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List:       openjdk-hotspot-runtime-dev
Subject:    RE: RFR(S): 8206003: SafepointSynchronize with TLH: StoreStore barriers should be moved out of the l
From:       "Doerr, Martin" <martin.doerr () sap ! com>
Date:       2018-06-29 10:12:22
Message-ID: 21dc215e155e40af8e0f3f34603ff4e7 () sap ! com
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Thank you for the reviews.

I've created a new webrev with a "_release" version instead of "_no_release":
http://cr.openjdk.java.net/~mdoerr/8206003_tlh_sync_membars/webrev.01/

Due to this change, SafepointMechanism::initialize_header doesn't use a release \
barrier anymore which should be fine.

Pushed to jdk/submit11 and our internal testing.

Best regards,
Martin


-----Original Message-----
From: David Holmes [mailto:david.holmes@oracle.com] 
Sent: Freitag, 29. Juni 2018 00:49
To: Erik Österlund <erik.osterlund@oracle.com>; Doerr, Martin \
<martin.doerr@sap.com>; hotspot-runtime-dev@openjdk.java.net; Robbin Ehn \
                <robbin.ehn@oracle.com>; Andrew Haley (aph@redhat.com) \
                <aph@redhat.com>
Subject: Re: RFR(S): 8206003: SafepointSynchronize with TLH: StoreStore barriers \
should be moved out of the loop

On 29/06/2018 1:28 AM, Erik Österlund wrote:
> Hi Martin,
> 
> This did catch my eye too. This looks good to me. But could you consider 
> having _release in the name of the setter that uses release, and no 
> postfix for the one using a plain store, instead of giving that one a 
> _no_release postfix. I don't need another webrev.

+1

I'm assuming that nothing may be tripped up (ie assertion somewhere) if 
the polling status of different threads can now be seen out-of-order.

Thanks,
David

> 
> Thanks,
> /Erik
> 
> On 2018-06-28 16:52, Doerr, Martin wrote:
> > Hi,
> > 
> > I have recently come across a bad placement of memory barriers in 
> > SafepointSynchronize::begin() and end() which were changed for JEP 
> > 312: Thread-Local Handshakes. They iterate over all JavaThreads and 
> > call SafepointMechanism::arm_local_poll or disarm_local_poll. 
> > Unfortunately, the release barriers are inside the latter functions.
> > 
> > Assume we have several 1000 JavaThreads. This means the code executes 
> > several 1000 release barriers on weak memory model platforms (PPC64 
> > and ARM/aarch64). Only one is needed.
> > 
> > A goal of JEP 312 was to minimize latency of safepoints which gets 
> > defeated by this issue to some extend on these platforms.
> > 
> > It could be fixed by this proposal:
> > http://cr.openjdk.java.net/~mdoerr/8206003_tlh_sync_membars/webrev.00/
> > 
> > Please review.
> > 
> > Best regards,
> > Martin
> > 
> 


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