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

[Daniel P. Berrangé]

On Fri, Mar 25, 2022 at 02:57:14PM +0100, Claudio Fontana wrote:
> On 3/25/22 2:54 PM, Daniel P. Berrangé wrote:
> > 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
> 
> INFO? DEBUG? Or nothing at all? Thanks again

I think DEBUG is sufficient for any logging in this code.


With regards,
Daniel
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