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Re: [Xen-users] Scheduling questions for I/O activity

  • To: "Amitayu Das" <amitayudas@xxxxxxxxx>
  • From: "Diwaker Gupta" <diwaker.lists@xxxxxxxxx>
  • Date: Tue, 28 Feb 2006 17:14:17 -0800
  • Cc: xen-users <xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 01 Mar 2006 01:14:57 +0000
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Lets not cross-post. xen-devel is for development issues regarding
Xen, xen-users is for users of Xen. Pertinent threads may migrate from
one list to another. Further, a lot of Xen devs lurk around on the
user list anyways.

Now, onto your questions.

On 2/28/06, Amitayu Das <amitayudas@xxxxxxxxx> wrote:
>  1. Say, DomU1 started transmitting packets over the network and got
>  blocked. Dom0 is supposed to do the actual transmission and it did
>  that. Eventually, when the reply came from the other side, nither
>  Dom0 nor DomU1 is scheduled. How does hypervisor react to this
>  interrupt? I can think of the following situations and you may be
>  able to tell me which is correct:
>  a. hypervisor preempts the currently running entity (which is
>  neither DomU1/Dom0) and schedules Dom0 to complete the I/O. When
>  Dom0 is done,
>  the preempted guy is scheduled again.
>  b. hypervisor preempts the currently running entity (which is
>  neither
>  DomU1/Dom0) and schedules Dom0 to complete the I/O. When Dom0 is
>  done,
>  DomU1 is scheduled.
>  c. hypervisor does not do any preemption; schedules Dom0/DomU1
>  according to the predefined order.

IIUC (and I might be wrong), its neither of the above 3. When a packet
comes in, Xen takes the interrupts and figures out which domain is to
handle the interrupt (in this case, Dom0).

*Whenever* Dom0 is scheduled next, it takes this interrupt and posts a
virtual interrupt to the destination domain (Dom1). Again, *whenever*
Dom1 is scheduled next, it deals with the packet. Depending on what
you meant by "predefined" order, this might actually look like (c),
except that Xen does preempt the originally running VM to take the
interrupt. It then calls into the scheduler to figure out which domain
to run next, just as it would normally.

Hope that helps,
Web/Blog/Gallery: http://floatingsun.net/blog

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