netdev - Re: small RPS cache for fragments?

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Message-ID: <1305664875.8149.945.camel@tardy>
Date:	Tue, 17 May 2011 13:41:15 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: small RPS cache for fragments?

On Tue, 2011-05-17 at 13:17 -0700, Rick Jones wrote:
> On Tue, 2011-05-17 at 13:02 -0700, Tom Herbert wrote:
> > I like it!  And this sounds like the sort of algorithm that NICs might
> > be able to implement to solve the UDP/RSS unpleasantness, so even
> > better.
> 
> Do (m)any devices take "shortcuts" with UDP datagrams these days?  By
> that I mean that back in the day, the HP-PB and "Slider" FDDI
> cards/drivers did checksum offload for fragmented UDP datagrams by
> sending the first fragment, the one with the UDP header and thus
> checksum, last.  It did that to save space on the card and make use of
> the checksum accumulator.

Even if no devices (mis)behave like that today, ordering of fragments
sent via a mode-rr bond is far from a sure thing.

rick

> 
> rick jones
> 
> > 
> > Tom
> > 
> > On Tue, May 17, 2011 at 11:33 AM, David Miller <davem@...emloft.net> wrote:
> > >
> > > It seems to me that we can solve the UDP fragmentation problem for
> > > flow steering very simply by creating a (saddr/daddr/IPID) entry in a
> > > table that maps to the corresponding RPS flow entry.
> > >
> > > When we see the initial frag with the UDP header, we create the
> > > saddr/daddr/IPID mapping, and we tear it down when we hit the
> > > saddr/daddr/IPID mapping and the packet has the IP_MF bit clear.
> > >
> > > We only inspect the saddr/daddr/IPID cache when iph->frag_off is
> > > non-zero.
> > >
> > > It's best effort and should work quite well.
> > >
> > > Even a one-behind cache, per-NAPI instance, would do a lot better than
> > > what happens at the moment.  Especially since the IP fragments mostly
> > > arrive as one packet train.


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