RFR(XL): 8185640: Thread-local handshakes

Doerr, Martin martin.doerr at sap.com
Wed Oct 18 14:05:49 UTC 2017

Hi Robbin,

thanks for the quick reply and for doing additional benchmarks.
Please note that t->does_dispatch() was just a first idea, but doesn't really fit for the purpose because it's false for conditional branch bytecodes for example. I just didn't find an appropriate quick check in the existing code.
I guess you will notice a performance impact when benchmarking with -Xint. (I don't know if Oracle usually runs startup performance benchmarks.)

Best regards,

-----Original Message-----
From: Robbin Ehn [mailto:robbin.ehn at oracle.com] 
Sent: Mittwoch, 18. Oktober 2017 15:58
To: Doerr, Martin <martin.doerr at sap.com>; hotspot-dev developers <hotspot-dev at openjdk.java.net>
Subject: Re: RFR(XL): 8185640: Thread-local handshakes

Hi Martin,

On 2017-10-18 12:11, Doerr, Martin wrote:
> Hi Robbin,
> so you would like to push your version first (as it does not break other platforms) and then help us to push non-Oracle platform implementations which change shared code again?
> I'd be fine with that, too.

Yes, great!

> While thinking a little longer about the interpreter implementation, a new idea came into my mind.
> I think we could significantly reduce impact on interpreter code size and performance by using safepoint polls only in a subset of bytecodes. E.g., we could use only bytecodes which perform any kind of jump by implementing something like
> if (SafepointMechanism::uses_thread_local_poll() && t->does_dispatch()) generate_safepoint_poll();
> in TemplateInterpreterGenerator::generate_and_dispatch.

We have not seen any performance regression in simple benchmark with this.
I will do a better benchmark and compare what difference it makes.

Thanks, Robbin

> Best regards,
> Martin
> -----Original Message-----
> From: Robbin Ehn [mailto:robbin.ehn at oracle.com]
> Sent: Mittwoch, 18. Oktober 2017 11:07
> To: Doerr, Martin <martin.doerr at sap.com>; hotspot-dev developers <hotspot-dev at openjdk.java.net>
> Subject: Re: RFR(XL): 8185640: Thread-local handshakes
> 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.
> Great!
>> 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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