On 3/28/22 13:10, Daniel P. Berrangé wrote: > 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@xxxxxxx> >>> --- >>> 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 Yes, in that case we could call virReportSystemError(), but the way the code is currently written doesn't make much sense. Anyway, let me post a follow up patch that does report error. Michal
