Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

Eric Dumazet
2015-04-15T17:29:20Z

On Wed, 2015-04-15 at 18:23 +0100, George Dunlap wrote:
> On 04/15/2015 05:38 PM, Eric Dumazet wrote:
> > My thoughts that instead of these long talks you should guys read the
> > code :
> >
> > /* TCP Small Queues :
> > * Control number of packets in qdisc/devices to two packets / or ~1 ms.
> > * This allows for :
> > * - better RTT estimation and ACK scheduling
> > * - faster recovery
> > * - high rates
> > * Alas, some drivers / subsystems require a fair amount
> > * of queued bytes to ensure line rate.
> > * One example is wifi aggregation (802.11 AMPDU)
> > */
> > limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
> > limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);
> >
> >
> > Then you'll see that most of your questions are already answered.
> >
> > Feel free to try to improve the behavior, if it does not hurt critical workloads
> > like TCP_RR, where we we send very small messages, millions times per second.
>
> First of all, with regard to critical workloads, once this patch gets
> into distros, *normal TCP streams* on every VM running on Amazon,
> Rackspace, Linode, &c will get a 30% hit in performance *by default*.
> Normal TCP streams on xennet *are* a critical workload, and deserve the
> same kind of accommodation as TCP_RR (if not more). The same goes for
> virtio_net.
>
> Secondly, according to Stefano's and Jonathan's tests,
> tcp_limit_output_bytes completely fixes the problem for Xen.
>
> Which means that max(2*skb->truesize, sk->sk_pacing_rate >>10) is
> *already* larger for Xen; that calculation mentioned in the comment is
> *already* doing the right thing.
>
> As Jonathan pointed out, sysctl_tcp_limit_output_bytes is overriding an
> automatic TSQ calculation which is actually choosing an effective value
> for xennet.
>
> It certainly makes sense for sysctl_tcp_limit_output_bytes to be an
> actual maximum limit. I went back and looked at the original patch
> which introduced it (46d3ceabd), and it looks to me like it was designed
> to be a rough, quick estimate of "two packets outstanding" (by choosing
> the maximum size of the packet, 64k, and multiplying it by two).
>
> Now that you have a better algorithm -- the size of 2 actual packets or
> the amount transmitted in 1ms -- it seems like the default
> sysctl_tcp_limit_output_bytes should be higher, and let the automatic
> TSQ you have on the first line throttle things down when necessary.

I asked you guys to make a test by increasing
sysctl_tcp_limit_output_bytes

You have no need to explain me the code I wrote, thank you.