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

From: Eric Dumazet
Date: Wed Apr 15 2015 - 12:39:42 EST

On Wed, 2015-04-15 at 14:43 +0100, George Dunlap wrote:
> On Mon, Apr 13, 2015 at 2:49 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> > On Mon, 2015-04-13 at 11:56 +0100, George Dunlap wrote:
> >
> >> Is the problem perhaps that netback/netfront delays TX completion?
> >> Would it be better to see if that can be addressed properly, so that
> >> the original purpose of the patch (fighting bufferbloat) can be
> >> achieved while not degrading performance for Xen? Or at least, so
> >> that people get decent perfomance out of the box without having to
> >> tweak TCP parameters?
> >
> > Sure, please provide a patch, that does not break back pressure.
> >
> > But just in case, if Xen performance relied on bufferbloat, it might be
> > very difficult to reach a stable equilibrium : Any small change in stack
> > or scheduling might introduce a significant difference in 'raw
> > performance'.
> So help me understand this a little bit here. tcp_limit_output_bytes
> limits the amount of data allowed to be "in-transit" between a send()
> and the wire, is that right?
> And so the "bufferbloat" problem you're talking about here are TCP
> buffers inside the kernel, and/or buffers in the NIC, is that right?
> So ideally, you want this to be large enough to fill the "pipeline"
> all the way from send() down to actually getting out on the wire;
> otherwise, you'll have gaps in the pipeline, and the machinery won't
> be working at full throttle.
> And the reason it's a problem is that many NICs now come with large
> send buffers; and effectively what happens then is that this makes the
> "pipeline" longer -- as the buffer fills up, the time between send()
> and the wire is increased. This increased latency causes delays in
> round-trip-times and interferes with the mechanisms TCP uses to try to
> determine what the actual sustainable rate of data trasmission is.
> By limiting the number of "in-transit" bytes, you make sure that
> neither the kernel nor the NIC are going to have packets queues up for
> long lengths of time in buffers, and you keep this "pipeline" as close
> to the actual minimal length of the pipeline as possible. And it
> sounds like for your 40G NIC, 128k is big enough to fill the pipeline
> without unduly making it longer by introducing buffering.
> Is that an accurate picture of what you're trying to achieve?
> But the problem for xennet (and a number of other drivers), as I
> understand it, is that at the moment the "pipeline" itself is just
> longer -- it just takes a longer time from the time you send a packet
> to the time it actually gets out on the wire.
> So it's not actually accurate to say that "Xen performance relies on
> bufferbloat". There's no buffering involved -- the pipeline is just
> longer, and so to fill up the pipeline you need more data.
> Basically, to maximize throughput while minimizing buffering, for
> *any* connection, tcp_limit_output_bytes should ideally be around
> (min_tx_latency * max_bandwidth). For physical NICs, the minimum
> latency is really small, but for xennet -- and I'm guessing for a lot
> of virtualized cards -- the min_tx_latency will be a lot higher,
> requiring a much higher ideal tcp_limit_output value.
> Rather than trying to pick a single value which will be good for all
> NICs, it seems like it would make more sense to have this vary
> depending on the parameters of the NIC. After all, for NICs that have
> low throughput -- say, old 100MiB NICs -- even 128k may still
> introduce a significant amount of buffering.
> Obviously one solution would be to allow the drivers themselves to set
> the tcp_limit_output_bytes, but that seems like a maintenance
> nightmare.
> Another simple solution would be to allow drivers to indicate whether
> they have a high transmit latency, and have the kernel use a higher
> value by default when that's the case.
> Probably the most sustainable solution would be to have the networking
> layer keep track of the average and minimum transmit latencies, and
> automatically adjust tcp_limit_output_bytes based on that. (Keeping
> the minimum as well as the average because the whole problem with
> bufferbloat is that the more data you give it, the longer the apparent
> "pipeline" becomes.)
> Thoughts?

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.


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