Smoother tiered compilation thread ergonomics

[Igor Veresov]

Claes,

Sure, I agree. Since you seem to be already looking at the problem, could you experiment with smoother functions and check how it affects the startup?

Thanks,
igor

> On Aug 18, 2015, at 5:02 AM, Claes Redestad <claes.redestad at oracle.com> wrote:
> 
> Hi,
> 
> I noticed the thread ergonomics for tiered compilation have a few odd jumps
> that perhaps could be improved.
> 
> The calculation used to derive CICompilerCount for Tiered in 
> vm/runtime/advancedThresholdPolicy.cpp: 
> 
>     int log_cpu = log2_intptr(os::active_processor_count());
>     int loglog_cpu = log2_intptr(MAX2(log_cpu, 1));
>     count = MAX2(log_cpu * loglog_cpu, 1) * 3 / 2;
> 
> Seems to evaluate to:
> 
> #CPUs   CICompilerCount
>    <4                 2
>     4                 3
>     8                 4
>    16                12
>    32                15
>    64                18
>   128                21
>   256                36
>   512                40
>  1024                45
>  2048                49
> 
> The jump from 4 to 12 threads at 16 processors doesn't look very elegant (there's
> a small bump going from 128->256, too). It seems reasonable the ratio of compiler 
> threads to actual CPUs should diminish as CPUs increase, but going from 8 (or 15) 
> to 16 actually increases the ratio. 
> 
> If we'd replace log2_intptr with some non-discrete function instead of using 
> log2_intptr, we could smooth out the curve, which I think would be beneficial 
> for the rather common cases where systems have somewhere between 8 and 32 CPUs. 
> 
> /Claes