[Qemu-devel] Re: [PATCH 0/4] Improve -icount, fix it with iothread

[Edgar E. Iglesias]

On Mon, Feb 21, 2011 at 09:51:22AM +0100, Paolo Bonzini wrote:
> This series redoes the way time spent waiting for I/O is accounted to
> the vm_clock.
> 
> The current code is advancing qemu_icount before waiting for I/O.
> Instead, after the patch qemu_icount is left aside (it is a pure
> instruction counter) and qemu_icount_bias is changed according to
> the actual amount of time spent in the wait.  This is more
> accurate, and actually works in the iothread case as well.
> 
> (I started this as an experiment while trying to understand what was
> going on.  But it fixes the bug and does not break the no-iothread
> case, so hey...).
> 
> Patch 1 is a cleanup to Edgar's commit 225d02c (Avoid deadlock whith
> iothread and icount, 2011-01-23).

Thanks, this one was a good cleanup, I've applied it.


> Patch 2 fixes another misunderstanding in the role of qemu_next_deadline.
> 
> Patches 3 and 4 implement the actual new accounting algorithm.

Sorry, I don't know the code well enough to give any sensible feedback
on patch 2 - 4. I did test them with some of my guests and things seem
to be OK with them but quite a bit slower.
I saw around 10 - 20% slowdown with a cris guest and -icount 10.

The slow down might be related to the issue with super slow icount together
with iothread (adressed by Marcelos iothread timeout patch).


> With these patches, iothread "-icount N" doesn't work when the actual
> execution speed cannot keep up with the requested speed; the execution
> in that case is not deterministic.  It works when the requested speed
> is slow enough.

Sorry, would you mind explaning this a bit?

For example, if I have a machine and guest sw that does no IO. It runs
the CPU and only uses devices that use the virtual time (e.g timers
and peripherals that compute stuff). Can I expect the guest (with
fixed icount speed "-icount N") to run deterministically regardless of
host speed?

Cheers