On Sat, Sep 28, 2013 at 10:29 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Sat, 28 Sep 2013, Ming Lei wrote: > >> On Wed, Sep 25, 2013 at 3:12 AM, Markus Rechberger >> <mrechberger@xxxxxxxxx> wrote: >> > This patch adds memory mapping support to USBFS for isochronous and bulk >> > data transfers, it allows to pre-allocate usb transfer buffers. >> > >> > The CPU usage decreases 1-2% on my 1.3ghz U7300 notebook when >> > transferring 20mbyte/sec, it should be more interesting to see those >> > statistics on embedded systems where copying data is more expensive. >> >> Given USB3 is becoming popular and throughput is increased much, zero >> copy should be charming. >> >> And another approach is to use direct I/O method(SG DMA to pages >> allocated to user space directly), which should be more flexible, and >> user don't need to use mmap/munmap, so should be easier to use. >> >> At least, wrt. usb mass storage test, both CPU utilization and throughput >> can be improved with direct I/O. > > For zero-copy to work, on many systems the pages have to be allocated > in the first 4 GB of physical memory. How can the userspace program It depends if device can DMA to/from 4GB above physical memory. > make sure this will happen? That can't be guaranteed but we can handle it with page bounce, just like block device. Actually I observed both throughput and cpu utilization can be improved with the 4GB of DMA limit on either 32bit arch or 64bit arch, wrt. direct I/O over usb mass storage block device. Thanks, -- Ming Lei -- 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
