Re: [Qemu-devel] Network Performance between Win Host and Linux

[Kenneth Duda]

Thanks, Leo.  It appears your patch or something similar has made it
into 0.8.0.  I have already merged the select loops, but it didn't
help as much as I hoped, maybe 10%.  A much bigger improvement was
made by fixing the badly hacked slirp DELACK behavior.  Believe it or
not, slirp delays all TCP acks *unless* the segment data starts with
an escape character, I kid you not.  I threw that out, and have made
slirp's tcp_input rfc2581 compliant (to my shallow reading of the rfc)
and that boosted throughput from vm->host by 3.5x, to 56 megabits
(from 16 megabits).  The performance from host->vm was helped less,
and that was because of another hack in slirp that was causing it to
get the wrong MSS --- it was sending 512 byte segments.  Now, I'm
looking at excessive numbers of retransmissions (believe it or not)
--- I suspect the ne2000 ring buffer is overflowing but I'm not yet
sure.  I will post a patch including all of these things when I'm
done.  I'm expecting a significant aggregate improvement.

     -Ken

On 4/11/06, Leonardo E. Reiter <address@hidden> wrote:
> Hi Ken,
>
> I'm attaching a pretty old patch I made (from the 0.7.1 days), which did
> a quick and dirty merge of the select's.  It's not something that is
> clean and it will need adapting to 0.8.0... but, I figure you could draw
> some quick hints on how to merge the 2.  Basically it fills the select
> bitmaps when it walks through the fd's the first time, then calls select
> instead of poll.  It also has slirp fill its own bits (fd's) in before
> calling select.  So this is condensed to 1 select call.
>
> Do what you want with the code - like I said, it's messy and old.  But
> maybe you can at least use it to quickly test your hypothesis.  I'd be
> interested in learning about any benchmarks you come up with if you
> merge the select+poll.  Also, it may not be valid at all on Windows
> hosts since there is a question about select() being interrupted
> properly on those hosts - it should work on Linux/BSD.
>
> Regards,
>
> Leo Reiter
>
> P.S. this patch should be applied with -p1, not -p0 like my newer
> patches are applied.  Sorry for that - like I said, it's quite old.
>