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

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Date:	Tue, 9 Aug 2011 09:57:19 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Per Forlin <per.forlin@...aro.org>
cc:	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

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>

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