Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

[Tejun Heo]

Hello, Austin.

On Mon, Aug 24, 2015 at 11:47:02AM -0400, Austin S Hemmelgarn wrote:
> >Just to learn more, what sort of hypervisor support threads are we
> >talking about?  They would have to consume considerable amount of cpu
> >cycles for problems like this to be relevant and be dynamic in numbers
> >in a way which letting them competing against vcpus makes sense.  Do
> >IO helpers meet these criteria?
> >
> Depending on the configuration, yes they can.  VirtualBox has some rather
> CPU intensive threads that aren't vCPU threads (their emulated APIC thread
> immediately comes to mind), and so does QEMU depending on the emulated

And the number of those threads fluctuate widely and dynamically?

> hardware configuration (it gets more noticeable when the disk images are
> stored on a SAN and served through iSCSI, NBD, FCoE, or ATAoE, which is
> pretty typical usage for large virtualization deployments).  I've seen cases
> first hand where the vCPU's can make no reasonable progress because they are
> constantly getting crowded out by other threads.

That alone doesn't require hierarchical resource distribution tho.
Setting nice levels reasonably is likely to alleviate most of the
problem.

> The use of the term 'hypervisor support threads' for this is probably not
> the best way of describing the contention, as it's almost always a full
> system virtualization issue, and the contending threads are usually storage
> back-end access threads.
> 
> I would argue that there are better ways to deal properly with this (Isolate
> the non vCPU threads on separate physical CPU's from the hardware emulation
> threads), but such methods require large systems to be practical at any
> scale, and many people don't have the budget for such large systems, and
> this way of doing things is much more flexible for small scale use cases
> (for example, someone running one or two VM's on a laptop under QEMU or
> VirtualBox).

I don't know.  "Someone running one or two VM's on a laptop under
QEMU" doesn't really sound like the use case which absolutely requires
hierarchical cpu cycle distribution.

Thanks.

-- 
tejun
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