netdev - Re: Very low latency TCP for clusters

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Message-ID: <1279561319.2553.153.camel@edumazet-laptop>
Date:	Mon, 19 Jul 2010 19:41:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Very low latency TCP for clusters

Le lundi 19 juillet 2010 à 10:05 -0700, Tom Herbert a écrit :
> We have been looking at best case TCP latencies that might be achieved
> within a cluster (low loss fabric).  The goal is to have latency
> numbers roughly comparable to that which can be produced using RDMA/IB
> in a low latency configuration  (<5 usecs round trip on netperf TCP_RR
> test with one byte data for directly connected hosts as a starting
> point).  This would be without changing sockets API, fabric, and
> preferably not using TCP offload or a user space stack.
> 
> I think there are at least two techniques that will drive down TCP
> latency: per connection queues and polling queues.  Per connection
> queues (supported by device) should eliminate costs of connection
> look-up, hopefully some locking.  Polling becomes viable as core
> counts on systems increase, and burning a few CPUs for networking
> polling on behalf of very low-latency threads would be reasonable.
> 
> Are there any efforts in progress to integrate per connection queues
> in the stack or integrate polling of queues?

aka "net channel" ;)

What a nightmare...

Anyway, 5 us roundtrip TCP_RR (including user thread work), seems a bit
utopic right now.

Even on loopback



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