Re: Network performance regression between kvm-29 and kvm-31

[Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>]

On Sat, Jul 21, 2007 at 12:25:45AM +0200, Aurelien Jarno wrote:
> Hi,
> 
> I have just noticed that there is a huge performance regression in
> network transfers between kvm-29 and kvm-31. Using the RTL8139 emulated
> card, I have seen the network speed has been divided by 27!!! I haven't
> made precise speed measurements with other emulated card, but the 
> network speed has been reduced by the same order.
> 
> Those tests have been done on a Core 2 CPU.
> 
> After some tests I have seen that the problem lies in the user part. I
> will do a git bisect tomorrow to find the patch that has caused the
> problem, unless somebody already find it.

I finally didn't resist to run git bisect before going to bed...

The patch that causes the regression is:

commit 7d2e674835492040c9baddf0989a95abea9a8d0f
Author: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
Date:   Sun Jul 15 15:08:23 2007 +0300

    kvm: qemu: synchronous signal handling
    
    in order to ensure that signals are handled with the global lock held, we
    block them, and dequeue them with sigtimedwait().  we can then call the
    handlers directly.
    
    this ensures that the device model and internal qemu data structures are not
    corrupted by cpu 0 thread handling a signal while cpu 1 is servicing a pio
    or mmio request.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32-8fiUuRrzOP0dnm+yROfE0A@public.gmane.org         | aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org
   `-    people.debian.org/~aurel32 | www.aurel32.net

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