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

[Stefano Garzarella]

On Thu, Oct 21, 2021 at 04:18:41PM +0100, Stefan Hajnoczi wrote:
> 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?

It's a little simpler to avoid having to recalculate the minimum for 
each attach/release of blockdevs.

When the blockdev does submit or unplug, the queue is flushed if the 
number of requests in the batch is greater or equal then the smallest 
aio-max-batch value of the blockdev and the AIO context.

> 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. ...

Whath about this:

  maximum number of requests to batch together into a single submission
  in the AIO backend. The smallest value between this and AIO context's 
  aio-max-batch value is chosen. ...

Thanks,
Stefano