On Wed, Apr 1, 2015 at 8:06 AM, Peter Grehan <gre...@freebsd.org> wrote:

> 1. Can anyone tell me what the cause might be? What may be happening?
>> 2. Do you know if there is currently any work in investigation this
>> problem? Or anything related?
>> 3. Is Gang Scheduling or Coscheduling implemented in FreeBSD?
>> 4. Do you know of any other solution to this kind of problem?
>> 5. Can you recommend me any papers/videos/links in anyway related to this?
>>
>
>  I answered these in the FreeBSD forums post, but reproduced again here
> for the list:
>
>  1. The main issue is 'lock holder preemption', where a vCPU that is
> holding a spinlock has been pre-empted by the host scheduler, resulting in
> other vCPUs that are trying to acquire that lock to spin for full quantums.
>
> Booting is a variant of this for FreeBSD since the AP spins on a memory
> location waiting for a BSP to start up.
>
> 2. There's some minor investigation going on.
>
> 3. No.
>
> 4. I don't know that 'classic' gang scheduling is the answer (see 5). What
> has been thought of for bhyve at least is to a) have the concept of vCPU
> 'groups' in the scheduler, b) provide metrics to assist the scheduler in
> trying to spread out threads associated with a vCPU group so they don't end
> up on the same physical CPU (avoidance of lock-holder preemption), and c)
> implement pause-loop exits (see the Intel SDM, 24.6.13) in the hypervisor
> and provide that information to the scheduler so it can give a temporary
> priority boost to vCPUs that have been preempted but aren't currently
> running.
>
> 5. The classic reference on this is VMWare's scheduler paper:
> www.vmware.com/files/pdf/techpaper/VMware-vSphere-CPU-Sched-Perf.pdf
>

I am seeing similar behavior on Windows host w/ vmware workstation
recently. Guest boots several times slower than before. And console message
apparently scrolls slower after APs were all started. Booting natively is
not slow as much.

Guess that's related to recent SMP changes to bring APs online early?
Wondering if APs can be halted or be brought up later after kernel can
really begin to schedule tasks onto them.


-Jia-Shiun
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