Re: [PATCH 1/3] file-posix: add `aio-max-batch` option

[Stefan Hajnoczi]

On Thu, Sep 23, 2021 at 04:30:58PM +0200, Stefano Garzarella wrote:
> Commit d7ddd0a161 ("linux-aio: limit the batch size using
> `aio-max-batch` parameter") added a way to limit the batch size
> of Linux AIO backend for the entire AIO context.
> 
> The same AIO context can be shared by multiple devices, so
> latency-sensitive devices may want to limit the batch size even
> more to avoid increasing latency.
> 
> For this reason we add the `aio-max-batch` option to the file
> backend, which will be used by the next commits to limit the size of
> batches including requests generated by this device.
> 
> Suggested-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  qapi/block-core.json | 5 +++++
>  block/file-posix.c   | 9 +++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index c8ce1d9d5d..1a8ed325bc 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -2851,6 +2851,10 @@
>  #              for this device (default: none, forward the commands via 
> SG_IO;
>  #              since 2.11)
>  # @aio: AIO backend (default: threads) (since: 2.8)
> +# @aio-max-batch: maximum number of requests in an AIO backend batch that
> +#                 contains request from this block device. 0 means that the

The first sentence is a little unclear. I guess s/request/requests/ but
that still doesn't doesn't fully explain how this works.

Does the AIO backend use the minimum aio-max-batch value of all its
blockdevs?

Maybe:

  maximum number of requests to batch together into a single submission
  in the AIO backend. If multiple BlockdevOptionsFile sharing an AIO
  backend have different values the smallest value is chosen. ...

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