RFR(XL): 8185640: Thread-local handshakes

Robbin Ehn robbin.ehn at oracle.com
Wed Oct 18 09:06:57 UTC 2017

Thanks for looking at this.

On 2017-10-17 19:58, Doerr, Martin wrote:
> Hi Robbin,
> my first impression is very good. Thanks for providing the webrev.


> I only don't like that "poll_page_val | poll_bit()" is used in shared code. I'd prefer to use either one or the other mechanism.
> Would it be ok to move the decision between what to use to platform code?
> (Some platforms could still use both if this is beneficial.)
> E.g. on PPC64, we'd like to use conditional trap instructions with special bit patterns if UseSIGTRAP is on. Would be excellent if we could implement set functions for _poll_armed_value and _poll_disarmed_value in platform code. poll_bit() also fits better into platform code in my opinion.

I see no issue with this.
Maybe SafepointMechanism::local_poll_armed should be possibly platform specific.
Can we do this incremental when adding the platform support for PPC64?

Thanks, Robbin

> Best regards,
> Martin
> -----Original Message-----
> From: hotspot-dev [mailto:hotspot-dev-bounces at openjdk.java.net] On Behalf Of Robbin Ehn
> Sent: Mittwoch, 11. Oktober 2017 15:38
> To: hotspot-dev developers <hotspot-dev at openjdk.java.net>
> Subject: RFR(XL): 8185640: Thread-local handshakes
> Hi all,
> Starting the review of the code while JEP work is still not completed.
> JEP: https://bugs.openjdk.java.net/browse/JDK-8185640
> This JEP introduces a way to execute a callback on threads without performing a global VM safepoint. It makes it both possible and cheap to stop individual threads and not
> just all threads or none.
> Entire changeset:
> http://cr.openjdk.java.net/~rehn/8185640/v0/flat/
> Divided into 3-parts,
> SafepointMechanism abstraction:
> http://cr.openjdk.java.net/~rehn/8185640/v0/SafepointMechanism-0/
> Consolidating polling page allocation:
> http://cr.openjdk.java.net/~rehn/8185640/v0/PollingPage-1/
> Handshakes:
> http://cr.openjdk.java.net/~rehn/8185640/v0/Handshakes-2/
> A handshake operation is a callback that is executed for each JavaThread while that thread is in a safepoint safe state. The callback is executed either by the thread
> itself or by the VM thread while keeping the thread in a blocked state. The big difference between safepointing and handshaking is that the per thread operation will be
> performed on all threads as soon as possible and they will continue to execute as soon as it’s own operation is completed. If a JavaThread is known to be running, then a
> handshake can be performed with that single JavaThread as well.
> The current safepointing scheme is modified to perform an indirection through a per-thread pointer which will allow a single thread's execution to be forced to trap on the
> guard page. In order to force a thread to yield the VM updates the per-thread pointer for the corresponding thread to point to the guarded page.
> Example of potential use-cases:
> -Biased lock revocation
> -External requests for stack traces
> -Deoptimization
> -Async exception delivery
> -External suspension
> -Eliding memory barriers
> All of these will benefit the VM moving towards becoming more low-latency friendly by reducing the number of global safepoints.
> Platforms that do not yet implement the per JavaThread poll, a fallback to normal safepoint is in place. HandshakeOneThread will then be a normal safepoint. The supported
> platforms are Linux x64 and Solaris SPARC.
> Tested heavily with various test suits and comes with a few new tests.
> Performance testing using standardized benchmark show no signification changes, the latest number was -0.7% on Linux x64 and +1.5% Solaris SPARC (not statistically
> ensured). A minor regression for the load vs load load on x64 is expected and a slight increase on SPARC due to the cost of ‘materializing’ the page vs load load.
> The time to trigger a safepoint was measured on a large machine to not be an issue. The looping over threads and arming the polling page will benefit from the work on
> JavaThread life-cycle (8167108 - SMR and JavaThread Lifecycle: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2017-October/024773.html) which puts all
> JavaThreads in an array instead of a linked list.
> Thanks, Robbin

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