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xen-devel
Re: [Xen-devel] Credit scheduler
At 21:46 12/07/2007, pak333@xxxxxxxxxxx wrote:
Hi Atsushi
I am still confused, so let me explain what i think should happen
and understand from you why it may not happen.
I have 2 cpu intensive VMs and 1 IO intensive VM. My system has 4
physical cpus and 8 virtual cpus.
Now using default parameters for the credit scheduler, the cpu
intersive vcpu should run for 30 millsec. It does not, it runs for
less (few microsecs).
You said this could be because of softirq which are raised after a
system call. Can i disable the softirqs. What happens if i do? If i
cannot disable is there a way to see what VM is raising the softirq.
If the cpu-VM raises the softirq it gets preempted or does it
continue to run. How can I montior the softirqs raised by this VM.
Can I tell if the premption is from a softirq or from something else.
I don't believe you can "disable" a softirq - you could of course
avoid doing the call to "do_softirq", but that is most likely just
going to completely remove any scheduling capability at all in the
system, making the system "single-tasking". Not a recommended way, in
my opinion.
Also, if the IO VM requests an IO, it will block and dom0 wakes up
and gets scheduled to run to service the IO. Will it preempt the cpu
intensive VM. If so why? Shouldn't the cpuintensiveV M get its
quantum of time.
Or does the IOVM get higher priority to preempt the cpu intensive
VM. How does the scheduler pick which cpu to run dom0 on ( if all
the vcpus running are cpu intensive)? If there is a mix of cpu-vcpus
and Io-vcpus, which will be the victim
The whole idea of having separate queues for IO and CPU intensive VMs
(or processes in a normal kernel scenario) is to allow the
IO-intensive tasks to avoid waiting for the next time-slot when a
CPU-hogging VM/process is running [1]. The normal behaviour for the
scheduler is to determine dynamically if the current VM/process is IO
or CPU intensive.
Unless Dom0 is used for doing some silly CPU-intensive task
(calculating PI with a million decimal points or compiling Xen +
Linux kernel, for example), it will most likely look to the scheduler
like a IO-intensive VM. So it will have the same priority as any
other IO-intensive VM (assuming Dom0 has equal scheduler parameters
as other guests, which I'm pretty sure is the default behaviour).
If, like in your example, there are a number of processors busy with
CPU-intensive tasks, and others with IO-intensive tasks, the most
likely scenario is that any new IO-request will go be run on a
"previously CPU-intensive" CPU - but on the other hand, if you have a
lot of IO-intensive processing, there should be some processor(s)
that are "asleep" - unless all CPU's are busy running CPU-intensive tasks...
Note that scheduling is a complex subject, and there have been dozens
of PhD dissertations written on the subject.
[1] The idea being that if we process the IO-request that just
completed, we can set up another IO-request, and this will get better
IO-throughput than waiting a long time (relatively speaking 30ms is
an eternity to the processor).
--
Mats
As you see i have a lot of questions, so please bear with me and
help me understand the scheduling behavior of Xen
Thanks
Prabha
-------------- Original message --------------
From: Atsushi SAKAI <sakaia@xxxxxxxxxxxxxx>
> Hi Prabha
>
> Please check do_softirq in assembler.
> for x86
> call do_softirq
> is in.
>
> And usually do_softirq is executed at the end of system call.
>
> Thanks
> Atsushi SAKAI
>
>
>
> pak333@xxxxxxxxxxx wrote:
>
> > Thanks for the explanation, i am not sure i understood it correctly.
> >
> >
> > when does softirq get set. So when softirq is set a processor enters the
> scheduler every 3 micro secs? where in the source code is this
handled. please
> could you send me some pointers
> >
> > thanks
> > -Prabha
> >
> >
> >
> >
> > -------------- Or iginal message --------------
> > From: Atsushi SAKAI
> >
> > > Hi,$B!!(BPrabha
> > >
> > > $B!!(B30msec is the maximum time slice.
> > > And I guess your 3 microsecond is the response after
> > > SCHEDULE_SOFTIRQ is set.
> > > As you know,
> > > If SCHEDULE_SOFTIRQ is set,
> > > it waits softirq calls schedule(). (this time interval is 3microsec)
> > >
> > > And I/O intensive has higher priority than CPU intensive.
> > > So I/O intensive job is first dispatched domain in runq.
> > > This is because latency improvement for I/O intensive guest.
> > >
> > > So your behavior is not strange.
> > >
> > > Thanks
> > > Atsushi SAKAI
> > >
> > >
> > > pak333@xxxxxxxxxxx wrote:
> &g
t; >
> > > > Hi
> > > >
> > > > I have noticed that VMs are typically spending 3 microsecs
or less before
> they
> > > are being prempted. I thought that the credit schduler time
slice was 30 ms.
> I
> > > have 4 VMs running and they are all cpu intensive except for
1 (which is IO
> > > intensive) but having a VM spend max 3 micro secs before
being kicked out
> seems
> > > strange.
> > > >
> > > > Is there something else going on that i am not aware of. Is
the time slice
> > > really 30 millisecs? I am using default parameters of the
credit scheduler.
> > > >
> > > > Thanks
> > > > -Prabha
> > >
> > >
>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxx source.com
> http://lists.xensource.com/xen-devel
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