Re: [PATCH v3 1/4] net_dma: simple removal

From: saeed bishara
Date: Fri Jan 17 2014 - 15:16:36 EST


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 <dan.j.williams@xxxxxxxxx> wrote:
> On Wed, Jan 15, 2014 at 1:31 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> On Wed, Jan 15, 2014 at 1:20 PM, saeed bishara <saeed.bishara@xxxxxxxxx> 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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