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Re: [Qemu-devel] [RFC][PATCH] qemu-timer: Run timers in alarm timer hand
From: |
Jan Kiszka |
Subject: |
Re: [Qemu-devel] [RFC][PATCH] qemu-timer: Run timers in alarm timer handler |
Date: |
Thu, 23 Aug 2012 15:01:28 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
On 2012-08-23 14:24, Paolo Bonzini wrote:
> Il 23/08/2012 14:10, Jan Kiszka ha scritto:
>>> Can you expand on this?
>>
>> Well, this patch removes an indirection from timer event deliveries. So
>> it reduces overhead, though only noticeable if you have high-rate timers.
>
> Actually, timers (and bottom halves) are always run after iohandlers.
> So the qemu_notify_event should already be completely useless for Unix,
> even if we leave the host_alarm_handler indirection.
Is there anything that requires this ordering for timers?
>
> But this leaves out Windows, where your next task of (IIUC) having
> multiple instances of struct qemu_alarm_timer would be complicated by
> the qemu_notify_event. I guess this is the original reason for your patch.
I'm not heading for multi-instance alarm timers or any kind of
optimization on Windows. It should just continue to work. Windows is
neither a high-performance nor a real-time platform for QEMU, IMHO.
>
> So, in order to remove the qemu_notify_event completely, what about not
> using signals anymore for timers? You could just tweak the select
> timeout and drop all the -clock madness. Zero syscalls, practically no
> overhead. If this is not precise enough, use timerfd on Linux only
Need to think about it. At least, real-time tasks will get proper
precision on Linux. Not sure if it will be sufficient on other hosts.
> (BTW, switching to an absolute deadline would be useful too).
Why? We aren't affected by clock adjustment with relative timeouts.
Jan
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