On 2012-05-30 22:23, Luigi Rizzo wrote: > Hi, > while testing qemu with netmap (see [Note 1] for details) on e1000 > emulation, i noticed that my sender program using a custom backend > [Note 2] could reach 1 Mpps (million packets per second) but on the > receive side i was limited to 50 Kpps (and CPU always below 5%). > > The problem was fixed by the following one-line addition to > hw/e1000.c :: e1000_mmio_write() , to wakeup the qemu mainloop and > check that some buffers might be available. > > --- hw/e1000.c.orig 2012-02-17 20:45:39.000000000 +0100 > +++ hw/e1000.c 2012-05-30 20:01:52.000000000 +0200 > @@ -919,6 +926,7 @@ > DBGOUT(UNKNOWN, "MMIO unknown write > addr=0x%08x,val=0x%08"PRIx64"\n", > index<<2, val); > } > + qemu_notify_event(); > } > > static uint64_t > > With this fix, the read throughput reaches 1 Mpps matching the write > speed. Now the system becomes CPU-bound, but this opens the way to > more optimizations in the emulator. > > The same problem seems to exist on other network drivers, e.g. > hw/rtl8139.c and others. The only one that seems to get it > right is virtio-net.c > > I think it would be good if this change could make it into > the tree. > > [Note 1] Netmap ( http://info.iet.unipi.it/~luigi/netmap ) > is an efficient mechanism for packet I/O that bypasses > the network stack and provides protected access to the > network adapter from userspace. > It works especially well on top of qemu because the > kernel needs only to trap a single register access > for each batch of packets. > > [Note 2] the custom backend is a virtual local ethernet > called VALE, implemented as a kernel module on the host, > that extends netmap to implement communication > between virtual machines. > VALE is extremely efficient, currently delivering about > 10~Mpps with 60-byte frames, and 5~Mpps with 1500-byte frames. > The 1 Mpps rates i mentioned are obtained between qemu instances > running in userspace on FreeBSD (no kernel acceleration whatsoever) > and using VALE as a communication mechanism.
"Custom backend" == you patched QEMU? Or what backend are you using? This sounds a lot like [1] and suggests that you are either a) using slirp in a version that doesn't contain that fix yet (before 1.1-rcX) or b) wrote a backend that suffers from a similar bug. Jan [1] http://thread.gmane.org/gmane.comp.emulators.qemu/144433
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