Re: Major network performance regression in 3.7

Willy Tarreau
2013-01-06T09:24:42Z

On Sat, Jan 05, 2013 at 11:35:24PM -0800, Eric Dumazet wrote:
> On Sun, 2013-01-06 at 03:52 +0100, Willy Tarreau wrote:
>
> > OK so I observed no change with this patch, either on the loopback
> > data rate at >16kB MTU, or on the myri. I'm keeping it at hand for
> > experimentation anyway.
> >
>
> Yeah, there was no bug. I rewrote it for net-next as a cleanup/optim
> only.

I have re-applied your last rewrite and noticed a small but nice
performance improvement on a single stream over the loopback :

1 session 10 sessions
- without the patch : 55.8 Gbps 68.4 Gbps
- with the patch : 56.4 Gbps 70.4 Gbps

This was with the loopback reverted to 16kB MTU of course.

> > Concerning the loopback MTU, I find it strange that the MTU changes
> > the splice() behaviour and not send/recv. I thought that there could
> > be a relation between the MTU and the pipe size, but it does not
> > appear to be the case either, as I tried various sizes between 16kB
> > and 256kB without achieving original performance.
>
>
> It probably is related to a too small receive window, given the MTU was
> multiplied by 4, I guess we need to make some adjustments

In fact even if I set it to 32kB it breaks.

I have tried to progressively increase the loopback's MTU from the default
16436, by steps of 4096 :

tcp_rmem = 256 kB tcp_rmem = 256 kB
pipe size = 64 kB pipe size = 256 kB

16436 : 55.8 Gbps 65.2 Gbps
20532 : 32..48 Gbps unstable 24..45 Gbps unstable
24628 : 56.0 Gbps 66.4 Gbps
28724 : 58.6 Gbps 67.8 Gbps
32820 : 54.5 Gbps 61.7 Gbps
36916 : 56.8 Gbps 65.5 Gbps
41012 : 57.8..58.2 Gbps ~stable 67.5..68.8 Gbps ~stable
45108 : 59.4 Gbps 70.0 Gbps
49204 : 61.2 Gbps 71.1 Gbps
53300 : 58.8 Gbps 70.6 Gbps
57396 : 60.2 Gbps 70.8 Gbps
61492 : 61.4 Gbps 71.1 Gbps

tcp_rmem = 1 MB tcp_rmem = 1 MB
pipe size = 64 kB pipe size = 256 kB

16436 : 16..34 Gbps unstable 49.5 or 65.2 Gbps (unstable)
20532 : 7..15 Gbps unstable 15..32 Gbps unstable
24628 : 36..48 Gbps unstable 34..61 Gbps unstable
28724 : 40..51 Gbps unstable 40..69 Gbps unstable
32820 : 40..55 Gbps unstable 59.9..62.3 Gbps ~stable
36916 : 38..51 Gbps unstable 66.0 Gbps
41012 : 30..42 Gbps unstable 47..66 Gbps unstable
45108 : 59.5 Gbps 71.2 Gbps
49204 : 61.3 Gbps 74.0 Gbps
53300 : 63.1 Gbps 74.5 Gbps
57396 : 64.6 Gbps 74.7 Gbps
61492 : 61..66 Gbps unstable 76.5 Gbps

So as long as we maintain the MTU to n*4096 + 52, performance is still
almost OK. It is interesting to see that the transfer rate is unstable
at many values and that it depends both on the rmem and pipe size, just
as if some segments sometimes remained stuck for too long.

And if I pick a value which does not match n*4096+52, such as
61492+2048 = 63540, then the transfer falls to about 50-100 Mbps again.

So there's clearly something related to the copy of segments from
incomplete pages instead of passing them via the pipe.

It is possible that this bug has been there for a long time and that
we never detected it because nobody plays with the loopback MTU.

I have tried with 2.6.35 :

16436 : 31..33 Gbps
61492 : 48..50 Gbps
63540 : 50..53 Gbps => so at least it's not affected

Even forcing the MTU to 16384 maintains 30..33 Gbps almost stable.

On 3.5.7.2 :

16436 : 23..27 Gbps
61492 : 61..64 Gbps
63540 : 40..100 Mbps => the problem was already there.

Since there were many splice changes in 3.5, I'd suspect that the issue
appeared there though I could be wrong.

> You also could try :
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1ca2536..b68cdfb 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1482,6 +1482,9 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
> break;
> }
> used = recv_actor(desc, skb, offset, len);
> + /* Clean up data we have read: This will do ACK frames. */
> + if (used > 0)
> + tcp_cleanup_rbuf(sk, used);
> if (used < 0) {
> if (!copied)
> copied = used;

It does not change anything to the tests above unfortunately. It did not
even stabilize the unstable runs.

I'll check if I can spot the original commit which caused the regression
for MTUs that are not n*4096+52.

But before that I'll try to find the recent one causing the myri10ge to
slow down, it should take less time to bisect.

Regards,
Willy