linux-aio: limit the batch size using `aio-max-batch` parameter

linux-aio: limit the batch size using `aio-max-batch` parameter

When there are multiple queues attached to the same AIO context,
some requests may experience high latency, since in the worst case
the AIO engine queue is only flushed when it is full (MAX_EVENTS) or
there are no more queues plugged.

Commit 2558cb8dd4 ("linux-aio: increasing MAX_EVENTS to a larger
hardcoded value") changed MAX_EVENTS from 128 to 1024, to increase
the number of in-flight requests. But this change also increased
the potential maximum batch to 1024 elements.

When there is a single queue attached to the AIO context, the issue
is mitigated from laio_io_unplug() that will flush the queue every
time is invoked since there can't be others queue plugged.

Let's use the new `aio-max-batch` IOThread parameter to mitigate
this issue, limiting the number of requests in a batch.

We also define a default value (32): this value is obtained running
some benchmarks and it represents a good tradeoff between the latency
increase while a request is queued and the cost of the io_submit(2)
system call.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20210721094211.69853-4-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>