netdev - Re: >10% performance degradation since 2.6.18

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Date:	Mon, 6 Jul 2009 10:45:37 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, andi@...stfloor.org,
	arjan@...radead.org, matthew@....cx, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, douglas.w.styner@...el.com,
	chinang.ma@...el.com, terry.o.prickett@...el.com,
	matthew.r.wilcox@...el.com, Eric.Moore@....com,
	DL-MPTFusionLinux@....com, netdev@...r.kernel.org
Subject: Re: >10% performance degradation since 2.6.18

> That seems to presume it is impossible to reprogram the NIC RX queue 
> selection rules?
> 
> If you can add a new 'flow' to a NIC hardware RX queue, surely you can 
> imagine a remove + add operation for a migrated 'flow'...  and thus, at 
> least on the NIC hardware level, flows can follow processes.

The standard on modern NIC hardware is stateless hashing: as in you
don't program in flows, but the hardware just uses a fixed hash
to map the header to a rx queue. You can't really significantly influence
the hash on a per flow basis there (in theory you could chose
specific local port numbers, but you can't do that for the remote
ports or for well known sockets)

There are a few highend NICs that optionally support arbitary flow matching,
but it typically only supports a very limited number of flows
(so you need some fallback anyways) and of course it would
be costly to reprogram the NIC on every socket state change.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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