Re: [Qemu-discuss] Using 2.5 multicore and the qemu (qemu-system-ppc64) is never sleeping

[Jakob Bohm]

On 04/07/2016 16:04, address@hidden wrote:
> I am using QEMU 2.5.0 with a 4 core set up and was expecting that the 
> load of the system would be minimal, as none of the 4 core cpus are 
> really doing anything. Each is awating a core specific timer 
> interrupt, writes a few characters to the QEMU screen, and then waits 
> for the next timer interrupt.
>
> What I see is that the QEMU-process is running at full rate (12,5% = 1 
> entire core-cpu out of 8 in a Windows7 environment).
> The previous QEMU version I ran with (0.14.50) was hardly showing any 
> load, so it must have managed to put the QEMU core(s) to sleep, but 
> 2.5 just hammers off cpu time, probably polling rather than 
> "Windows-waiting".
>
> I have tried to track what is going on, but had to give up, as this 
> appears to be very complex.
> The _cpus.c_ source in the qemu_wait_io_event_common() function has 
> this code:
>
>      while (all_cpu_threads_idle()) {
>        /* Start accounting real time to the virtual clock if the CPUs
>           are idle.  */
> qemu_clock_warp(QEMU_CLOCK_VIRTUAL);
> qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
>     }
>
> When all 4 cores are idle the qemu_clock_warp() function decides that 
> the clock can 'warp', but here I would have thought that the loop 
> should have been exited, so we could get to the nearest 
> 'waiting-point' to fall asleep, until an interrupt would wake us up 
> again. Instead we stay inside this loop, until one of the cpus is not 
> idle anymore.
> I have experimentally tried to exit the while-loop when the 
> qemu_clock_warp() function finds out that it can 'warp', but it made 
> no difference.
>
> The only place I can see where the cpus might be sleeping is inside 
> _main-loop.c_ inside the os_host_main_loop_wait() function (or called 
> from that), but the word 'poll' is all over the place, so I fear it is 
> polling until the wait is done, and not awaiting a Windows selective 
> wait call for interrupts or other events.
>
> My question is therefore: Does 2.5 offer a way at all for the cores to 
> Windows-sleep (wait) until an event occurs?
> As written above this used to work with 0.14.50, so I would think it 
> was possible.
> Right now this appears to be an unnecessary waste of power, and my i7 
> laptop gets noticably hot when the core QEMU is running inside is 
> totally monopolized.
>
> Kind regards
>
>         Thorkil B. Rasmussen
I am not completely familiar with the qemu source code, however
qemu_cond_wait() looks a lot like the POSIX call
pthread_cond_wait(), which should release the host CPU until
"halt_cond" is signalled by another thread.  If this is it's
purpose, but if it keeps looping anyway, then maybe there is
something wrong with the Windows port of qemu_cond_wait().

Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
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