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.
cheers
luigi
-----------------------------------------+-------------------------------
Prof. Luigi RIZZO, ri...@iet.unipi.it . Dip. di Ing. dell'Informazione
http://www.iet.unipi.it/~luigi/ . Universita` di Pisa
-----------------------------------------+-------------------------------
