RFR: 8224816: Provide os::processor_id() implementation for Mac OS

[David Holmes]

Hi Erik,

On 27/05/2019 8:05 pm, Erik Österlund wrote:
> Hi,
> 
> In order to port ZGC to Mac OS, we need to implement os::processor_id(), 
> due to the use of CPU locals in ZGC.
> I built an implementation that uses CPUID to get the APIC id, and from 
> that determine a unique CPU id.
> Note though that the APIC ids are split over a potentially larger number 
> space than the number of processors.
> In order to reliably translate APIC ids to CPU ids, I first use CPUID to 
> query how many bits are being used
> for APIC ids, and then create an array of integers covering that space. 
> The array maps possible APIC id positions
> to a unique processor id. The array is lazily populated with APIC id -> 
> processor id mappings.

So you aren't actually querying a processor id as such but rather 
creating a mapping from APIC ID to [0, num_processors). This processor 
id may be quite different to what the OS might consider the "processor 
id" - right?

It's a shame this stuff is so painful. We have a number of uses of cpuid 
to try and query topology information (some of which is incorrect - see 
JDK-8219434) and it is all nearly unfathomable code to understand and 
thus debug.

Cheers,
David

> The specific CPUID leaf used to query this topology information is 
> intel-specific, but that should be okay as all macs are Intel based.
> I have tried this out on a number of macs with a mac port of ZGC (to be 
> upstreamed soon), and it worked well.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8224816
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8224816/webrev.00/
> 
> Thanks,
> /Erik