On Thu, May 31, 2012 at 9:38 AM, Paolo Bonzini <pbonz...@redhat.com> wrote:
> Il 31/05/2012 00:53, Luigi Rizzo ha scritto: > > The image contains my fast packet generator "pkt-gen" (a stock > > traffic generator such as netperf etc. is too slow to show the > > problem). pkt-gen can send about 1Mpps in this configuration using > > -net netmap in the backend. The qemu process in this case takes 100% > > CPU. On the receive side, i cannot receive more than 50Kpps, even if i > > flood the bridge with a a huge amount of traffic. The qemu process stays > > at 5% cpu or less. > > > > Then i read on the docs in main-loop.h which says that one case where > > the qemu_notify_event() is needed is when using > > qemu_set_fd_handler2(), which is exactly what my backend uses > > (similar to tap.c) > > The path is a bit involved, but I think Luigi is right. The docs say > "Remember to call qemu_notify_event whenever the [return value of the > fd_read_poll callback] may change from false to true." Now net/tap.c has > ... > So as a conservative approximation, you need to fire qemu_notify_event > whenever you write to RDH, RDT, RDLEN and RCTL, or when check_rxov becomes > zero. In practice, only RDT, RCTL and check_rxov matter. Luigi, does this > patch work for you? > it almost surely works (cannot check today), as my (guest) driver modifies RDT to notify the hardware of read packets. I know/mentioned that the notification can be optimized and sent only in specific case, as you describe above (i might be missing the check_rxov). But i think it would be more robust to make fewer assumptions on what the guest does, and send the notify on all register writes (those are relatively rare in a driver, and the datapath touches exactly the registers we ought to be interested in), and possibly on those reads that may have side effects, such as interrupt registers, together with a big comment in the code explaining why we need to call qemu_notify_event(). Actually, what would pay even more is devise a way to avoid the write() syscall in qemu_notify_event if the other process (or is it a thread ?) has data queued. This is not so important in my netmap driver, but a standard driver is likely to update the RDT on every single packet, which would pretty much kill performance. cheers luigi -- -----------------------------------------+------------------------------- Prof. Luigi RIZZO, ri...@iet.unipi.it . Dip. di Ing. dell'Informazione http://www.iet.unipi.it/~luigi/ . Universita` di Pisa TEL +39-050-2211611 . via Diotisalvi 2 Mobile +39-338-6809875 . 56122 PISA (Italy) -----------------------------------------+-------------------------------
