netdev - Re: [PATCH net-next] udp: under rx pressure, try to condense skbs

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Message-ID: <1481213334.4930.106.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 08 Dec 2016 08:08:54 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Rick Jones <rick.jones2@....com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] udp: under rx pressure, try to condense skbs

On Thu, 2016-12-08 at 07:36 -0800, Rick Jones wrote:
> On 12/08/2016 07:30 AM, Eric Dumazet wrote:
> > On Thu, 2016-12-08 at 10:46 +0100, Jesper Dangaard Brouer wrote:
> >
> >> Hmmm... I'm not thrilled to have such heuristics, that change memory
> >> behavior when half of the queue size (sk->sk_rcvbuf) is reached.
> >
> > Well, copybreak drivers do that unconditionally, even under no stress at
> > all, you really should complain then.
> 
> Isn't that behaviour based (in part?) on the observation/belief that it 
> is fewer cycles to copy the small packet into a small buffer than to 
> send the larger buffer up the stack and have to allocate and map a 
> replacement?

If properly done yes ;)

Some drivers do a copybreak, but throw away the original page frag and
reallocates a fresh one anyway.

Like if you have a PAGE_SIZE=65536, it is split in ~32 frags, and
drivers might not bother trying to reuse 1 frag.