linux-kernel - Re: [PATCH v4] usb: gadget: storage_common: make FSG_NUM

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Date:	Thu, 18 Aug 2011 13:31:29 +0200
From:	Per Forlin <per.forlin@...aro.org>
To:	balbi@...com
Cc:	Per Forlin <per.forlin@...ricsson.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Michal Nazarewicz <mina86@...a86.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linaro-dev@...ts.linaro.org
Subject: Re: [PATCH v4] usb: gadget: storage_common: make FSG_NUM_BUFFERS
 variable size

On 18 August 2011 13:14, Felipe Balbi <balbi@...com> wrote:
> Hi,
>
> On Thu, Aug 18, 2011 at 11:28:46AM +0200, Per Forlin wrote:
>> From: Per Forlin <per.forlin@...aro.org>
>>
>> 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. If selecting USB_DEBUG this value may be
>> set by a module parameter as well.
>>
>> Signed-off-by: Per Forlin <per.forlin@...aro.org>
>
> Alan, are you ok with the change ? I'm not sure tying the parameter to
> CONFIG_USB_DEBUG is a good idea, maybe CONFIG_USB_GADGET_DEBUG or
> CONFIG_USB_FILE_STORAGE_TEST is better ?
>
The reason for the module parameter is to enable runtime calibration
of the optimal num_buffers. If the DEBUG-code adds
overhead that affects the performance the calibration will be
incorrect. I haven't measured the overhead of USB_DEBUG.

Regards,
Per
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