GSO (was: Please enter issues into the issue tracker - Issue system organisation needed)

[Eric Dumazet]

Le vendredi 25 février 2011 à 12:21 +0100, Jesper Dangaard Brouer a
écrit :
> On Thu, 2011-02-24 at 20:29 +0100, Eric Dumazet wrote:
> > - Its important to set TSO off (ethtool -K eth0 tso off), or else we
> > send big packets (up to 64Kbytes) and this used to break SFQ fairness.
> > This can really hurt latencies of interactive flows.
> 
> Don't you mean "GSO" Generic-Segmentation-Offload (ethtool -K eth0 gso
> off) as this happens in the stack.  While TSO Tcp-Segmentation-Offload
> happens in hardware, and you will not see it in the SFQ qdisc?
> 

I definitly see big packets if TSO is enabled, for localy generated
trafic. (You probably are concerned by routers, where all trafic is
forwarded, so TSO is not used, even if enabled)

> I recommend that both is turned off, on small bandwidth links where
> latency matters.
> 

Sure.

> I'm wondering if LRO (Large-Receive-Offload) affect you, when you are
> using SFQ on ingress?
> 
> 

GRO/LRO can have an impact, for sure. But most 'current' kernels dont
have GRO/LRO by default. I mean, kernels in use by 2-3 years old
distros.


> Recently had some "funny" issues with GRO, where a 100 Mbit/s customer
> could "only" get approx 90 Mbit/s throughput to our speed test server
> (other customers, in another appartment building could get approx 96
> Mbit/s). The issue was resolved by disabling GSO on the speed test
> server.  The theory is that some switch on the path cannot handle the
> bursts generated by GSO, which is max 64K (I think, correct me if I'm
> wrong).
> 

Thats right. One 64K packet with standard MTU means some spikes on wire,
but if your switches cant resist to this... Is TCP SACK active on the
customer side (and speed test server) ?


> When adjusting buffer sizes, its important to take this bursty TCP
> behavior into account, which is created by both GSO and TSO.  I'm not
> saying that the queue size needs to be above 64K.  For smaller links, it
> might make sense to set it, significantly below 64K, to avoid a GSO
> enabled Linux machine to ramp up its window size, which makes it capable
> of bursting.
> 

TSO basically hurts SFQ or other AQM, unless you use big/fast pipes.

For a router workload anyway, I would say its better to not try to
coalesce frames in software level, just handle them one by one.