Re: [libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance

the first user is the qemu driver, virsh save/resume would slow to a crawl with a default pipe size (64k). This improves the situation by 400%. Going through io_helper still seems to incur in some penalty (~15%-ish) compared with direct qemu migration to a nc socket to a file. Signed-off-by: Claudio Fontana <cfontana(a)suse.de&gt; --- src/qemu/qemu_driver.c | 6 +++--- src/qemu/qemu_saveimage.c | 11 ++++++----- src/util/virfile.c | 12 ++++++++++++ src/util/virfile.h | 1 + 4 files changed, 22 insertions(+), 8 deletions(-) Hello, I initially thought this to be a qemu performance issue, so you can find the discussion about this in qemu-devel: "Re: bad virsh save /dev/null performance (600 MiB/s max)" https://lists.gnu.org/archive/html/qemu-devel/2022-03/msg03142.html RFC since need to validate idea, and it is only lightly tested: save - about 400% benefit in throughput, getting around 20 Gbps to /dev/null, and around 13 Gbps to a ramdisk. By comparison, direct qemu migration to a nc socket is around 24Gbps. restore - not tested, _should_ also benefit in the "bypass_cache" case coredump - not tested, _should_ also benefit like for save Thanks for your comments and review, Claudio diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index c1b3bd8536..be248c1e92 100644 --- a/src/qemu/qemu_driver.c +++ b/src/qemu/qemu_driver.c @@ -3044,7 +3044,7 @@ doCoreDump(virQEMUDriver *driver, virFileWrapperFd *wrapperFd = NULL; int directFlag = 0; bool needUnlink = false; - unsigned int flags = VIR_FILE_WRAPPER_NON_BLOCKING; + unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE; const char *memory_dump_format = NULL; g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver); g_autoptr(virCommand) compressor = NULL; @@ -3059,7 +3059,7 @@ doCoreDump(virQEMUDriver *driver, /* Create an empty file with appropriate ownership. */ if (dump_flags & VIR_DUMP_BYPASS_CACHE) { - flags |= VIR_FILE_WRAPPER_BYPASS_CACHE; + wrapperFlags |= VIR_FILE_WRAPPER_BYPASS_CACHE; directFlag = virFileDirectFdFlag(); if (directFlag < 0) { virReportError(VIR_ERR_OPERATION_FAILED, "%s", @@ -3072,7 +3072,7 @@ doCoreDump(virQEMUDriver *driver, &needUnlink)) < 0) goto cleanup; - if (!(wrapperFd = virFileWrapperFdNew(&fd, path, flags))) + if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) goto cleanup; if (dump_flags & VIR_DUMP_MEMORY_ONLY) { diff --git a/src/qemu/qemu_saveimage.c b/src/qemu/qemu_saveimage.c index c0139041eb..1b522a1542 100644 --- a/src/qemu/qemu_saveimage.c +++ b/src/qemu/qemu_saveimage.c @@ -267,7 +267,7 @@ qemuSaveImageCreate(virQEMUDriver *driver, int fd = -1; int directFlag = 0; virFileWrapperFd *wrapperFd = NULL; - unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING; + unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING | VIR_FILE_WRAPPER_BIG_PIPE; /* Obtain the file handle. */ if ((flags & VIR_DOMAIN_SAVE_BYPASS_CACHE)) { @@ -463,10 +463,11 @@ qemuSaveImageOpen(virQEMUDriver *driver, if ((fd = qemuDomainOpenFile(cfg, NULL, path, oflags, NULL)) < 0) return -1; - if (bypass_cache && - !(*wrapperFd = virFileWrapperFdNew(&fd, path, - VIR_FILE_WRAPPER_BYPASS_CACHE))) - return -1; + if (bypass_cache) { + unsigned int wrapperFlags = VIR_FILE_WRAPPER_BYPASS_CACHE | VIR_FILE_WRAPPER_BIG_PIPE; + if (!(*wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) + return -1; + } data = g_new0(virQEMUSaveData, 1); diff --git a/src/util/virfile.c b/src/util/virfile.c index a04f888e06..fdacd17890 100644 --- a/src/util/virfile.c +++ b/src/util/virfile.c @@ -282,6 +282,18 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags) ret->cmd = virCommandNewArgList(iohelper_path, name, NULL); + if (flags & VIR_FILE_WRAPPER_BIG_PIPE) { + /* + * virsh save/resume would slow to a crawl with a default pipe size (usually 64k). + * This improves the situation by 400%, although going through io_helper still incurs + * in a performance penalty compared with a direct qemu migration to a socket. + */ + int pipe_sz, rv = virFileReadValueInt(&pipe_sz, "/proc/sys/fs/pipe-max-size");

+ if (rv != 0) { + pipe_sz = 1024 * 1024; /* common default for pipe-max-size */ + } + fcntl(pipefd[output ? 0 : 1], F_SETPIPE_SZ, pipe_sz); + }