Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

Stefano Garzarella
2022-01-20T15:08:52Z

On Fri, Jan 14, 2022 at 2:40 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Jan 14, 2022 at 02:38:16PM +0100, Stefano Garzarella wrote:
> > On Fri, Jan 14, 2022 at 07:45:35AM -0500, Michael S. Tsirkin wrote:
> > > On Fri, Jan 14, 2022 at 10:05:08AM +0100, Stefano Garzarella wrote:
> > > > In vhost_enable_notify() we enable the notifications and we read
> > > > the avail index to check if new buffers have become available in
> > > > the meantime.
> > > >
> > > > We are not caching the avail index, so when the device will call
> > > > vhost_get_vq_desc(), it will find the old value in the cache and
> > > > it will read the avail index again.
> > > >
> > > > It would be better to refresh the cache every time we read avail
> > > > index, so let's change vhost_enable_notify() caching the value in
> > > > `avail_idx` and compare it with `last_avail_idx` to check if there
> > > > are new buffers available.
> > > >
> > > > Anyway, we don't expect a significant performance boost because
> > > > the above path is not very common, indeed vhost_enable_notify()
> > > > is often called with unlikely(), expecting that avail index has
> > > > not been updated.
> > > >
> > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > >
> > > ... and can in theory even hurt due to an extra memory write.
> > > So ... performance test restults pls?
> >
> > Right, could be.
> >
> > I'll run some perf test with vsock, about net, do you have a test suite or
> > common step to follow to test it?
> >
> > Thanks,
> > Stefano
>
> You can use the vhost test as a unit test as well.

Thanks for the advice, I did indeed use it!

I run virtio_test (with vhost_test.ko) using 64 as batch to increase the
chance of the path being taken. (I changed bufs=0x1000000 in
virtio_test.c to increase the duration).

I used `perf stat` to take some numbers, running this command:

taskset -c 2 perf stat -r 10 --log-fd 1 -- ./virtio_test --batch=64

- Linux v5.16 without the patch applied

Performance counter stats for './virtio_test --batch=64' (10 runs):

2,791.70 msec task-clock # 0.996 CPUs utilized ( +- 0.36% )
23 context-switches # 8.209 /sec ( +- 2.75% )
0 cpu-migrations # 0.000 /sec
79 page-faults # 28.195 /sec ( +- 0.41% )
7,249,926,989 cycles # 2.587 GHz ( +- 0.36% )
7,711,999,656 instructions # 1.06 insn per cycle ( +- 1.08% )
1,838,436,806 branches # 656.134 M/sec ( +- 1.44% )
3,055,439 branch-misses # 0.17% of all branches ( +- 6.22% )

2.8024 +- 0.0100 seconds time elapsed ( +- 0.36% )

- Linux v5.16 with this patch applied

Performance counter stats for './virtio_test --batch=64' (10 runs):

2,753.36 msec task-clock # 0.998 CPUs utilized ( +- 0.20% )
24 context-switches # 8.699 /sec ( +- 2.86% )
0 cpu-migrations # 0.000 /sec
76 page-faults # 27.545 /sec ( +- 0.56% )
7,150,358,721 cycles # 2.592 GHz ( +- 0.20% )
7,420,639,950 instructions # 1.04 insn per cycle ( +- 0.76% )
1,745,759,193 branches # 632.730 M/sec ( +- 1.03% )
3,022,508 branch-misses # 0.17% of all branches ( +- 3.24% )

2.75952 +- 0.00561 seconds time elapsed ( +- 0.20% )

The difference seems minimal with a slight improvement.

To try to stress the patch more, I modified vhost_test.ko to call
vhost_enable_notify()/vhost_disable_notify() on every cycle when calling
vhost_get_vq_desc():

- Linux v5.16 modified without the patch applied

Performance counter stats for './virtio_test --batch=64' (10 runs):

4,126.66 msec task-clock # 1.006 CPUs utilized ( +- 0.25% )
28 context-switches # 6.826 /sec ( +- 3.41% )
0 cpu-migrations # 0.000 /sec
85 page-faults # 20.721 /sec ( +- 0.44% )
10,716,808,883 cycles # 2.612 GHz ( +- 0.25% )
11,804,381,462 instructions # 1.11 insn per cycle ( +- 0.86% )
3,138,813,438 branches # 765.153 M/sec ( +- 1.03% )
11,286,860 branch-misses # 0.35% of all branches ( +- 1.23% )

4.1027 +- 0.0103 seconds time elapsed ( +- 0.25% )

- Linux v5.16 modified with this patch applied

Performance counter stats for './virtio_test --batch=64' (10 runs):

3,953.55 msec task-clock # 1.001 CPUs utilized ( +- 0.33% )
29 context-switches # 7.345 /sec ( +- 2.67% )
0 cpu-migrations # 0.000 /sec
83 page-faults # 21.021 /sec ( +- 0.65% )
10,267,242,653 cycles # 2.600 GHz ( +- 0.33% )
7,972,866,579 instructions # 0.78 insn per cycle ( +- 0.21% )
1,663,770,390 branches # 421.377 M/sec ( +- 0.45% )
16,986,093 branch-misses # 1.02% of all branches ( +- 0.47% )

3.9489 +- 0.0130 seconds time elapsed ( +- 0.33% )

In this case the difference is bigger, with a reduction in execution
time (3.7 %) and fewer branches and instructions. It should be the
branch `if (vq->avail_idx == vq->last_avail_idx)` in vhost_get_vq_desc()
that is not taken.

Should I resend the patch adding some more performance information?

Thanks,
Stefano