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

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On Mon, Mar 14, 2022 at 5:06 PM Michal Prívozník <mprivozn@xxxxxxxxxx> wrote:
>
> On 3/12/22 17:30, 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
>
> Hey, I like this idea, but couple of points below.
>
> >
> >
> > 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) {
>
>
> I believe we don't need this flag. I mean, the plain fact that
> virFileWrapper is used means that caller wants to avoid VFS because it's
> interested in speed. Therefore, this code could be done unconditionally.
>
> > +        /*
> > +         * 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.
> > +         */
>
> This belongs into the commit message. This code has no knowledge about
> qemu. What you can mention here is the performance benefit.
> Also, QEMU migrating straight to a socket is going to have performance
> benefit but only in a few cases, because if it's a migration into a file
> then VFS (and thus caching) is involved. Thus, if you migrate into a
> file and have enough free RAM for caches then yes, it's going to be
> faster. But if you don't have free RAM then it's going to be way slower.
>
> > +        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);
>
> Alternative implementation would be to call fcntl() only if we know
> we've succeeded in reading /proc/.../pipe-max-size, like this:
>
> int pipe_sz;
> int rv = virFileReadValueInt(&pipe_size, "/proc/sys/fs/pipe-max-size");
>
> if (rv >= 0)
>   fcntl(pipefd[output ? 0 : 1], F_SETPIPE_SZ, pipe_sz);
>
> (notice I've declared variables on separate lines, we like it that way)
>
> Now, what can we do about that ternary operator? It doesn't look nice.
> But I guess we can hardcode just one end of the pipe, because it's the
> actual pipe and not FD we are modifying here.
>
> Lastly, let's add some error checking:
>
> int pipe_sz;
> int rv = virFileReadValueInt(&pipe_size, "/proc/sys/fs/pipe-max-size");
>
> if (rv == -2) {
>   /* file doesn't exist */

Yes here we need to distinguish between error on linux vs legit error
on non-linux systems.





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