Re: [PATCH v6] usb: gadget: storage: make FSG_NUM_BUFFERS variablesize

[Alan Stern]

On Fri, 19 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_governor 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_GADGET_DEBUG_FILES
> this value may be set by a module parameter as well.
> 
> Signed-off-by: Per Forlin <per.forlin@xxxxxxxxxx>

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

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