RFR: Refactor allocation failure and explicit GC handling

[Aleksey Shipilev]

On 01/16/2018 11:16 PM, Roman Kennke wrote:
> I am unsure about:
> 
>> Behavioral change: Explicit GC no longer cancels the concurrent cycle, instead it waits for another
>> control loop iteration to start explicit GC. This is for the best, because it both simplifies our
>> handling logic, and allows requesters to wait for their own cycle. This is interesting when
>> concurrent cycle is running, ExplicitGCInvokesConcurrent is enabled and System.gc() is called: the
>> requesting thread would wait for one complete GC cycle to start and finish.
> 
> That does mean that System.gc() at the beginning of marking would wait until
> marking+evac(+updaterefs?) finishes, then does the full-gc, and only then is the Java thread allowed
> to progress? 

Yes. Think about it like the event loop, where Full GC request gets queued while Conc GC is being
processed at the moment, and the Full GC requester waits its place in line. Basically shifts
System.gc() from being "OMG, drop everything" to being "Noted, take a number, we shall do this at
our convenience".

> I guess it does not really matter very much, but what is the point to wait for current
> cycle completion if it goes into full-gc anyway? 

There is little performance point, I guess, and there are no performance guarantees for System.gc
either :) There are two off-the-bat considerations: a) the abrupt explicit GC in the middle of
regular cycle can wreck up inflight compaction decisions of smarter relocation heuristics; b) as
concurrent GC cycle runs, we have more chances to coalesce explicit GC-s from multiple threads, and
do one Full GC at once, not many quick back-to-back cancellations.

But ultimately, this thing is really the implementation convenience: it makes cancellations *only*
happen during allocation failures, which simplifies reasoning about the whole thing. This helps a
lot with Degenerated GC, because cancellation is then the sole route to Degenerate GC (which can
then be upgraded to Full GC), without the need to figure out if that cancellation was due to
explicit GC.

-Aleksey