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SubjectRe: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks
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On 12/11/18 9:18 AM, Dave Hansen wrote:
> On 12/10/18 4:24 PM, Aubrey Li wrote:
>> The tracking turns on the usage flag at the next context switch of
>> the task, but requires 3 consecutive context switches with no usage
>> to clear it. This decay is required because well-written AVX-512
>> applications are expected to clear this state when not actively using
>> AVX-512 registers.
>
> One concern about this: Given a HZ=1000 system, this means that the
> flag needs to get scanned every ~3ms. That's a pretty good amount of
> scanning on a system with hundreds or thousands of tasks running around.
>
> How many tasks does this scale to until you're eating up an entire CPU
> or two just scanning /proc?
>

The reason why I believe that an exponential decay average, or some
other kind of weighted decay of the AVX512 usage is better.

Tim

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