|
On 2019/8/13 18:55, Michael S. Tsirkin wrote:
On Tue, Jul 23, 2019 at 12:05:03PM +0000, 冉 jiang wrote:
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
Looks good to me.
Pls post for inclusion in -net.
Thx. I've just posted another thread "[PATCH] virtio-net: lower min ring num_free for efficiency" for inclusion.
Jiang
|
