Re: [libvirt RFCv3] virfile: set pipe size in virFileWrapperFdNew to improve throughput

On Fri, Mar 25, 2022 at 02:52:05PM +0100, Claudio Fontana wrote: > On 3/25/22 2:13 PM, Daniel P. Berrangé wrote: >> On Fri, Mar 25, 2022 at 01:54:51PM +0100, 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; >>> --- >>> >>> see v2 at >>> https://listman.redhat.com/archives/libvir-list/2022-March/229423.html >>> >>> 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) >>> >>> >>> src/util/virfile.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++ >>> 1 file changed, 49 insertions(+) >>> >>> diff --git a/src/util/virfile.c b/src/util/virfile.c >>> index a04f888e06..876b865974 100644 >>> --- a/src/util/virfile.c >>> +++ b/src/util/virfile.c >>> @@ -201,6 +201,51 @@ 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. >>> + * >>> + * 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 sz; >>> + >>> + for (sz = 1024 * 1024; sz >= 64 * 1024; sz /= 2) { >>> + int rv = fcntl(fd, F_SETPIPE_SZ, sz); >>> + if (rv < 0 && errno == EPERM) { >>> + continue; /* retry with half the size */ >>> + } >>> + if (rv < 0) { >>> + break; >>> + } >>> + VIR_INFO("fd %d pipe size adjusted to %d", fd, sz); >>> + return 0; >>> + } >>> + VIR_WARN("failed to set pipe size to %d (errno=%d)", sz, errno); >>> + return -1; >>> +} >>> + >>> +#else /* !__linux__ */ >>> +static int virFileWrapperSetPipeSize(int fd) >>> +{ >>> + return 0; >>> +} >>> +#endif /* !__linux__ */ >>> + >>> + >>> /** >>> * virFileWrapperFdNew: >>> * @fd: pointer to fd to wrap >>> @@ -282,6 +327,10 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags) >>> >>> ret->cmd = virCommandNewArgList(iohelper_path, name, NULL); >>> >>> + if (virFileWrapperSetPipeSize(pipefd[!output]) < 0) { >>> + virReportError(VIR_ERR_SYSTEM_ERROR, "%s", _("unable to set pipe size, data transfer might be slow")); >> >> Push this into virFileWrapperSetPipeSize instead of the VIR_WARN >> there, and use virReportSystemError passing in the errno value too. > > > ok, what about also warning on EPERM? In the normal case we should succeed on the first try I think. We generally try to avoid any VIR_WARN in cases that we expect to be still functional. Users tend to complain when they get warnings for

these kind of things. I think coping with smaller max size is a normal situation, so its merely a perf factor, not a functional problem. With regards, Daniel