On Fri, Nov 13, 2015 at 12:00:53AM +0100, Steinar H. Gunderson wrote: > Interesting. I hacked libusb to get the fd. Then I did USBDEVFS_ALLOC_MEMORY, > which succeeded (well, as soon as I filled mem.size before calling), > then mmap on it as described (which also succeeded), and everything _works_, > but I don't think it's actually using zerocopy, because I still see > copy_user_enhanced_fast_string() using a lot of CPU > (with libusb_handle_events() in the call stack).
OK, so I did some more testing. It does actually seem to do _something_. In particular, memset_erms() all but disappeared from my profile, which was stated in the thread as one advantage of the patch. I ran overnight for eight hours (one card, continuous stream at about 900 Mbit/sec, 128 kB buffers) and it didn't get any page allocation features. Before the patch, it was unlikely but not impossible to run for that long. I've kept it running, but it's weak evidence of solving the problem. As for general performance, I'm unsure; from a quick glance at perf, I _may_ have more L3 misses when reading the data back in userspace, but I won't say anything for sure here without a closer look. So, in general I think it's good news, although I don't fully understand why I still need the kernel-to-userspace copy for isochronous transfer. /* Steinar */ -- Homepage: https://www.sesse.net/ -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
