RFR: 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack Processing [v8]

[Erik Österlund]

On Tue, 6 Oct 2020 02:57:00 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Hi Erik,
> Can you give an overview of the use of the "poll word" and its relation to the "poll page" please?
> Thanks,
> David

Hi David,

Thanks for reviewing this code.

There are various polls in the VM. We have runtime transitions, interpreter transitions, transitions at returns, native
wrappers, transitions in nmethods... and sometimes they are a bit different.

The "poll word" encapsulates enough information to be able to poll for returns (stack watermark barrier), or poll for
normal handshakes/safepoints, with a conditional branch. So really, we could use the "poll word" for every single poll.
A low order bit is a boolean saying if handshake/safepoint is armed, and the rest of the word denotes the watermark for
which frame has armed returns.

The "poll page" is for polls that do not use conditional branches, but instead uses an indirect load. It is used still
in nmethod loop polls, because I experimentally found it to perform worse with conditional branches on one machine, and
did not want to risk regressions. It is also used for VM configurations that do not yet support stack watermark
barriers, such as Graal, PPC, S390 and 32 bit platforms. They will hopefully eventually support this mechanism, but
having the poll page allows a more smooth transition. And unless it is crystal clear that the performance of the
conditional branch loop poll really is fast enough on sufficiently many machines, we might keep it until that changes.

Hope this makes sense.

Thanks,

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PR: https://git.openjdk.java.net/jdk/pull/296