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virfile: set pipe size in virFileWrapperFdNew to improve throughput

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currently the only user of virFileWrapperFdNew is the qemu driver;
virsh save is very slow with a default pipe size.
This change improves throughput by ~400% on fast nvme or ramdisk.

Best value currently measured is 1MB, which happens to be also
the kernel default for the pipe-max-size.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
This commit is contained in:
Claudio Fontana 2022-03-25 16:10:19 +01:00 committed by Michal Privoznik
parent 4d7bb0177a
commit c61d1e9ba0
1 changed files with 50 additions and 0 deletions
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50
src/util/virfile.c
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@ -201,6 +201,54 @@ struct _virFileWrapperFd {
};
#ifndef WIN32
#ifdef __linux__
/**
* virFileWrapperSetPipeSize:
* @fd: the fd of the pipe
*
* Set best pipe size on the passed file descriptor for bulk transfers of data.
*
* default pipe size (usually 64K) is generally not suited for large transfers
* to fast devices. A value of 1MB has been measured to improve virsh save
* by 400% in ideal conditions. We retry multiple times with smaller sizes
* on EPERM to account for possible small values of /proc/sys/fs/pipe-max-size.
*
* OS note: only for linux, on other OS this is a no-op.
*/
static void
virFileWrapperSetPipeSize(int fd)
{
int sz;
for (sz = 1024 * 1024; sz >= 64 * 1024; sz /= 2) {
int rv = fcntl(fd, F_SETPIPE_SZ, sz);
if (rv < 0 && errno == EPERM) {
VIR_DEBUG("EPERM trying to set fd %d pipe size to %d", fd, sz);
continue; /* retry with half the size */
}
if (rv < 0) {
break;
}
VIR_DEBUG("fd %d pipe size adjusted to %d", fd, sz);
return;
}
VIR_WARN("unable to set pipe size, data transfer might be slow: %s",
g_strerror(errno));
}
#else /* !__linux__ */
static void
virFileWrapperSetPipeSize(int fd G_GNUC_UNUSED)
{
return;
}
#endif /* !__linux__ */
/**
* virFileWrapperFdNew:
* @fd: pointer to fd to wrap
@ -282,6 +330,8 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
virFileWrapperSetPipeSize(pipefd[output]);
if (output) {
virCommandSetInputFD(ret->cmd, pipefd[0]);
virCommandSetOutputFD(ret->cmd, fd);