On Sat, 28 Sep 2013, Ming Lei wrote: > 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. Very few non-xHCI controllers can do DMA above the 4 GB limit. > > make sure this will happen? > > That can't be guaranteed but we can handle it with page bounce, just like > block device. Obviously. But if we have to bounce the pages, it isn't zero-copy any more. > 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. This may depend more on the host controller capabilities than on the CPU architecture. Alan Stern -- 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
