netdev - Re: small RPS cache for fragments?

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Date:	Mon, 06 Jun 2011 14:06:09 -0700
From:	Jay Vosburgh <fubar@...ibm.com>
To:	rick.jones2@...com
cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

Rick Jones <rick.jones2@...com> wrote:

>On Mon, 2011-06-06 at 12:22 -0700, David Miller wrote:
>> From: Rick Jones <rick.jones2@...com>
>> Date: Mon, 06 Jun 2011 10:08:52 -0700
>> 
>> > Mode-rr bonding reorders TCP segments all the time. 
>> 
>> Oh well, then don't use this if you care about performance at all.
>> And therefore it's not even worth considering for our RPS fragment
>> cache.
>
>Heh - the (or at least a) reason people use mode-rr is to make a single
>(TCP) stream go faster :)  Without buying the next-up NIC speed.

	Right, the common use case for balance-rr (round robin) is to
maximize TCP throughput for one connection, over a set of whatever
network devices are available (or are cheap) by striping that connection
across multiple interfaces.  The tcp_reordering sysctl is set to some
large value so that TCP will deal with the reordering as best it can.

	Since TCP generally won't fragment, I don't see that the RPS
frag cache is going to matter for this usage anyway.

	If somebody out there is using round robin for some UDP-based
application that doesn't care about packet ordering (but might create
fragmented datagrams), I've not heard about it.

	-J

---
	-Jay Vosburgh, IBM Linux Technology Center, fubar@...ibm.com
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