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

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&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. 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