Re: tx queue hashing hot-spots and poor performance (multiq, ixgbe)

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <dada1@cosmosbay.com>
Date: Fri, 01 May 2009 08:14:03 +0200

> [PATCH] net: skb_tx_hash() improvements
> 
> When skb_rx_queue_recorded() is true, we dont want to use jash distribution
> as the device driver exactly told us which queue was selected at RX time.
> jhash makes a statistical shuffle, but this wont work with 8 static inputs.
> 
> Later improvements would be to compute reciprocal value of real_num_tx_queues
> to avoid a divide here. But this computation should be done once,
> when real_num_tx_queues is set. This needs a separate patch, and a new
> field in struct net_device.
> 
> Reported-by: Andrew Dickinson <andrew@whydna.net>
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

Applied, except that I changed the commit message header line to more
reflect that this is in fact a bug fix.

BTW, you don't need the reciprocol when num-tx-queues <= num-rx-queues
(you can just use the RX queue recording as the hash, straight) and
that's the kind of check what I intended to add to net-2.6 had you not
beaten me to this patch.

Also, thanks for giving me absolutely no credit for this whole thing
in your commit message.  I know I do that to you all the time :-/ How
can you forget so quickly that I'm the one that even suggested the
exact code change for Andrew to test in the first place?