Phillip Ezolt wrote:
> Does flock/fcntl wake up all of the processes that are waiting on the lock?
>
> If so, has the thundering herd problem just been pushed into flock/fcntl
> instead of accept?
>
> >
> > I'm willing to accept any well founded explanation, and this is where
> > most of the concern has been coming from.
yes, as can be demonstrated by running the attached program, eg:
slock -s 60
will start 60 contending processes, and the load average will
tend towards 60. the load should tend to 1 since at most one
process can actually hold the file lock.
at a first glance this would appear tricky to fix. fixing the accept()
race looks to be trivial if we had a decent semaphore implementation.
who's working on that?
jan-simon.
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Content-Type: text/plain; charset=us-ascii;
name="slock.c"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
filename="slock.c"
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/file.h>
#include <string.h>
static int
forkn(int n)
{
int i;
pid_t pid;
for (i = 0; i < n; i++) {
pid = fork();
switch (pid) {
case -1:
fprintf(stderr, "slock: fork: %s\n", strerror(errno));
break;
case 0:
return (i);
default:
break;
}
}
return (n);
}
int
main(int c, char *v[])
{
int fd = STDIN_FILENO;
int ch, id, nservers, nxid;
char xid[12];
nservers = 1;
while ((ch = getopt(c, v, "s:")) != EOF) {
switch (ch) {
case 's':
nservers = atoi(optarg);
if (nservers < 1)
nservers = 1;
break;
}
}
id = forkn(nservers);
nxid = sprintf(xid, "%d ", id);
for (;;) {
flock(fd, LOCK_EX);
flock(fd, LOCK_UN);
/*write(1, xid, nxid);*/
}
}
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