Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

From: Michael S. Tsirkin
Date: Fri Jul 19 2019 - 12:14:08 EST

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On Fri, Jul 19, 2019 at 03:31:29PM +0000, å jiang wrote:
>
> On 2019/7/19 22:29, Jiang wrote:
> >
> > On 2019/7/19 10:36, Jason Wang wrote:
> >>
> >> On 2019/7/18 äå10:43, Michael S. Tsirkin wrote:
> >>> On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
> >>>> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
> >>>>> On 2019/7/18 äå9:04, Michael S. Tsirkin wrote:
> >>>>>> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
> >>>>>>> This change makes ring buffer reclaim threshold num_free
> >>>>>>> configurable
> >>>>>>> for better performance, while it's hard coded as 1/2 * queue now.
> >>>>>>> According to our test with qemu + dpdk, packet dropping happens
> >>>>>>> when
> >>>>>>> the guest is not able to provide free buffer in avail ring timely.
> >>>>>>> Smaller value of num_free does decrease the number of packet
> >>>>>>> dropping
> >>>>>>> during our test as it makes virtio_net reclaim buffer earlier.
> >>>>>>>
> >>>>>>> At least, we should leave the value changeable to user while the
> >>>>>>> default value as 1/2 * queue is kept.
> >>>>>>>
> >>>>>>> Signed-off-by: jiangkidd<jiangkidd@xxxxxxxxxxx>
> >>>>>> That would be one reason, but I suspect it's not the
> >>>>>> true one. If you need more buffer due to jitter
> >>>>>> then just increase the queue size. Would be cleaner.
> >>>>>>
> >>>>>>
> >>>>>> However are you sure this is the reason for
> >>>>>> packet drops? Do you see them dropped by dpdk
> >>>>>> due to lack of space in the ring? As opposed to
> >>>>>> by guest?
> >>>>>>
> >>>>>>
> >>>>> Besides those, this patch depends on the user to choose a suitable
> >>>>> threshold
> >>>>> which is not good. You need either a good value with demonstrated
> >>>>> numbers or
> >>>>> something smarter.
> >>>>>
> >>>>> Thanks
> >>>> I do however think that we have a problem right now: try_fill_recv can
> >>>> take up a long time during which net stack does not run at all.
> >>>> Imagine
> >>>> a 1K queue - we are talking 512 packets. That's exceessive.
> >>
> >>
> >> Yes, we will starve a fast host in this case.
> >>
> >>
> >>>> ÂÂ napi poll
> >>>> weight solves a similar problem, so it might make sense to cap this at
> >>>> napi_poll_weight.
> >>>>
> >>>> Which will allow tweaking it through a module parameter as a
> >>>> side effect :) Maybe just do NAPI_POLL_WEIGHT.
> >>> Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
> >>> experiment, measure performance and let the list know
> >>>
> >>>> Need to be careful though: queues can also be small and I don't
> >>>> think we
> >>>> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
> >>>> Definitely must not exceed the full queue size.
> >>
> >>
> >> Looking at intel, it uses 16 and i40e uses 32.Â It looks to me
> >> NAPI_POLL_WEIGHT/2 is better.
> >>
> >> Jiang, want to try that and post a new patch?
> >>
> >> Thanks
> >>
> >>
> >>>>
> >>>> --
> >>>> MST
> >
> > We did have completed several rounds of test with setting the value to
> > budget (64 as the default value). It does improve a lot with pps is
> > below 400pps for a single stream. Let me consolidate the data and will
> > send it soon. Actually, we are confident that it runs out of free
> > buffer in avail ring when packet dropping happens with below systemtap:
> >
> > Just a snippet:
> >
> > probe module("virtio_ring").function("virtqueue_get_buf")
> > {
> > ÂÂÂ x = (@cast($_vq, "vring_virtqueue")->vring->used->idx)-
> > (@cast($_vq, "vring_virtqueue")->last_used_idx) ---> we use this one
> > to verify if the queue is full, which means guest is not able to take
> > buffer from the queue timely
> >
> > ÂÂÂ if (x<0 && (x+65535)<4096)
> > ÂÂÂ ÂÂÂ x = x+65535
> >
> > ÂÂÂ if((x==1024) && @cast($_vq, "vring_virtqueue")->vq->callback ==
> > callback_addr)
> > ÂÂÂ ÂÂÂ netrxcount[x] <<< gettimeofday_s()
> > }
> >
> >
> > probe module("virtio_ring").function("virtqueue_add_inbuf")
> > {
> > ÂÂÂ y = (@cast($vq, "vring_virtqueue")->vring->avail->idx)-
> > (@cast($vq, "vring_virtqueue")->vring->used->idx) ---> we use this one
> > to verify if we run out of free buffer in avail ring
> > ÂÂÂ if (y<0 && (y+65535)<4096)
> > ÂÂÂ ÂÂÂ y = y+65535
> >
> > ÂÂÂ if(@2=="debugon")
> > ÂÂÂ {
> > ÂÂÂ ÂÂÂ if(y==0 && @cast($vq, "vring_virtqueue")->vq->callback ==
> > callback_addr)
> > ÂÂÂ ÂÂÂ {
> > ÂÂÂ ÂÂÂ ÂÂÂ netrxfreecount[y] <<< gettimeofday_s()
> >
> > ÂÂÂ ÂÂÂ ÂÂÂ printf("no avail ring left seen, printing most recent 5
> > num free, vq: %lx, current index: %d\n", $vq, recentfreecount)
> > ÂÂÂ ÂÂÂ ÂÂÂ for(i=recentfreecount; i!=((recentfreecount+4) % 5);
> > i=((i+1) % 5))
> > ÂÂÂ ÂÂÂ ÂÂÂ {
> > ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ printf("index: %d, num free: %d\n", i, recentfree[$vq,
> > i])
> > ÂÂÂ ÂÂÂ ÂÂÂ }
> >
> > ÂÂÂ ÂÂÂ ÂÂÂ printf("index: %d, num free: %d\n", i, recentfree[$vq, i])
> > ÂÂÂ ÂÂÂ ÂÂÂ //exit()
> > ÂÂÂ ÂÂÂ }
> > ÂÂÂ }
> > }
> >
> >
> > probe
> > module("virtio_net").statement("virtnet_receive@drivers/net/virtio_net.c:732")
> > {
> > ÂÂÂ recentfreecount++
> > ÂÂÂ recentfreecount = recentfreecount % 5
> > ÂÂÂ recentfree[$rq->vq, recentfreecount] = $rq->vq->num_free --->
> > record the num_free for the last 5 calls to virtnet_receive, so we can
> > see if lowering the bar helps.
> > }
> >
> >
> > Here is the result:
> >
> > no avail ring left seen, printing most recent 5 num free, vq:
> > ffff9c13c1200000, current index: 1
> > index: 1, num free: 561
> > index: 2, num free: 305
> > index: 3, num free: 369
> > index: 4, num free: 433
> > index: 0, num free: 497
> > no avail ring left seen, printing most recent 5 num free, vq:
> > ffff9c13c1200000, current index: 1
> > index: 1, num free: 543
> > index: 2, num free: 463
> > index: 3, num free: 469
> > index: 4, num free: 476
> > index: 0, num free: 479
> > no avail ring left seen, printing most recent 5 num free, vq:
> > ffff9c13c1200000, current index: 2
> > index: 2, num free: 555
> > index: 3, num free: 414
> > index: 4, num free: 420
> > index: 0, num free: 427
> > index: 1, num free: 491
> >
> > You can see in the last 4 calls to virtnet_receive before we run out
> > of free buffer and start to relaim, num_free is quite high. So if we
> > can do the reclaim earlier, it will certainly help.
> >
> > Meanwhile, the patch I proposed actually keeps the default value as
> > 1/2 * queue. So the default behavior remains and only leave the
> > interface to advanced users, who really understands what they are
> > doing. Also, the best value may vary in different environment. Do you
> > still think hardcoding this is better option?
> >
> >
> > Jiang
> >
> Here is the snippet from our test result. Test1 was done with default
> driver with the value of 1/2 * queue, while test2 is with my patch and
> min_numfree set to 64 (the default budget value). We can see average
> drop packets do decrease a lot in test2. Let me know if you need the
> full testing data.
>
> test1TimeÂÂÂ avgDropPacketsÂÂÂ test2TimeÂÂÂ avgDropPacketsÂÂÂ pps
>
> > 16:21.0ÂÂÂ 12.295ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 17:19.1ÂÂÂ 15.244ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 18:17.5ÂÂÂ 18.789ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 19:15.1ÂÂÂ 14.208ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 20:13.2ÂÂÂ 20.818ÂÂÂ 56:50.4ÂÂÂ 0.267ÂÂÂ 300k
> > 21:11.2ÂÂÂ 12.397ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 22:09.3ÂÂÂ 12.599ÂÂÂ 56:50.4ÂÂÂ 0ÂÂÂ 300k
> > 23:07.3ÂÂÂ 15.531ÂÂÂ 57:48.4ÂÂÂ 0ÂÂÂ 300k
> > 24:05.5ÂÂÂ 13.664ÂÂÂ 58:46.5ÂÂÂ 0ÂÂÂ 300k
> > 25:03.7ÂÂÂ 13.158ÂÂÂ 59:44.5ÂÂÂ 4.73ÂÂÂ 300k
> > 26:01.1ÂÂÂ 2.486ÂÂÂ 00:42.6ÂÂÂ 0ÂÂÂ 300k
> > 26:59.1ÂÂÂ 11.241ÂÂÂ 01:40.6ÂÂÂ 0ÂÂÂ 300k
> > 27:57.2ÂÂÂ 20.521ÂÂÂ 02:38.6ÂÂÂ 0ÂÂÂ 300k
> > 28:55.2ÂÂÂ 30.094ÂÂÂ 03:36.7ÂÂÂ 0ÂÂÂ 300k
> > 29:53.3ÂÂÂ 16.828ÂÂÂ 04:34.7ÂÂÂ 0.963ÂÂÂ 300k
> > 30:51.3ÂÂÂ 46.916ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 31:49.3ÂÂÂ 56.214ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 32:47.3ÂÂÂ 58.69ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 33:45.3ÂÂÂ 61.486ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 34:43.3ÂÂÂ 72.175ÂÂÂ 05:32.8ÂÂÂ 0.598ÂÂÂ 400k
> > 35:41.3ÂÂÂ 56.699ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 36:39.3ÂÂÂ 61.071ÂÂÂ 05:32.8ÂÂÂ 0ÂÂÂ 400k
> > 37:37.3ÂÂÂ 43.355ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
> > 38:35.4ÂÂÂ 44.644ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
> > 39:33.4ÂÂÂ 72.336ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
> > 40:31.4ÂÂÂ 70.676ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
> > 41:29.4ÂÂÂ 108.009ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
> > 42:27.4ÂÂÂ 65.216ÂÂÂ 06:30.8ÂÂÂ 0ÂÂÂ 400k
>
>
> Jiang

OK I find this surprising but I accept what you see.
I'm inclined not to add a tunable and just select
a value ourselves.
I'm also fine with using the napi poll module parameter
which will give you a bit of tunability.

--
MST

Next message: Oleg Nesterov: "Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling"
Previous message: Eric W. Biederman: "Re: [PATCH ghak90 V6 02/10] audit: add container id"
In reply to: Jason Wang: "Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive"
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