Re: [PATCH v3] usb: gadget: storage_common: make FSG_NUM_BUFFERSvariable size
From: Alan Stern
Date: Tue Aug 09 2011 - 09:57:26 EST
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
> 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@xxxxxxxxxx>
> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
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