Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device

[Avi Kivity]

On 02/25/2010 05:06 PM, Paul Brook wrote:
> > > Idle bottom halves (i.e. qemu_bh_schedule_idle) are just bugs waiting to
> > > happen, and should never be used for anything.
> > >        
> > Idle bottom halves make considerable more sense than the normal bottom
> > halves.
> >
> > The fact that rescheduling a bottom half within a bottom half results in
> > an infinite loop is absurd.  It is equally absurd that bottoms halves
> > alter the select timeout.  The result of that is that if a bottom half
> > schedules another bottom half, and that bottom half schedules the
> > previous, you get a tight infinite loop.  Since bottom halves are used
> > often times deep within functions, the result is very subtle infinite
> > loops (that we've absolutely encountered in the past).
> >      
> I disagree. The "select timeout" is a completely irrelevant implementation
> detail. Anything that relies on it is just plain wrong. If you require a delay
> then you should be using a timer. If scheduling a BH directly then you should
> expect it to be processed without delay.
>    

I agree.  Further, once we fine-grain device threading, the iothread 
essentially disappears and is replaced by device-specific threads.  
There's no "idle" anymore.

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.