On 2019/7/19 22:29, Jiang wrote:
>
> On 2019/7/19 10:36, Jason Wang wrote:
>>
>> On 2019/7/18 下午10:43, 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 下午9:04, 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<jiangk...@hotmail.com>
>>>>>> 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.
>>
>>
>>>> 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. 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")
> {
> 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
>
> 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.
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
Jiang
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