linux-kernel - Re: [PATCH v3 2/4] videobuf2-dma-streaming: new videobuf2 memory allocator

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Message-ID: <20130108073130.38a8cc3d@lwn.net>
Date:	Tue, 8 Jan 2013 07:31:30 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Alessandro Rubini <rubini@...dd.com>, federico.vaga@...il.com,
	mchehab@...radead.org, pawel@...iak.com, hans.verkuil@...co.com,
	giancarlo.asnaghi@...com, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, s.nawrocki@...sung.com
Subject: Re: [PATCH v3 2/4] videobuf2-dma-streaming: new videobuf2 memory
 allocator

On Tue, 08 Jan 2013 07:50:41 +0100
Marek Szyprowski <m.szyprowski@...sung.com> wrote:

> > Couldn't this performance difference be due to the usage of GFP_DMA inside
> > the VB2 code, like Federico's new patch series is proposing?
> >
> > If not, why are there a so large performance penalty?  
> 
> Nope, this was caused rather by a very poor CPU access to non-cached (aka
> 'coherent') memory and the way the video data has been accessed/read 
> with CPU.

Exactly.  Uncached memory *hurts*, especially if you're having to touch it
all with the CPU.

jon
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