Re: [PATCH] virtio-net: lower min ring num_free for efficiency

[Jason Wang <jasowang@redhat.com>]

On 2019/8/15 上午11:11, 冉 jiang wrote:
> On 2019/8/15 11:01, Jason Wang wrote:
>> On 2019/8/14 上午10:06, ? jiang wrote:
>>> This change lowers ring buffer reclaim threshold from 1/2*queue to
>>> budget
>>> for better performance. According to our test with qemu + dpdk, packet
>>> dropping happens when the guest is not able to provide free buffer in
>>> avail ring timely with default 1/2*queue. The value in the patch has
>>> been
>>> tested and does show better performance.
>>
>> Please add your tests setup and result here.
>>
>> Thanks
>>
>>
>>> Signed-off-by: jiangkidd <jiangkidd@hotmail.com>
>>> ---
>>>    drivers/net/virtio_net.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 0d4115c9e20b..bc08be7925eb 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -1331,7 +1331,7 @@ static int virtnet_receive(struct receive_queue
>>> *rq, int budget,
>>>            }
>>>        }
>>>    -    if (rq->vq->num_free > virtqueue_get_vring_size(rq->vq) / 2) {
>>> +    if (rq->vq->num_free > min((unsigned int)budget,
>>> virtqueue_get_vring_size(rq->vq)) / 2) {
>>>            if (!try_fill_recv(vi, rq, GFP_ATOMIC))
>>>                schedule_delayed_work(&vi->refill, 0);
>>>        }
> Sure, here are the details:


Thanks for the details, but I meant it's better if you could summarize 
you test result in the commit log in a compact way.

Btw, some comments, see below:


>
>
> Test setup & result:
>
> ----------------------------------------------------
>
> Below 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. We
> can see average
> drop packets do decrease a lot in test2.
>
> test1Time    avgDropPackets    test2Time    avgDropPackets pps
>
> 16:21.0    12.295    56:50.4    0 300k
> 17:19.1    15.244    56:50.4    0    300k
> 18:17.5    18.789    56:50.4    0    300k
> 19:15.1    14.208    56:50.4    0    300k
> 20:13.2    20.818    56:50.4    0.267    300k
> 21:11.2    12.397    56:50.4    0    300k
> 22:09.3    12.599    56:50.4    0    300k
> 23:07.3    15.531    57:48.4    0    300k
> 24:05.5    13.664    58:46.5    0    300k
> 25:03.7    13.158    59:44.5    4.73    300k
> 26:01.1    2.486    00:42.6    0    300k
> 26:59.1    11.241    01:40.6    0    300k
> 27:57.2    20.521    02:38.6    0    300k
> 28:55.2    30.094    03:36.7    0    300k
> 29:53.3    16.828    04:34.7    0.963    300k
> 30:51.3    46.916    05:32.8    0    400k
> 31:49.3    56.214    05:32.8    0    400k
> 32:47.3    58.69    05:32.8    0    400k
> 33:45.3    61.486    05:32.8    0    400k
> 34:43.3    72.175    05:32.8    0.598    400k
> 35:41.3    56.699    05:32.8    0    400k
> 36:39.3    61.071    05:32.8    0    400k
> 37:37.3    43.355    06:30.8    0    400k
> 38:35.4    44.644    06:30.8    0    400k
> 39:33.4    72.336    06:30.8    0    400k
> 40:31.4    70.676    06:30.8    0    400k
> 41:29.4    108.009    06:30.8    0    400k
> 42:27.4    65.216    06:30.8    0    400k


Why there're difference in test time? Could you summarize them like:

Test setup: e.g testpmd or pktgen to generate packets to guest

avg packets drop before: XXX

avg packets drop after: YYY(-ZZZ%)

Thanks


>
>
> Data to prove why the patch helps:
>
> ----------------------------------------------------
>
> 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. 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")
> {
>        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
>
>        if (x<0 && (x+65535)<4096)
>            x = x+65535
>
>        if((x==1024) && @cast($_vq, "vring_virtqueue")->vq->callback ==
> callback_addr)
>            netrxcount[x] <<< gettimeofday_s()
> }
>
>
> probe module("virtio_ring").function("virtqueue_add_inbuf")
> {
>        y = (@cast($vq, "vring_virtqueue")->vring->avail->idx)-
> (@cast($vq, "vring_virtqueue")->vring->used->idx) ---> we use this one
> to verify if we run out of free buffer in avail ring
>        if (y<0 && (y+65535)<4096)
>            y = y+65535
>
>        if(@2=="debugon")
>        {
>            if(y==0 && @cast($vq, "vring_virtqueue")->vq->callback ==
> callback_addr)
>            {
>                netrxfreecount[y] <<< gettimeofday_s()
>
>                printf("no avail ring left seen, printing most recent 5
> num free, vq: %lx, current index: %d\n", $vq, recentfreecount)
>                for(i=recentfreecount; i!=((recentfreecount+4) % 5);
> i=((i+1) % 5))
>                {
>                    printf("index: %d, num free: %d\n", i, recentfree[$vq,
> i])
>                }
>
>                printf("index: %d, num free: %d\n", i, recentfree[$vq, i])
>                //exit()
>            }
>        }
> }
>
>
> probe
> module("virtio_net").statement("virtnet_receive@drivers/net/virtio_net.c:732")
>
> {
>        recentfreecount++
>        recentfreecount = recentfreecount % 5
>        recentfree[$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
>
> We 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.
>
> Jiang


Right, but I think there's no need to put those thing in the commit log.

Thanks