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

On 3/14/22 6:17 PM, Daniel P. Berrangé wrote: > On Sat, Mar 12, 2022 at 05:30:01PM +0100, Claudio Fontana wrote: >> 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"); > > This is fine as an experiment but I don't think it is that safe > to use in the real world. There could be a variety of reasons why > an admin can enlarge this value, and we shouldn't assume the max > size is sensible for libvirt/QEMU to use. > > I very much suspect there are diminishing returns here in terms > of buffer sizes. > > 64k is obvious too small, but 1 MB, may be sufficiently large > that the bottleneck is then elsewhere in our code. IOW, If the > pipe max size is 100 MB, we shouldn't blindly use it. Can you > do a few tests with varying sizes to see where a sensible > tradeoff falls ? Hi Daniel, this is a very good point. Actually I see very diminishing returns after the default pipe-max-size (1MB). The idea was that beyond allowing larger size, the admin could have set a _smaller_ pipe-max-size, so we want to use that in that case, otherwise an attempt to use 1MB would result in EPERM, if the process does not have CAP_SYS_RESOURCE or CAP_SYS_ADMIN. I am not sure if used with Kubevirt, for example, CAP_SYS_RESOURCE or CAP_SYS_ADMIN would be available...? So maybe one idea could be to use the minimum between /proc/sys/fs/pipe-max-size and for example 1MB, but will do more testing to see where the actual break point is.