[libvirt RFC v2] virfile: set pipe size in virFileWrapperFdNew to improve throughput

[Daniel P. Berrangé]

On Mon, Mar 21, 2022 at 09:13:20AM +0100, Claudio Fontana wrote:
> virsh save is very slow with a default pipe size, so set a larger one.
> 
> This change improves throughput by ~400% on fast nvme or ramdisk,
> for the current only user of virFileWrapperFdNew: the qemu driver.
> 
> Best value currently measured is 1MB, which happens to be also
> the kernel default for the pipe-max-size.
> 
> We do not try to use a pipe buffer larger than what the setting
> of /proc/sys/fs/pipe-max-size currently allows.
> 
> Signed-off-by: Claudio Fontana <cfontana at suse.de>
> ---
>  src/util/virfile.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 69 insertions(+)
> 
> see v1 at
> https://listman.redhat.com/archives/libvir-list/2022-March/229252.html
> 
> 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..13bdd42c68 100644
> --- a/src/util/virfile.c
> +++ b/src/util/virfile.c
> @@ -201,6 +201,71 @@ struct _virFileWrapperFd {
>  };
>  
>  #ifndef WIN32
> +
> +#ifdef __linux__
> +/**
> + * virFileWrapperGetBestPipeSize:
> + *
> + * get the best pipe size to use with virFileWrapper.
> + *
> + * We first check the maximum we are allowed by the system pipe-max-size,
> + * and then use the minimum between that and our tested best value.
> + * This is because a request beyond pipe-max-size may fail with EPERM.
> + * If we are unable to read pipe-max-size, use the kernel default (1MB).
> + *
> + * Return value is the pipe size to use.
> + */
> +
> +static int virFileWrapperGetBestPipeSize(void)
> +{
> +    const char path[] = "/proc/sys/fs/pipe-max-size";
> +    int best_sz = 1024 * 1024; /* good virsh save results with this size */
> +    int max_sz;
> +
> +    if (virFileReadValueInt(&max_sz, path) < 0) {
> +        max_sz = 1024 * 1024; /* this is the kernel default pipe-max-size */
> +        VIR_WARN("failed to read %s, trying default %d", path, max_sz);
> +    } else if (max_sz > best_sz) {
> +        max_sz = best_sz;
> +    }
> +    return max_sz;
> +}
> +
> +/**
> + * 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. This has been measured to improve virsh save by 400%
> + * in ideal conditions.
> + *
> + * 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 pipe_sz = virFileWrapperGetBestPipeSize();

I wonder if we shouldn't just ignore the proc setting and instead

   for (sz = 1024 * 1024 ; sz >= 64 * 1024; sz /= 2) {
      int rv = fcntl(fd, F_SETPIPE_SZ, sz);
      if (rv < 0 && errno == EPERM) {
        continue;
      }
      if (rv < 0) {
         virReportError(...)
	 return -1;
      }
      
      VIR_INFO("fd %d pipe size adjusted to %d", fd, sz);
      return 0;
   }


We'll only have 1 loop iteration in the default case, and 4 iterations
in the worst case, and gracefully leave it on the default if the last
ieratino fails

With regards,
Daniel
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