netdev - Re: Linux Route Cache performance tests

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Date:	Mon, 7 Nov 2011 13:42:44 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Paweł Staszewski <pstaszewski@...are.pl>,
	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: Linux Route Cache performance tests

On Sun, 2011-11-06 at 20:38 +0100, Eric Dumazet wrote:
> Le dimanche 06 novembre 2011 à 20:20 +0100, Paweł Staszewski a écrit :
[...]
> > So the point of this test was figure out how much of route cache entries 
> > Linux can handle without dropping performance.
> 
> No need to even do a bench, its pretty easy to understand how a hash
> table is handled.
> 
> Allowing long chains is not good.
> 
> With your 512k slots hash table, you cannot expect handling 1.4M routes
> with optimal performance. End of story.
> 
> Since route hash table is allocated at boot time, only way to change its
> size is using "rhash_entries=2097152" boot parameter.
> 
> If it still doesnt fly, try with "rhash_entries=4194304"

A routing cache this big is not going to fit in the processor caches,
anyway; in fact even the hash table may not.  So a routing cache hit is
likely to involve processor cache misses.  After David's work to make
cacheless operation faster, I suspect that such a 'hit' can be a net
loss.  But it *is* necessary to run a benchmark to answer this (and the
answer will obviously vary between systems).

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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