[libvirt RFCv4] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Michal Prívozník]

On 3/25/22 16:10, 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>
> ---
>  src/util/virfile.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> see v2 at
> https://listman.redhat.com/archives/libvir-list/2022-March/229423.html
> 
> Changes v3 -> v4:
> 
> * changed INFO and WARN messages to DEBUG (Daniel)
> 
> 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)
> 
> 
> diff --git a/src/util/virfile.c b/src/util/virfile.c
> index a04f888e06..87539be0b9 100644
> --- a/src/util/virfile.c
> +++ b/src/util/virfile.c
> @@ -201,6 +201,50 @@ 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.
> + *
> + * OS note: only for linux, on other OS this is a no-op.
> + */
> +static void
> +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) {
> +            VIR_DEBUG("EPERM trying to set fd %d pipe size to %d", fd, sz);
> +            continue; /* retry with half the size */
> +        }
> +        if (rv < 0) {
> +            break;
> +        }
> +        VIR_DEBUG("fd %d pipe size adjusted to %d", fd, sz);
> +        return;
> +    }
> +    virReportSystemError(errno, "%s", _("unable to set pipe size, data transfer might be slow"));

This should have been VIR_WARN(). It's weird to report an error when the
function returns void.

> +}
> +
> +#else /* !__linux__ */
> +static void virFileWrapperSetPipeSize(int fd)
The @fd argument is unused and thus has to be marked as such.

> +{
> +    return;
> +}
> +#endif /* !__linux__ */
> +
> +
>  /**
>   * virFileWrapperFdNew:
>   * @fd: pointer to fd to wrap
> @@ -282,6 +326,8 @@ virFileWrapperFdNew(int *fd, const char *name, unsigned int flags)
>  
>      ret->cmd = virCommandNewArgList(iohelper_path, name, NULL);
>  
> +    virFileWrapperSetPipeSize(pipefd[!output]);

This feels weird, because just a few lines below the pipefd[!output]) is
closed. As I said earlier, it doesn't matter what end of the pipe we set
the size on, therefore, let's switch over to pipefd[output].

> +
>      if (output) {
>          virCommandSetInputFD(ret->cmd, pipefd[0]);
>          virCommandSetOutputFD(ret->cmd, fd);

Reviewed-by: Michal Privoznik <mprivozn at redhat.com>

and pushed.

Michal