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


From: Stefano Garzarella
Subject: Re: [PATCH 1/3] file-posix: add `aio-max-batch` option
Date: Mon, 25 Oct 2021 12:20:06 +0200

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




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