Linux Virtualization - Jul 2019 - [PATCH] virtio-net: parameterize min ring num_free for virtio receive

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 ??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 at 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.  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.
> 
> -- 
> MST

On 2019/7/18 ??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 ??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 at 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

On 2019/7/19 10:36, Jason Wang wrote:
>
> On 2019/7/18 ??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 ??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 at
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 at
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

On 2019/7/19 22:29, Jiang wrote:
>
> On 2019/7/19 10:36, Jason Wang wrote:
>>
>> On 2019/7/18 ??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 ??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 at
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[$vq, i])
> ??? ??? ??? //exit()
> ??? ??? }
> ??? }
> }
>
>
> probe 
> module("virtio_net").statement("virtnet_receive at
drivers/net/virtio_net.c:732")
> {
> ??? recentfreecount++
> ??? recentfreecount = recentfreecount % 5
> ??? recentfree[$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

On Fri, Jul 19, 2019 at 03:31:29PM +0000, ? jiang wrote:
> 
> On 2019/7/19 22:29, Jiang wrote:
> >
> > On 2019/7/19 10:36, Jason Wang wrote:
> >>
> >> On 2019/7/18 ??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 ??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 at
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[$vq, i])
> > ??? ??? ??? //exit()
> > ??? ??? }
> > ??? }
> > }
> >
> >
> > probe 
> > module("virtio_net").statement("virtnet_receive at
drivers/net/virtio_net.c:732")
> > {
> > ??? recentfreecount++
> > ??? recentfreecount = recentfreecount % 5
> > ??? recentfree[$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.

-- 
MST