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Re: [Xen-devel] [PROPOSAL] Event channel for SMP-VMs: per-vCPU or per-OS?

On Tue, Oct 29, 2013 at 11:21 PM, David Vrabel <dvrabel@xxxxxxxxxx> wrote:
On 28/10/2013 15:26, Luwei Cheng wrote:
> This following idea was first discussed with George Dunlap, David Vrabel
> and Wei Liu in XenDevSummit13. Many thanks for their encouragement to
> post this idea to the community for a wider discussion.
> [Current Design]
> Each event channel is associated with only “one” notified vCPU: one-to-one.
> [Problem]
> Some events are per-vCPU (such as local timer interrupts) while some others
> are per-OS (such as I/O interrupts: network and disk).
> For SMP-VMs, it is possible that when one vCPU is waiting in the scheduling
> queue, another vCPU is running. So, if the I/O events can be dynamically
> routed to the running vCPU, the events can be processed quickly, without
> suffering from VM scheduling delays (tens of milliseconds). On the other
> hand, no reschedule operations are introduced.
> Though users can set IRQ affinity in the guest OS, the current
> implementation forces to bind the IRQ to the first vCPU of the
> affinity mask [events.c: set_affinity_irq].
> If the hypervisor delivers the event to a different vCPU, the event
> will get lost because the guest OS has masked out this event in all
> non-notified vCPUs [events.c: bind_evtchn_to_cpu].
> [New Design]
> For per-OS event channel, add “vCPU affinity” support: one-to-many.
> The “affinity” should be consistent with the ‘/proc/irq/#/smp_affinity’
> of the
> guest OS and users can change the mapping at runtime. But by default,
> all vCPUs should be enabled to serve I/O.
> When such flexibility is enabled, I/O balancing among vCPUs can be
> offloaded to the hypervisor. “irqbalance” is designed for physical
> SMP systems, not virtual SMP systems.

Thanks for your echoing, David. 
It's an interesting idea but I'm not sure how useful it will be in
practise as often work is deferred to threads in the guest rather than
done directly in the interrupt handler.
Sure, but if the interrupt handler is not called timely, no irq threads will
be created.
I don't see any way this could be implemented using the 2-level ABI.
Probably the implementation does not need to bother 2-level ABI.

With the FIFO ABI, queues cannot move between VCPUs without some
additional locking (dequeuing an event is only safe with a single
consumer) but it may be possible (when an event is set pending) for Xen
to pick a queue from a set of queues, instead of always using the same

I don't think this would result in balanced I/O between VCPUs, but the
opposite -- events would crowd onto the few VCPUs that are currently
I think it is the hypervisor who plays the role of deciding which vCPU should
be kicked to serve I/O. Different routing policies results in different results.
Since all vCPUs are symmetrically scheduled, the events can therefore be 
evenly distributed onto them. At one moment, vCPUx is running, while at 
another moment, vCPUy is running. So, the events will not always crowd to
very few ones.

Currently, all I/O events are bound to vCPU0, which is just like what you said:
events would crowd onto that vCPU. As a result, vCPU0 consumes much more
CPU cycles than other ones, leading to unfairness. If some workload can be 
dynamically migrated to other vCPUs, I believe more or less we can get 
some benefit.

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