On 2019/7/20 0:13, Michael S. Tsirkin wrote:
> On Fri, Jul 19, 2019 at 03:31:29PM +0000, 冉 jiang wrote:
>> 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<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.
>>>>
>>>>
>>>>>> 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
>
> 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.
OK, kindly take a look if you prefer the below code change. I tested
budget/2 and the result is almost the same as budget when pps below
400k, but a little better when it goes beyond 400k in my environment.
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);
}
Jiang
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