LKML Archive on
help / color / mirror / Atom feed
From: Ingo Molnar <>
To: Dor Laor <>
Cc: Avi Kivity <>,
	kvm-devel <>,
	linux-kernel <>
Subject: Re: [kvm-devel] kvm & dyntick
Date: Sun, 14 Jan 2007 09:51:16 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

* Dor Laor <> 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 

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 

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


 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.


      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]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \
    --subject='Re: [kvm-devel] kvm & dyntick' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).