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From: Ingo Molnar <mingo@elte.hu>
To: Dor Laor <dor.laor@qumranet.com>
Cc: Avi Kivity <avik@qumranet.com>,
	kvm-devel <kvm-devel@lists.sourceforge.net>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [kvm-devel] kvm & dyntick
Date: Sun, 14 Jan 2007 09:51:16 +0100	[thread overview]
Message-ID: <20070114085116.GD2913@elte.hu> (raw)
In-Reply-To: <64F9B87B6B770947A9F8391472E0321609F7A0E2@ehost011-8.exch011.intermedia.net>


* Dor Laor <dor.laor@qumranet.com> wrote:

> Afterwards we'll need to compensate the lost alarm signals to the 
> guests by using one of
>  - hrtimers to inject the lost interrupts for specific guests. The 
> problem this will increase the overall load.
>  - Injecting several virtual irq to the guests one after another 
> (using interrupt window exit). The question is how the guest will be 
> effected from this unfair behavior.

well, the most important thing would be to fix qemu to:

 - not use a 1024 /dev/rtc stream of signals as its clock emulation 
   source

i hacked that out of qemu, only to find out that qemu then uses periodic 
itimers. Instead of that it should use one-shot itimers, driven by the 
expiry time of the next clock. I.e. this code in vl.c, in 
host_alarm_handler():

    if (qemu_timer_expired(active_timers[QEMU_TIMER_VIRTUAL],
                           qemu_get_clock(vm_clock)) ||
        qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
                           qemu_get_clock(rt_clock))) {

should start an itimer with an expiry time of:

 active_timers[QEMU_TIMER_VIRTUAL]->expire_time - qemu_get_clock(vm_clock)

or:

 active_timers[QEMU_TIMER_REALTIME]->expire_time - qemu_get_clock(rt_clock)

whichever is smaller. Furthermore, whenever timer->expire_time is 
changed in qemu_mod_timer(), this set-the-next-itimer-expiry-time code 
needs to be called. Would anyone like to try that?

this will reduce the host Qemu wakeup rate from 1000-1100/sec to the 
guest's 4-5/sec wakeup rate - resulting in 0.01% CPU overhead from a 
single idle guest. Current unmodified Qemu causes 10-20% CPU overhead 
from a single idle guest.

	Ingo

      parent reply	other threads:[~2007-01-14  8:55 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-11 16:08 Avi Kivity
2007-01-12  5:34 ` Rik van Riel
2007-01-12  6:20 ` Ingo Molnar
2007-01-12 10:19   ` Ingo Molnar
2007-01-12 17:04     ` Avi Kivity
2007-01-14  8:39       ` Ingo Molnar
2007-01-12 23:25     ` [kvm-devel] " Dor Laor
2007-01-12 23:39       ` Thomas Gleixner
2007-01-14  8:51       ` Ingo Molnar [this message]

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