[libvirt RFCv3] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Daniel P. Berrangé]

On Fri, Mar 25, 2022 at 02:52:05PM +0100, Claudio Fontana wrote:
> On 3/25/22 2:13 PM, Daniel P. Berrangé wrote:
> > On Fri, Mar 25, 2022 at 01:54:51PM +0100, Claudio Fontana wrote:
> >> currently the only user of virFileWrapperFdNew is the qemu driver;
> >> virsh save is very slow with a default pipe size.
> >> This change improves throughput by ~400% on fast nvme or ramdisk.
> >>
> >> Best value currently measured is 1MB, which happens to be also
> >> the kernel default for the pipe-max-size.
> >>
> >> Signed-off-by: Claudio Fontana <cfontana at suse.de>
> >> ---
> >>
> >> see v2 at
> >> https://listman.redhat.com/archives/libvir-list/2022-March/229423.html
> >>
> >> Changes v2 -> v3:
> >>
> >> * removed reading of max-pipe-size from procfs,
> >>   instead make multiple attempts on EPERM with smaller sizes.
> >>   In the regular case, this should succeed on the first try.
> >>   (Daniel)
> >>
> >> Changes v1 -> v2:
> >>
> >> * removed VIR_FILE_WRAPPER_BIG_PIPE, made the new pipe resizing
> >>   unconditional (Michal)
> >>
> >> * moved code to separate functions (Michal)
> >>
> >> * removed ternary op, disliked in libvirt (Michal)
> >>
> >> * added #ifdef __linux__ (Ani Sinha)
> >>
> >> * try smallest value between currently best measured value (1MB)
> >>   and the pipe-max-size setting. If pipe-max-size cannot be read,
> >>   try kernel default max (1MB). (Daniel)
> >>
> >>
> >> src/util/virfile.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 49 insertions(+)
> >>
> >> diff --git a/src/util/virfile.c b/src/util/virfile.c
> >> index a04f888e06..876b865974 100644
> >> --- a/src/util/virfile.c
> >> +++ b/src/util/virfile.c
> >> @@ -201,6 +201,51 @@ struct _virFileWrapperFd {
> >>  };
> >>  
> >>  #ifndef WIN32
> >> +
> >> +#ifdef __linux__
> >> +
> >> +/**
> >> + * virFileWrapperSetPipeSize:
> >> + * @fd: the fd of the pipe
> >> + *
> >> + * Set best pipe size on the passed file descriptor for bulk transfers of data.
> >> + *
> >> + * default pipe size (usually 64K) is generally not suited for large transfers
> >> + * to fast devices. A value of 1MB has been measured to improve virsh save
> >> + * by 400% in ideal conditions. We retry multiple times with smaller sizes
> >> + * on EPERM to account for possible small values of /proc/sys/fs/pipe-max-size.
> >> + *
> >> + * Return value is 0 on success, -1 and errno set on error.
> >> + * OS note: only for linux, on other OS this is a no-op.
> >> + */
> >> +static int
> >> +virFileWrapperSetPipeSize(int fd)
> >> +{
> >> +    int sz;
> >> +
> >> +    for (sz = 1024 * 1024; sz >= 64 * 1024; sz /= 2) {
> >> +        int rv = fcntl(fd, F_SETPIPE_SZ, sz);
> >> +        if (rv < 0 && errno == EPERM) {
> >> +            continue; /* retry with half the size */
> >> +        }
> >> +        if (rv < 0) {
> >> +            break;
> >> +        }
> >> +        VIR_INFO("fd %d pipe size adjusted to %d", fd, sz);
> >> +        return 0;
> >> +    }
> >> +    VIR_WARN("failed to set pipe size to %d (errno=%d)", sz, errno);
> >> +    return -1;
> >> +}
> >> +
> >> +#else /* !__linux__ */
> >> +static int virFileWrapperSetPipeSize(int fd)
> >> +{
> >> +    return 0;
> >> +}
> >> +#endif /* !__linux__ */
> >> +
> >> +
> >>  /**
> >>   * virFileWrapperFdNew:
> >>   * @fd: pointer to fd to wrap
> >> @@ -282,6 +327,10 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
> >>  
> >>      ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
> >>  
> >> +    if (virFileWrapperSetPipeSize(pipefd[!output]) < 0) {
> >> +        virReportError(VIR_ERR_SYSTEM_ERROR, "%s", _("unable to set pipe size, data transfer might be slow"));
> > 
> > Push this into virFileWrapperSetPipeSize instead of the VIR_WARN
> > there, and use virReportSystemError passing in the errno value too.
> 
> 
> ok, what about also warning on EPERM? In the normal case we should succeed on the first try I think.

We generally try to avoid any VIR_WARN in cases that we expect to be
still functional. Users tend to complain when they get warnings for
these kind of things. I think coping with smaller max size is a normal
situation, so its merely a perf factor, not a functional problem.

With regards,
Daniel
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