netdev - Re: [PATCH net-next 2/2] udp: Reset flow table for flows over unconnected sockets

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Date:	Mon, 27 Oct 2014 18:09:25 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/2] udp: Reset flow table for flows over
 unconnected sockets

On Mon, Oct 27, 2014 at 4:19 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2014-10-27 at 12:36 -0700, Tom Herbert wrote:
>
>> Please try this patch and provide real data to support your points.
>>
>
> Yep. This is not good, I confirm my fear.
>
> Google servers are shifting to serve both TCP & UDP traffic (QUIC
> protocol), with an increasing UDP load.
>
> Millions of packets per second per host, from millions of different
> sources...
>
This indicates nothing about the merits of this patch. Nevertheless,
in order to avoid further rat-holing and since this patch does change
a long standing behavior I'll will respin to make it enabled only by
sysctl.

Tom

> And your patch voids the RFS table, adds another cache miss in fast path
> for UDP rx path which is already too expensive.
>
>
>> If a TCP connection is hot it will continually refresh the table for
>> that connection, if connection becomes idle it only takes one received
>> packet to restore the CPU. The only time there could be a persistent
>> problem is if collision rate is high (which probably means table is
>> too small).
>
>
> RFS already has a low hit/miss rate, this patch does not help neither
> UDP or TCP.
>
> Ideally, RFS should be enabled on a protocol base, not an agnostic u32
> flow hash.
>
> Whatever strategy you implement, as long as different protocols share a
> common hash table, it wont be perfect for mixed workloads.
>
> Fundamental problem is that when an UDP packet comes, its not possible
> to know if its a 'flow' or 'not', unless we perform an expensive lookup,
> and then RPS/RFS cost becomes prohibitive.
>
> While for TCP, the current RFS cache miss is good enough, because about
> all packets are for connected flows. We eventually have bad steering for
> <not yet established> flows where the stack performs poorly anyway.
>
>
>
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