Re: [PATCH v4 0/10] bql: Byte Queue Limits

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 29 novembre 2011 à 05:23 +0100, Dave Taht a écrit :
> > In this test 100 netperf TCP_STREAMs were started to saturate the link.
> > A single instance of a netperf TCP_RR was run with high priority set.
> > Queuing discipline in pfifo_fast, NIC is e1000 with TX ring size set to
> > 1024.  tps for the high priority RR is listed.
> >
> > No BQL, tso on: 3000-3200K bytes in queue: 36 tps
> > BQL, tso on: 156-194K bytes in queue, 535 tps
> 
> > No BQL, tso off: 453-454K bytes int queue, 234 tps
> > BQL, tso off: 66K bytes in queue, 914 tps
> 
> 
> Jeeze. Under what circumstances is tso a win? I've always
> had great trouble with it, as some e1000 cards do it rather badly.
> 
> I assume these are while running at GigE speeds?
> 
> What of 100Mbit? 10GigE? (I will duplicate your tests
> at 100Mbit, but as for 10gigE...)
> 

TSO on means a low priority 65Kbytes packet can be in TX ring right
before the high priority packet. If you cant afford the delay, you lose.

There is no mystery here.

If you want low latencies :
- TSO must be disabled so that packets are at most one ethernet frame. 
- You adjust BQL limit to small value
- You even can lower MTU to get even more better latencies.

If you want good throughput from your [10]GigE and low cpu cost, TSO
should be enabled.

If you want to be smart, you could have a dynamic behavior :

Let TSO on as long as no high priority low latency producer is running
(if low latency packets are locally generated)