Re: [libvirt RFC v2] virfile: set pipe size in virFileWrapperFdNew to improve throughput

On Mon, Mar 21, 2022 at 09:13:20AM +0100, Claudio Fontana wrote: > virsh save is very slow with a default pipe size, so set a larger one. > > This change improves throughput by ~400% on fast nvme or ramdisk, > for the current only user of virFileWrapperFdNew: the qemu driver. > > Best value currently measured is 1MB, which happens to be also > the kernel default for the pipe-max-size. > > We do not try to use a pipe buffer larger than what the setting > of /proc/sys/fs/pipe-max-size currently allows. > > Signed-off-by: Claudio Fontana <cfontana(a)suse.de&gt; > --- > src/util/virfile.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 69 insertions(+) > > see v1 at > https://listman.redhat.com/archives/libvir-list/2022-March/229252.html > > 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..13bdd42c68 100644 > --- a/src/util/virfile.c > +++ b/src/util/virfile.c > @@ -201,6 +201,71 @@ struct _virFileWrapperFd { > }; > > #ifndef WIN32 > + > +#ifdef __linux__ > +/** > + * virFileWrapperGetBestPipeSize: > + * > + * get the best pipe size to use with virFileWrapper. > + * > + * We first check the maximum we are allowed by the system pipe-max-size, > + * and then use the minimum between that and our tested best value. > + * This is because a request beyond pipe-max-size may fail with EPERM. > + * If we are unable to read pipe-max-size, use the kernel default (1MB). > + * > + * Return value is the pipe size to use. > + */ > + > +static int virFileWrapperGetBestPipeSize(void) > +{ > + const char path[] = "/proc/sys/fs/pipe-max-size"; > + int best_sz = 1024 * 1024; /* good virsh save results with this size */ > + int max_sz; > + > + if (virFileReadValueInt(&max_sz, path) < 0) { > + max_sz = 1024 * 1024; /* this is the kernel default pipe-max-size */ > + VIR_WARN("failed to read %s, trying default %d", path, max_sz); > + } else if (max_sz > best_sz) { > + max_sz = best_sz; > + } > + return max_sz; > +} > + > +/** > + * 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. This has been measured to improve virsh save by 400% > + * in ideal conditions. > + * > + * Return value is 0 on success, -1 and errno set on error. > + * OS note: only for linux, on other OS this is a no-op. > + */ > +static int > +virFileWrapperSetPipeSize(int fd) > +{ > + int pipe_sz = virFileWrapperGetBestPipeSize(); I wonder if we shouldn't just ignore the proc setting and instead for (sz = 1024 * 1024 ; sz >= 64 * 1024; sz /= 2) { int rv = fcntl(fd, F_SETPIPE_SZ, sz); if (rv < 0 && errno == EPERM) { continue; } if (rv < 0) { virReportError(...) return -1; } VIR_INFO("fd %d pipe size adjusted to %d", fd, sz); return 0; } We'll only have 1 loop iteration in the default case, and 4 iterations in the worst case, and gracefully leave it on the default if the last ieratino fails With regards, Daniel