Re: [Qemu-devel] [Qemu-block] [PATCH] linux-aio: Cancel BH if not needed

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Jun 15, 2016 at 01:16:42PM +0200, Kevin Wolf wrote:
> linux-aio uses a BH in order to make sure that the remaining completions
> are processed even in nested event loops of completion callbacks in
> order to avoid deadlocks.
> 
> There is no need, however, to have the BH overhead for the first call
> into qemu_laio_completion_bh() or after all pending completions have
> already been processed. Therefore, this patch calls directly into
> qemu_laio_completion_bh() in qemu_laio_completion_cb() and cancels
> the BH after qemu_laio_completion_bh() has processed all pending
> completions.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/linux-aio.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

I ran 4 x iodepth=16 random 4KB read I/O benchmarks.  There might be an
improvement but it's within the error margin.  My benchmarking setup can
be noisy...

Anyway, this patch doesn't hurt performance.  Guest and host are RHEL 7.2.

$ ./analyze.py runs/
Name                                          IOPS   Error
linux-aio-bh-optimizations-ccb9dc1      12942616.0 ± 16.83%
linux-aio-bh-optimizations-ccb9dc1-2    13833110.4 ± 4.74%
linux-aio-bh-optimizations-off-23b0d9f  13303981.4 ± 2.21%

qemu-system-x86_64 -pidfile qemu.pid -daemonize \
                   -machine accel=kvm -cpu host \
		   -smp 4 -m 1024 \
		   -netdev user,id=netdev0,hostfwd=tcp::2222-:22 \
                   -object iothread,id=iothread0 \
		   -device virtio-net-pci,netdev=netdev0 \
		   -drive if=none,id=drive0,file=/var/lib/libvirt/images/test.img,format=raw,aio=native,cache=none \
                   -device virtio-blk-pci,drive=drive0 \
		   -drive if=none,id=drive1,file=/dev/nullb0,format=raw,aio=native,cache=none \
                   -device virtio-blk-pci,drive=drive1 \
		   -display none

$ cat fio.job
[global]
filename=/dev/vdb
ioengine=libaio
direct=1
runtime=60
ramp_time=5
gtod_reduce=1

[job1]
numjobs=4
iodepth=16
rw=randread
bs=4K

Stefan

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