10 RFR: 8169517: WhiteBox should provide concurrent GC phase control

[Aleksey Shipilev]

On 03/03/2017 10:31 PM, Kim Barrett wrote:
>> On Mar 3, 2017, at 5:16 AM, Aleksey Shipilev <shade at redhat.com> wrote:
> Thanks for your review. I'll be interested in how the Shenandoah
> adaptation goes.
> 
> Regarding the string-based API, this is a white box API after all.
> Doing something else, like an enum in the whitebox code, seemed to
> require maintaining information both in the whitebox code and in the
> various GCs. And which GCs should be supported by the whitebox code?
> Should the Shenandoah phase identifiers be there, for example?

No, I don't think so. I mean, I get that string-based API is the lesser evil
here. I am wondering though that if the API expects some predefined strings, it
is better to have a way to figure out what strings are accepted. I think it is a
common courtesy for the API to tell that, no matter how internal it is :)

In current code, this is possible by calling into the actual await_phase(...)
method, and hoping for the best.

> I thought about a function to return the set of valid phase names, but
> couldn't come up with a good use for it.  One can't write a test that
> just iterates through the phases, requesting each in turn.  A phase
> might not be reachable without additional setup; there is at least one
> phase like that in G1 already.

The example that immediately comes to mind is asserting that a target GC indeed
supports all the phases you are about to test. Having a 10ms sanity test that
polls phase name roster and checks everything is available is better than having
multi-second tries for each phase in turn. Good luck keeping calm debugging a
test that has a typo in 20th phase name in try order.

Thanks,
-Aleksey

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