On Thu, Jan 20, 2022 at 04:08:39PM +0100, Stefano Garzarella wrote: > On Fri, Jan 14, 2022 at 2:40 PM Michael S. Tsirkin <mst@xxxxxxxxxx> 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@xxxxxxxxxx> > > > > > > > > ... 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 Yea, pls do. You can just summarize it in a couple of lines. -- MST
