RFR (S): Parallel AlwaysPreTouch

[Aleksey Shipilev]

On 10/10/2016 02:55 PM, Roman Kennke wrote:
> Some little comments:
> - Why do we need our own flag? Can't we simply use the +AlwaysPreTouch
> flag?

Ah, I was about to explain that in comments, but forgot. VirtualSpace
code has this nasty thing:

static bool commit_expanded(char* start, size_t size, size_t alignment,
bool pre_touch, bool executable) {
  if (os::commit_memory(start, size, alignment, executable)) {
    if (pre_touch || AlwaysPreTouch) {
      pretouch_expanded_memory(start, start + size);
    }
    return true;
  }

...notice AlwaysPreTouch is unconditionally checked. This will get
memory touched during the initial storage allocation, even before we've
got a chance to wind up gang workers to parallelize it. So, if we want
to bypass this behavior, there are two options:
 a) Create our own VirtualSpace (like G1 is doing), and manage memory by
ourselves;
 b) Turn off AlwaysPreTouch, substitute it with a flag, and do this
little thing on our own.

I think (b) is much less hassle.

> - Instead of dividing memory into chunks, and adding another flag for
> it, you could just iterate over shenandoah's regions using
> ShenandoahHeapRegionSet::claim() and pretouch each region's memory?

The flag is coming from G1 changes, it seems we can leverage that. Also,
regions are not yet initialized at this point, and it would seem more
straightforward to do this on storage memory.

Thanks,
-Aleksey