Re: [Qemu-devel] [RFC PATCH 0/2] High downtime with 95+ throttle pct

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Yury Kotov (yury-kotov@yandex-team.ru) wrote:
> Hi,
> 
> I wrote a test for migration auto converge and found out a strange thing:
> 1. Enable auto converge
> 2. Set max-bandwidth 1Gb/s
> 3. Set downtime-limit 1ms
> 4. Run standard test (just writes a byte per page)
> 5. Wait for converge
> 6. It's converged with 99% throttle percentage
> 7. The result downtime was about 300-600ms   <<<<
> 
> It's much higher than expected 1ms. I figured out that cpu_throttle_thread()
> function sleeps for 100ms+ for high throttle percentage (>=95%) in VCPU thread.
> And it sleeps even after a cpu kick.
> 
> I tried to fix it by using timedwait for ms part of sleep.
> E.g timedwait(halt_cond, 1ms) + usleep(500).
> 
> But I'm not sure about using timedwait function here with qemu_global_mutex.
> The original function uses qemu_mutex_unlock_iothread + qemu_mutex_lock_iothread
> It differs from locking/unlocking (inside timedwait) qemu_global_mutex
> because of using qemu_bql_mutex_lock_func function which could be anything.
> This is why the series is RFC.
> 
> What do you think?

Would qemu_sem_timedwait work for your use?  I use it in
migration_thread for the bandwidth limiting and allowing that to be
woken up.

Dave

> Thanks!
> 
> Yury Kotov (2):
>   qemu-thread: Add qemu_cond_timedwait
>   cpus: Fix throttling during vm_stop
> 
>  cpus.c                   | 27 +++++++++++++++++++--------
>  include/qemu/thread.h    | 12 ++++++++++++
>  util/qemu-thread-posix.c | 40 ++++++++++++++++++++++++++++------------
>  util/qemu-thread-win32.c | 16 ++++++++++++++++
>  4 files changed, 75 insertions(+), 20 deletions(-)
> 
> -- 
> 2.22.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK