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

[Erik Österlund]

On Tue, 6 Oct 2020 07:35:16 GMT, Erik Österlund <eosterlund 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 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,

> _Mailing list message from [Andrew Haley](mailto:aph at redhat.com) on [hotspot-dev](mailto:hotspot-dev at openjdk.java.net):_
> 
> On 06/10/2020 08:22, Erik ?sterlund wrote:
> 
> > > This PR the implementation of "JEP 376: ZGC: Concurrent Thread-Stack Processing" (cf.
> > > https://openjdk.java.net/jeps/376).
> 
> One small thing: the couple of uses of lea(InternalAddress) should really be adr;
> this generates much better code.

Hi Andrew,

Thanks for having a look. I applied your patch. Having said that, this is run on the safepoint slow path, so should be
a rather cold path, where threads have to wear coats and gloves. But it does not hurt to optimize the encoding further,
I suppose.

Thanks,

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