Re: [Qemu-devel] [PATCH] virtio-blk: notify guest directly

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Dec 19, 2017 at 09:33:03PM +0800, sochin.jiang wrote:
> From: "sochin.jiang" <sochin.jiang@huawei.com>
> 
>  Till now, we've already notify guest as a batch mostly, an
>  extra BH won't decrease guest interrupts much, but cause a
>  significant notification loss. Generally, we could have 15%
>  or so performance lost in single queue IO models, as I tested.

I have CCed Ming Lei, who originally implemented batched notifications
in commit 5b2ffbe4d99843fd8305c573a100047a8c962327 ("virtio-blk:
dataplane: notify guest as a batch").  The original commit mentions a 13K
interrupt/sec reduction, which is significant.

Which host storage device are you benchmarking and what is the benchmark
configuration?

How many interrupts/sec does the guest report (cat /proc/interrupts)
before and after this patch?

In the past I've noticed performance can vary significantly depending on
QEMUBH ordering in the AioContext->first_bh list.  Can you measure the
latency from virtio_blk_data_plane_notify() to aio_bh_poll() and compare
against the latency from virtio_blk_data_plane_notify() to
notify_guest_bh()?

> @@ -75,7 +67,12 @@ static void notify_guest_bh(void *opaque)
>      }
>  }
>  
> -/* Context: QEMU global mutex held */

Please keep this doc comment for virtio_blk_data_plane_create().

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