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

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"));

+ } + if (output) { virCommandSetInputFD(ret->cmd, pipefd[0]); virCommandSetOutputFD(ret->cmd, fd);