netdev - Re: Very low latency TCP for clusters

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Message-ID: <AANLkTillCVWMHHBImGEeRYy2MJYqtGIvSPvacLKJnpP2@mail.gmail.com>
Date:	Mon, 19 Jul 2010 16:37:59 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Very low latency TCP for clusters

On Mon, Jul 19, 2010 at 3:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le lundi 19 juillet 2010 à 11:44 -0700, Tom Herbert a écrit :
>
>> I see about 7 usecs as best number on loopback, so I believe this is
>> in the ballpark.  As I mentioned above, this about "best case" latency
>> of a single thread, so we assume any amount of pinning or other
>> customized configuration to that purpose.
>
> Well, given I get 29 us on a ping between two machines (Gb link, no
> process involved on receiver, only softirq), I really doubt we can reach
> 5 us on a tcp test involving a user process on both side ;)
>
That's pretty pokey ;-) I see numbers around 25 usecs between to
machines, this is with TCP_NBRR.  With TCP_RR it's more like 35 usecs,
so eliminating the scheduler is already a big reduction.  That leaves
18 usecs in device time, interrupt processing, network, and cache
misses; 7 usecs in TCP processing, user space.  While 5 usecs is an
aggressive goal, I am not ready to concede that there's an
architectural limit in either NICs, TCP, or sockets that can't be
overcome.
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