On Mon, Mar 28, 2022 at 01:06:03PM +0200, Michal Prívozník wrote:
On 3/25/22 16:10, Claudio Fontana wrote:
> 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(a)suse.de>
> ---
> src/util/virfile.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 46 insertions(+)
>
> see v2 at
>
https://listman.redhat.com/archives/libvir-list/2022-March/229423.html
>
> Changes v3 -> v4:
>
> * changed INFO and WARN messages to DEBUG (Daniel)
>
> Changes v2 -> v3:
>
> * removed reading of max-pipe-size from procfs,
> instead make multiple attempts on EPERM with smaller sizes.
> In the regular case, this should succeed on the first try.
> (Daniel)
>
> Changes v1 -> v2:
>
> * removed VIR_FILE_WRAPPER_BIG_PIPE, made the new pipe resizing
> unconditional (Michal)
>
> * moved code to separate functions (Michal)
>
> * removed ternary op, disliked in libvirt (Michal)
>
> * added #ifdef __linux__ (Ani Sinha)
>
> * try smallest value between currently best measured value (1MB)
> and the pipe-max-size setting. If pipe-max-size cannot be read,
> try kernel default max (1MB). (Daniel)
>
>
> diff --git a/src/util/virfile.c b/src/util/virfile.c
> index a04f888e06..87539be0b9 100644
> --- a/src/util/virfile.c
> +++ b/src/util/virfile.c
> @@ -201,6 +201,50 @@ 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;
> + }
> + virReportSystemError(errno, "%s", _("unable to set pipe size,
data transfer might be slow"));
This should have been VIR_WARN(). It's weird to report an error when the
function returns void.
Actually I said to report an error in prvious version, as I figured we
were handling the expect EPERM, but I guess we could even fail the
last 64 KB iteration and stick with the default. So we need a slight
tweak:
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) {
virReportSystemError(errno, "%s", _("unable to set pipe
size"));
return -1;
}
VIR_DEBUG("fd %d pipe size adjusted to %d", fd, sz);
return 0;
}
VIR_WARN("Could set pipe size to 64 KB, leaving on default size");
return 0;
}
then the caller can treat -1 as fatal
With regards,
Daniel
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