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

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();