linux-kernel - Re: [PATCH v3] usb: gadget: storage_common: make FSG_NUM

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Date:	Tue, 9 Aug 2011 17:51:55 +0300
From:	Felipe Balbi <balbi@...com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Per Forlin <per.forlin@...aro.org>, Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	Michal Nazarewicz <mina86@...a86.com>
Subject: Re: [PATCH v3] usb: gadget: storage_common: make FSG_NUM_BUFFERS
 variable size

Hi,

On Tue, Aug 09, 2011 at 09:57:19AM -0400, Alan Stern wrote:
> On Mon, 8 Aug 2011, Per Forlin wrote:
> 
> > FSG_NUM_BUFFERS is set to 2 as default.
> > Usually 2 buffers are enough to establish a good buffering pipeline.
> > The number may be increased in order to compensate a for bursty VFS
> > behaviour.
> > 
> > Here follows a description of system that may require more than
> > 2 buffers.
> >  * CPU ondemand governor active
> >  * latency cost for wake up and/or frequency change
> >  * DMA for IO
> > 
> > Use case description.
> >  * Data transfer from MMC via VFS to USB.
> >  * DMA shuffles data from MMC and to USB.
> >  * The CPU wakes up every now and then to pass data in and out from VFS,
> >    which cause the bursty VFS behaviour.
> > 
> > Test set up
> >  * Running dd on the host reading from the mass storage device
> >  * cmdline: dd if=/dev/sdb of=/dev/null bs=4k count=$((256*100))
> >  * Caches are dropped on the host and on the device before each run
> > 
> > Measurements on a Snowball board with ondemand_govenor active.
> > 
> > FSG_NUM_BUFFERS 2
> > 104857600 bytes (105 MB) copied, 5.62173 s, 18.7 MB/s
> > 104857600 bytes (105 MB) copied, 5.61811 s, 18.7 MB/s
> > 104857600 bytes (105 MB) copied, 5.57817 s, 18.8 MB/s
> > 
> > FSG_NUM_BUFFERS 4
> > 104857600 bytes (105 MB) copied, 5.26839 s, 19.9 MB/s
> > 104857600 bytes (105 MB) copied, 5.2691 s, 19.9 MB/s
> > 104857600 bytes (105 MB) copied, 5.2711 s, 19.9 MB/s
> > 
> > There may not be one optimal number for all boards. This is why
> > the number is added to Kconfig.
> > 
> > Signed-off-by: Per Forlin <per.forlin@...aro.org>
> > Acked-by: Michal Nazarewicz <mina86@...a86.com>
> 
> Acked-by: Alan Stern <stern@...land.harvard.edu>

Thanks.

Hijacking this thread for a small note. I'll be back on applying patches
this friday. Until then I'll be focussed on some other issues which
we're tracking down.

Thanks for understanding ;-)

-- 
balbi

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