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

[Avi Kivity <avi@redhat.com>]

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.