On Mon, Mar 14, 2022 at 06:38:31PM +0100, Claudio Fontana wrote: > 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@xxxxxxx> > >> --- > >> 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. That's reasonable. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
