RFR(XL): 8185640: Thread-local handshakes

Wed Oct 25 12:53:38 UTC 2017

Hi Martin,

On 2017-10-24 19:08, Doerr, Martin wrote:
> Hi Robbin,
> 
> sounds good. Thanks a lot for doing it.
> The change looks good to me except that I'd expect a poll for wide_ret, too.

Yes, incremental:
http://cr.openjdk.java.net/~rehn/8185640/v6/Interpreter-Poll-Wide_Ret-8/webrev/index.html

Sanity tested, running big test job now.

Thanks!

/Robbin

> 
> Best regards,
> Martin
> 
> 
> -----Original Message-----
> From: Robbin Ehn [mailto:robbin.ehn at oracle.com]
> Sent: Dienstag, 24. Oktober 2017 16:54
> To: Karen Kinnear <karen.kinnear at oracle.com>; Doerr, Martin <martin.doerr at sap.com>
> Cc: hotspot-dev developers <hotspot-dev at openjdk.java.net>; Coleen Phillimore (coleen.phillimore at oracle.com) <coleen.phillimore at oracle.com>
> Subject: Re: RFR(XL): 8185640: Thread-local handshakes
> 
> Hi,
> 
> I did a fix for the interpreter performance regression, it's plain and simple, I
> kept the polling code inside dispatch_base but made it optional as the verify oop.
> 
> Incremental:
> http://cr.openjdk.java.net/~rehn/8185640/v5/Interpreter-Poll-7/webrev/index.html
> 
> Manual tested with jstack and it passes: hotspot_tier1, hotspot_handshake
> 
> It reduces the polling cost of 80%, sensitive benchmarks shows -0.44% regression
> vs TLH off. More insensitive benchmark show no regression.
> 
> Thanks, Robbin
> 
> On 2017-10-23 17:58, Karen Kinnear wrote:
>> Works for me
>>
>> Thanks,
>> Karen
>>
>>> On Oct 23, 2017, at 8:40 AM, Doerr, Martin <martin.doerr at sap.com> wrote:
>>>
>>> Hi Coleen and Robbin,
>>>
>>> I'm ok with putting it into a separate RFE. I understand that there are more fun activities than rebasing this XL change for a long time :-)
>>> So you don't need to delay it. It's acceptable for me.
>>>
>>> Thanks, Coleen, for sharing your proposal. I appreciate it.
>>>
>>> Best regards,
>>> Martin
>>>
>>>
>>> -----Original Message-----
>>> From: Robbin Ehn [mailto:robbin.ehn at oracle.com]
>>> Sent: Montag, 23. Oktober 2017 17:26
>>> 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 16:05, Doerr, Martin wrote:
>>>> 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.)
>>>
>>> Yes, we are seeing a performance regression, 2.5%-6% depending on benchmark.
>>> We are committed to fix this, but it might come as separate RFE/bug depending on
>>> the JEP's timeline.
>>>
>>> (If the fix, very unlikely, would not be done before next release, we would
>>> change the default to off)
>>>
>>> I hope this is an acceptable path?
>>>
>>> Thanks, Robbin
>>>
>>>>
>>>> Best regards,
>>>> Martin
>>>>
>>>>
>>>> -----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
>>>>>>
>>