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Re: [Xen-devel] Re: [Xen-users] About profiling xen

  • To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
  • From: Marco Tizzoni <marco.tizzoni@xxxxxxxxx>
  • Date: Thu, 1 Oct 2009 23:05:36 +0200
  • Cc: "Fajar A." <fajar@xxxxxxxxx>, Fasiha Ashraf <feehapk@xxxxxxxxxxx>, xen <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Thu, 01 Oct 2009 14:06:37 -0700
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On Thu, Oct 1, 2009 at 10:54 PM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 10/01/09 00:35, Marco Tizzoni wrote:
>> On Thu, Oct 1, 2009 at 1:23 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>>> Your clocksource is "xen", which is the best possible for a PV xen guest.
>>> However, a clocksource is for measuring elapsed time, not triggering
>>> timers.
>> Why? How would you solve the problem of raising events at a fixed rate?
> I'm not sure I follow you.  In the kernel, the clock*source* subsystem
> is for measuring time: its used to implement gettimeofday, as well as
> internal time accounting.  It isn't directly related to time events.


> The clock*event* mechanism is all about setting up timers to raise events.
> When running paravirtualized, we use Xen-specific versions of both.

Ok.  My problem is granularity. In xen I can't raise events more
frequently than 1000/s (with timer set to 1000hz).
Net test softwares such as netperf or hping use setitimer to send
packet at a fixed rate. On a non-xen box they works fine even  with
100k packets/sec (i.e. 100k alarms/sec). This can be problematic for
streaming software and more in general for soft real-time

>>> Unfortunately there doesn't seem to be a /sys file to show the
>>> current clockevent source in use, but if you have "xen" clocksource it's
>>> almost certainly the xen clockevent.
>>> However, this is only relevent if you have CONFIG_NO_HZ and
>> I've tried both set/unset but nothing change.
> Hm.  Its best to leave both enabled either way.
> Try the attached program to get some info about how well timers are
> working.  Compile with:

Thanks for your test, I tried and timers work as expected.


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