[libvirt RFC] virFile: new VIR_FILE_WRAPPER_BIG_PIPE to improve performance
Daniel P. Berrangé
berrange at redhat.com
Mon Mar 14 17:48:57 UTC 2022
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 at suse.de>
> >> ---
> >> 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
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