On 2012-05-30 23:23, Luigi Rizzo wrote:
>> On Wed, May 30, 2012 at 10:23:11PM +0200, Luigi Rizzo wrote:
>>> 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  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.
>>  http://thread.gmane.org/gmane.comp.emulators.qemu/144433
> my custom backend is the one in [Note 2] above.
> It replaces the -net pcap/user/tap/socket option which defines
> how qemu communicate with the host network device.