Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head() calls

Eric Dumazet
2012-04-19T18:00:45Z

On Thu, 2012-04-19 at 10:48 -0700, Rick Jones wrote:
> On 04/19/2012 10:25 AM, Eric Dumazet wrote:
> > On Thu, 2012-04-19 at 10:20 -0700, Rick Jones wrote:
> >
> >> By copying them to smaller buffers? Or just by altering truesize?
> >> Wasn't the whole point of fixing all the broken truesize settings to
> >> accurately account for memory consumed?
> >
> > I checked, their truesize are OK (1024+256) for ixgbe driver.
> > They could be little smaller, but not that much. (512 + 256)
> >
> > No, its only the sk_rcvbuf is small for a tcp sender,
> > and sk_add_backlog() makes sure we dont queue more than sk_rcvbuf
> > bytes in backlog.
>
> Sounds like a variation on the theme of wildly divergent
> inbound/outbound bandwidth and constraining ACKs constraining throughput
> - only with buffer sizes.
>
> 87380 is the default SO_RCVBUF right? That should have allowed
> 87380/1280 or 68 ACKs to be queued. Without ACK stretching from GRO
> that should have covered 68 * 2896 or 196928 bytes. Based on your
> previous 54 usec to transmit 64 KB that would be 162+ usecs to
> accumulate those ACKs, so I guess a question becomes if TCP can be
> held-off processing ACKs for > 162 usecs, and if so and that cannot be
> changed, the autotuning will have to start increasing SO_SNDBUF
> alongside so_sndbuf even if the endpoint is not receiving. As a
> handwave, since TCP does not know the buffer size(s) used by the driver,
> by 1 MSS for every 2 MSS it adds to SO_SNDBUF. Or find some way to do
> it "on demand" in the about to drop path.
>
> That or bare ACKs have to be excluded from the overhead checks somehow I
> guess, or there be more aggressive copying into smaller buffers.
>

Nope, we need a limit or risk OOM if a malicious peer send ACK flood ;)

To be clear, if I change the tcp_rmem[1] from 87380 to big value, I no
longer have ACK drops, but still poor performance, I am still
investigating.