[libvirt RFCv4] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Daniel P. Berrangé]

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 at 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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