RFR: bug: Timely Reducing Unused Committed Memory

[Thomas Schatzl]

Hi,

On Sun, 2018-08-19 at 22:25 +0200, Rodrigo Bruno wrote:
> Hi Thomas,
> 
> I am just wrapping up the patch with the new changes.
> 
> Three questions:
> 
> 1 - regarding "use Ticks/Tickspan to measure time"
> 
> Currently I was using CollectedHeap::last_full_collection to get the
> time of the last attemp to compact/shrink. Since we are now giving
> the opportunity to shrink the heap without a full GC, would it make
> sense to have a different counter (last_compaction_attemp or
> something) that would be updated
> inside resize_if_necessary_after_full_collection?

That seems fine.

> 
> 2 - what checks should we have inside should_idle_compaction
> (previous should_gc)?
> Right now we have:
> a) time since last compaction attempt, 
> b) committed memory above user-defined threshold,
> c) over committed mem is above used-defined threshold, and 
> d) load average is above user-defined threshold.

I think these are the checks as mentioned in the JEP. Do you see a
reason to skip one or the other?

> 3 - it can happen now that two Concurrent Cycles are triggered with
> no collection in between.

I.e. if the idle timeout is hit multiple times with the option to not
do a full compaction? Just to make sure I understand the situation.

> This breakes debug build asserts because we never
> call  G1ConcurrentMark::post_initial_mark.
> We cannot call directly from within CM main loop because we are
> outside a safepoint. Any ideas?
> Should we setup the next phase (i.e. call post_initial_mark) upon the
> Cleanup phase?

This will not work: G1ConcurrentMark::post_initial_mark() needs to be
called in a safepoint as the initial setup of marking needs to capture
the state of the heap atomically wrt to mutators. Forcing an initial-
mark gc (with safepoint) seems to be the way to go here.

Look at G1CollectedHeap::attempt_allocation_humongous() to see how this
is done for humongous allocations. Basically a call to
G1CollectedHeap::collect() with a new, specific
"GCCause::g1_idle_timeout_no_fullgc" (for example) value.

Thanks,
  Thomas