Re: Loopback and Nagle's algorithm

Re: Loopback and Nagle's algorithm

[Alejandro Riveira Fernández]

El Mon, 11 Apr 2011 22:37:49 -0400
"Adam McLaurin" <lkml@irotas.net> escribió:

 Just CCing netdev

> I understand that disabling Nagle's algorithm via TCP_NODELAY will
> generally degrade throughput. However, in my scenario (150 byte
> messages, sending as fast as possible), the actual throughput penalty
> over the network is marginal (maybe 10% at most).
> 
> However, when I disable Nagle's algorithm when connecting over loopback,
> the performance hit is *huge* - 10x reduction in throughput.
> 
> The question is, why is disabling Nagle's algorithm on loopback so much
> worse w.r.t. throughput? Is there anything I can do to reduce the
> incurred throughput penalty?
> 
> Thanks,
> Adam
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Re: Loopback and Nagle's algorithm

[Eric Dumazet]

Le mardi 12 avril 2011 à 13:54 +0200, Jiri Kosina a écrit :
> On Tue, 12 Apr 2011, Adam McLaurin wrote:
> 
> > > It may be caused by an increase in context switch rate, as both sender
> > > and receiver are on the same machine.
> > 
> > I'm not sure that's what's happening, since the box where I'm running
> > this test has 8 physical CPU's and 32 cores in total.
> 
> Have you tried firing up the testcase under perf, to see what it reveals 
> as the bottleneck?
> 
CC netdev

This rings a bell here.

I suspect we hit mod_timer() / lock_timer_base()  because of delack
timer constantly changing.

I remember raising this point last year :

http://kerneltrap.org/mailarchive/linux-netdev/2010/5/20/6277741

David answer : 
http://kerneltrap.org/mailarchive/linux-netdev/2010/6/2/6278430

I am afraid no change was done...