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

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&gt; > --- > 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.