Hi, On Tue, Aug 09, 2011 at 09:57:19AM -0400, Alan Stern wrote: > 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@xxxxxxxxxx> > > Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx> > > Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Thanks. Hijacking this thread for a small note. I'll be back on applying patches this friday. Until then I'll be focussed on some other issues which we're tracking down. Thanks for understanding ;-) -- balbi
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