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

[Austin S Hemmelgarn]

On 2015-08-22 14:29, Tejun Heo wrote:
> Hello, Paul.
>
> On Fri, Aug 21, 2015 at 12:26:30PM -0700, Paul Turner wrote:
> ...
> > A very concrete example of the above is a virtual machine in which you
> > want to guarantee scheduling for the vCPU threads which must schedule
> > beside many hypervisor support threads.   A hierarchy is the only way
> > to fix the ratio at which these compete.
>
> 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 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.

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).

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