Hi Erik,
On 6/10/2020 5:37 pm, Erik Österlund wrote:
On Tue, 6 Oct 2020 02:57:00 GMT, David Holmes <dhol...@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.
Yes but I am somewhat surprised. The conventional wisdom has always been
that polling based on the "poison page" approach far outperforms
explicit load-test-branch approaches.
Cheers,
David
Thanks,
-------------
PR: https://git.openjdk.java.net/jdk/pull/296
