Re: [Qemu-devel] Linux SIGIO handling changes

[Anthony Liguori]

Avi Kivity wrote:
> 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.

Unless you emulate signalfd() using a thread.  FWIW, I've been thinking 
of implementing something similar to posix-aio (using a thread-pool for 
AIO) for QEMU to get a bit more control for this sort of thing.

posix-aio is a pretty unfortunate interface as it doesn't provide a 
mechanism to do asynchronous fdatasync() nor individual vector requests.

Regards,

Anthony Liguori

Regards,

Anthony Liguori

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