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Date:	Fri, 17 Jan 2014 22:16:06 +0200
From:	saeed bishara <>
To:	Dan Williams <>
Cc:	"" <>,
	Alexander Duyck <>,
	Dave Jiang <>,
	Vinod Koul <>,
	"" <>,
	David Whipple <>,
	lkml <>,
	"David S. Miller" <>
Subject: Re: [PATCH v3 1/4] net_dma: simple removal


isn't this issue similar to direct io case?
can you please look at the following article

regarding performance improvement using NET_DMA, I don't have concrete
numbers, but it should be around 15-20%. my system is i/o coherent.


On Wed, Jan 15, 2014 at 11:33 PM, Dan Williams <> wrote:
> On Wed, Jan 15, 2014 at 1:31 PM, Dan Williams <> wrote:
>> On Wed, Jan 15, 2014 at 1:20 PM, saeed bishara <> wrote:
>>> Hi Dan,
>>> I'm using net_dma on my system and I achieve meaningful performance
>>> boost when running Iperf receive.
>>> As far as I know the net_dma is used by many embedded systems out
>>> there and might effect their performance.
>>> Can you please elaborate on the exact scenario that cause the memory corruption?
>>> Is the scenario mentioned here caused by "real life" application or
>>> this is more of theoretical issue found through manual testing, I was
>>> trying to find the thread describing the failing scenario and couldn't
>>> find it, any pointer will be appreciated.
>> Did you see the referenced commit?
>> This is a real issue in that any app that forks() while receiving data
>> can cause the dma data to be lost.  The problem is that the copy
>> operation falls back to cpu at many locations.  Any one of those
>> instance could touch a mapped page and trigger a copy-on-write event.
>> The dma completes to the wrong location.
> Btw, do you have benchmark data showing that NET_DMA is beneficial on
> these platforms?  I would have expected worse performance on platforms
> without i/o coherent caches.
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