netdev - Re: [RFC] New driver API to speed up small packets xmits

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Message-ID: <46439189.5090907@cosmosbay.com>
Date:	Thu, 10 May 2007 23:41:29 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Stevens <dlstevens@...ibm.com>
CC:	Rick Jones <rick.jones2@...com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Krishna Kumar2 <krkumar2@...ibm.com>, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org
Subject: Re: [RFC] New driver API to speed up small packets xmits

David Stevens a écrit :
> The word "small" is coming up a lot in this discussion, and
> I think packet size really has nothing to do with it. Multiple
> streams generating packets of any size would benefit; the
> key ingredient is a queue length greater than 1.
> 
> I think the intent is to remove queue lock cycles by taking
> the whole list (at least up to the count of free ring buffers)
> when the queue is greater than one packet, thus effectively
> removing the lock expense for n-1 packets.
> 

Yes, but on modern cpus, locked operations are basically free once the CPU 
already has the cache line in exclusive access in its L1 cache.

I am not sure adding yet another driver API will help very much.
It will for sure adds some bugs and pain.

A less expensive (and less prone to bugs) optimization would be to prefetch 
one cache line for next qdisc skb, as a cache line miss is far more expensive 
than a locked operation (if lock already in L1 cache of course)


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