netdev - Re: [PATCH v2] tcp: splice as many packets as possible at once

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Date:	Fri, 6 Feb 2009 12:10:15 +0100
From:	Willy Tarreau <w@....eu>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>, zbr@...emap.net,
	dada1@...mosbay.com, ben@...s.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH v2] tcp: splice as many packets as possible at once

On Fri, Feb 06, 2009 at 10:58:07AM +0000, Jarek Poplawski wrote:
> On Fri, Feb 06, 2009 at 09:28:22PM +1100, Herbert Xu wrote:
> > On Fri, Feb 06, 2009 at 09:51:20AM +0000, Jarek Poplawski wrote:
> > >
> > > OK, but since there is not so much difference, and we need to waste
> > > it in some cases anyway, plus handle it later some special way, I'm
> > > a bit in doubt.
> > 
> > Well the thing is cards that don't support SG on tx probably
> > don't support jumbo frames either.
> 
> ?? I mean this 128 byte chunk would be hard to reuse after copying
> to skb->data, and if reused, we could miss this for some NICs on TX,
> so the whole packed would need a copy.

couldn't we stuff up to 32 128-byte chunks in a page and use a 32-bit
map to indicate which slot is used and which one is free ? This would
just be a matter of calling ffz() to find one spare place in a page.
Also, that bitmap might serve as a refcount, because if it drops to
zero, it means all slots are unused. And -1 means all slots are used.

This would reduce wastage if wee need to allocate 128 bytes often.

Willy

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