RFR (S): JEP-142: Reduce Cache Contention on Specified Fields

[Martijn Verburg]

Hi Aleksey,

Hi Martijn,
>
> On 11/23/2012 01:55 AM, Martijn Verburg wrote:
> > Is this based on detecting the size of the cache line of the CPU the
> > JVM is running on and multiplying by two?
>
> Not yet. We use the constant 128 bytes, overridable with
> -XX:FieldPaddingWidth. The cache line detection is there in Hotspot, and
> I still need to wire this up into the patch. The really sad part is that
> we can not detect the presence of hardware prefetcher running without
> reading the MSRs, which might in the end require superuser privileges.
>

OK, that's fair - We have a Java tool that reads MSRs and graphs lots of
interesting data. It has similar supreruser requirements under Linux (and
don't even get me started on having to patch in Mac Hardware kernel
drivers) :-).  I'll get one of our engineers to jump in if they have a
brainwave on this.

> As an extra aside - I'm wondering if the IDE vendors could pick up
> > on this and actually visualise for you how your class will be laid
> > out. Would be useful when applying @Contented and making sure it's
> > padding as you'd want it to.
>
> Yup, as soon as class is loaded by the VM, it is actually pretty easy to
> do, see:
>   https://github.com/shipilev/java-field-layout


Awesome! I'm hoping that there's a lot of education and visualisation
around this as marking fields, classes and groups as @Contended will have
side effects that a day to day developer may not fully understand (like
me!).

Thanks again on working on this, it certainly will help with performance as
numbers of cores grows.

Cheers,
Martijn