
    }
    t2 = time_usec();

    close(fd);

    printf("%5d ns/signal (%s)\n", 1000 * (t2 - t1) / N, name);
}

int main(int ac, char **av)
{
    test("pipe", create_pipe(), 1);
    test("signalfd", create_signalfd(), 128);
    return 0;
}


-- 
error compiling committee.c: too many arguments to function