Re: [Qemu-devel] Proposed patch: huge RX speedup for hw/e1000.c

[Jan Kiszka]

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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