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

[Martijn Verburg]

Hi Aleksey,

A couple of small questions/comments from an enthusiastic amateur, take
with a grain of salt (inline).

So this in theory takes care of the False Sharing problem without resorting
to tricks, great!

<snip>


Note that we use 128 bytes, twice the cache line size on most hardware

to adjust for adjacent sector prefetchers extending the false sharing
> collisions to two cache lines.
>

Is this based on detecting the size of the cache line of the CPU the JVM is
running on and multiplying by two?

I can see architectures changing over time (bigger cache lines? Who knows).
Also, wouldn't a padding of 64 bytes be fine to avoid prefetchers for a 32
byte cache line based CPU whereas 128 is overkill?

*** IV. Contention groups
>
> There are cases where you want to separate the *group* of fields that
> are experiencing contention with everything else but not pairwise. This
> is the usual thing for some of the code updating two fields at once.
> While marking both with @Contended would be sufficient, we can optimize
> the memory footprint by not applying padding between them. In order to
> demarcate these groups, we have the parameter in the annotation
> describing the equivalence class for contention group.
>
> So that:
>
>     public static class ContendedTest5 {
>         @Contended("updater1")
>         private Object contendedField1;
>
>         @Contended("updater1")
>         private Object contendedField2;
>
>         @Contended("updater2")
>         private Object contendedField3;
>
>         private Object plainField5;
>         private Object plainField6;
>     }
>
> ...is laid out as:
>
>    TestContended$ContendedTest5: field layout
>      @ 12 --- instance fields start ---
>      @ 12 "plainField5" Ljava.lang.Object;
>      @ 16 "plainField6" Ljava.lang.Object;
>      @148 "contendedField1" Ljava.lang.Object; (contended, group = 12)
>      @152 "contendedField2" Ljava.lang.Object; (contended, group = 12)
>      @284 "contendedField3" Ljava.lang.Object; (contended, group = 15)
>      @416 --- instance fields end ---
>      @416 --- instance ends ---
>
> Note $contendedField1 and $contendedField2 are padded from everything
> else, but still densely packed with each other.
>

That's a nice addition.


> The code is known to work at least on Linux x86-64, tested with a few
> microtests. The layout of fields without @Contended is not affected, so
> this is presumably a safe change. I will try to run more tests against
> this implementation with JPRT, but will appreciate the design, API, and
> draft implementation review meanwhile...
>

I've forwarded this to a few folks who actually know what they're talking
about
and have complained about false sharing for some time, hopefully they'll
pop in.

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.

Cheers,
Martijn