Re: [Qemu-devel] Linux SIGIO handling changes

[Avi Kivity]

Anthony Liguori wrote:
> Anders Melchiorsen wrote:
> > I am resending this patch, split into two parts now.
> >
> > This cleans up SIGIO handling to improve latency:
> > - SIGALRM for alarm timers
> > - enable SIGIO on qemu_set_fd_handler2()
> >
> > The issue was found in KVM, where it is much more visible,
> > because there is no periodic timer. However, it has been
> > confirmed (by Aurelien Jarno) that even for qemu, this
> > approach "improves network transfers in a huge way".
> >
> > Please apply, or give a firm rejection so I can stop resending.
> >   
>
> Probably the right thing to do is the direction KVM is moving toward, 
> i.e. have a separate IO thread.
>
> Setting SIGIO on every file descriptor is really just a hack to break 
> out of the cpu exec loop.  It's unclear to me whether it's really 
> always the right thing to do for every file descriptor.

Even with a separate iothread one needs the signals, as there is no 
other race-free way to poll for both aio completions and fd readiness.

[well, pselect works, but I dislike it and it isn't present on all kernels]

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