Can Xen sleep or wake up a thread?

Can Xen sleep or wake up a thread?

[Xinxin Jin]

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Hi everyone,

I found in Xen, when synchronization between multiple threads is needed,
Xen always uses busy wait or simple spin lock. In linux, the kernel can
block a waiting thread and schedule other threads for execution. Why does
Xen do not have this mechanism? I guess it is because the performance
issue? A lot of appreciation for your answer !!


Xinxin

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Re: Can Xen sleep or wake up a thread?

[Dario Faggioli]

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On mar, 2013-10-08 at 08:11 -0700, Xinxin Jin wrote:
> Hi everyone,
> 
> 
> I found in Xen, when synchronization between multiple threads is
> needed, Xen always uses busy wait or simple spin lock.
>
Mmm... What exactly is a 'thread' here? Are you talking about Xen
internals? If yes, I don't think there's much multithreading involved
there...

> In linux, the kernel can block a waiting thread and schedule other
> threads for execution. Why does Xen do not have this mechanism? 
>
Well, Linux, as a general purpose kernel, does indeed schedule a lot of
different stuff, yes, but I still don't see where you think Xen
could/should do something similar...

> I guess it is because the performance issue? A lot of appreciation for
> your answer !!
> 
Perhaps you can:
 - show some example of what you mean
 - describe what is the issue that you are seeing/trying to solve

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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Re: Can Xen sleep or wake up a thread?

[George Dunlap]

On Tue, Oct 8, 2013 at 4:11 PM, Xinxin Jin <xinxinjin89@gmail.com> wrote:
> Hi everyone,
>
> I found in Xen, when synchronization between multiple threads is needed, Xen
> always uses busy wait or simple spin lock. In linux, the kernel can block a
> waiting thread and schedule other threads for execution. Why does Xen do not
> have this mechanism? I guess it is because the performance issue? A lot of
> appreciation for your answer !!

Linux keeps per-thread kernel stacks, so it can call schedule() and
block in the middle of kernel code.  Xen does not keep per-vcpu
hypervisor stacks, so in general it cannot block in the middle of
hypervisor code.

 -George

Re: Can Xen sleep or wake up a thread?

[Ian Campbell]

On Wed, 2013-10-09 at 12:15 +0100, George Dunlap wrote:
> On Tue, Oct 8, 2013 at 4:11 PM, Xinxin Jin <xinxinjin89@gmail.com> wrote:
> > Hi everyone,
> >
> > I found in Xen, when synchronization between multiple threads is needed, Xen
> > always uses busy wait or simple spin lock. In linux, the kernel can block a
> > waiting thread and schedule other threads for execution. Why does Xen do not
> > have this mechanism? I guess it is because the performance issue? A lot of
> > appreciation for your answer !!
> 
> Linux keeps per-thread kernel stacks, so it can call schedule() and
> block in the middle of kernel code.  Xen does not keep per-vcpu
> hypervisor stacks, so in general it cannot block in the middle of
> hypervisor code.

And since it does not deal with e.g. I/O which can take a long time
there is typically no need for the hypervisor to block anyway.

Ian.