On 02/25/2010 11:11 AM, Avi Kivity wrote:
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.
That's a nice idea, but how is io dispatch handled? Is everything
synchronous or do we continue to program asynchronously?
It's very difficult to mix concepts. I personally don't anticipate
per-device threading but rather anticipate re-entrant device models. I
would expect all I/O to be dispatched within the I/O thread and the VCPU
threads to be able to execute device models simultaneously with the I/O
thread.
Regards,
Anthony Liguori
