On 18 August 2011 13:14, Felipe Balbi <balbi@xxxxxx> wrote: > Hi, > > On Thu, Aug 18, 2011 at 11:28:46AM +0200, Per Forlin wrote: >> From: Per Forlin <per.forlin@xxxxxxxxxx> >> >> 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@xxxxxxxxxx> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
