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

On 2019/8/15 上午11:11, 冉 jiang wrote:

On 2019/8/15 11:01, Jason Wang wrote:

quoted

On 2019/8/14 上午10:06, ? jiang wrote:

quoted

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

quoted

Signed-off-by: jiangkidd <redacted>
---
   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