RFR(M): Memory ordering in taskqueue.hpp

[Lindenmaier, Goetz]

Hi David,

It's a prerequisite that thread two reads the new value of bottom.
It it doesn't, there is no problem.

Thread three will read the new value of age, because that is 
written with cmpxchg.  And if thread three sees a 
bottom older than thread two that is inconsistent.

Best regards,
  Goetz.






-----Original Message-----
From: David Holmes [mailto:david.holmes at oracle.com] 
Sent: Samstag, 9. März 2013 00:41
To: Volker Simonis
Cc: Lindenmaier, Goetz; Vitaly Davidovich; Simonis, Volker; hotspot-dev at openjdk.java.net
Subject: Re: RFR(M): Memory ordering in taskqueue.hpp

On 9/03/2013 2:08 AM, Volker Simonis wrote:
> On Fri, Mar 8, 2013 at 4:31 PM, Lindenmaier, Goetz
> <goetz.lindenmaier at sap.com> wrote:
>> Hi Vitaly,
>>
>> the writer has a sync after the write to bottom (see pop_local)
>> but the store and the sync are not one atomic operation.  Stuff
>> can happen before the sync is executed.  Therefore the reader
>> must sync, too.
>>
>
> Exactly - you have three threads. The first one executes the write
> to bottom but not the following sync. The second thread reads this
> new value (by chance) and uses it but the third thread still sees the
> old value because there's no multipl-copy-atomicity on PPC. So we
> really need the sync before the read to assure thread three is not seeing
> an older value than thread two.

That doesn't quite make sense to me in isolation because thread two 
could read the old value just before thread one writes the new one 
anyway. I would think there would have to be a reordering issue as well, 
and that is what the sync would fix. (I haven't read the paper.)

David

>> Anyways, in pop_global it should be cheaper than in pop_local.
>>
>> Best regards,
>>    Geotz.
>>
>> From: Vitaly Davidovich [mailto:vitalyd at gmail.com]
>> Sent: Freitag, 8. März 2013 16:13
>> To: Lindenmaier, Goetz
>> Cc: hotspot-dev at openjdk.java.net; David Holmes; John Cuthbertson; Simonis, Volker
>> Subject: RE: RFR(M): Memory ordering in taskqueue.hpp
>>
>>
>> Hmm, if load fences on the reader aren't sufficient, seems like the problem is missing order/fence on the writer? If the write isn't visible to all processors immediately on power, shouldn't there be a fence in the write to ensure all CPUs see it? Putting the fence in the reader seems odd, but maybe that's just me.
>>
>> Thanks
>>
>> Sent from my phone
>> On Mar 8, 2013 10:03 AM, "Lindenmaier, Goetz" <goetz.lindenmaier at sap.com<mailto:goetz.lindenmaier at sap.com>> wrote:
>> Hi Vitaly,
>>
>> No, the problem is not reordering.  The problem is that
>> _bottom, which is read after _age, might be older than _age because
>> another processor didn't write it back yet.  The fence (sync) makes the
>> current thread wait until it has the new _bottom.
>> On Power, a write is not visible to all other threads simultaneously
>> (no multipl-copy-atomicity).
>>
>> What you propose helps if you look at two processors, but not if
>> four threads on four processors are involved.
>>
>> Best regards,
>>    Goetz.
>>
>>
>> From: Vitaly Davidovich [mailto:vitalyd at gmail.com<mailto:vitalyd at gmail.com>]
>> Sent: Freitag, 8. März 2013 15:29
>> To: Lindenmaier, Goetz
>> Cc: David Holmes; hotspot-dev at openjdk.java.net<mailto:hotspot-dev at openjdk.java.net>; John Cuthbertson; Simonis, Volker
>> Subject: RE: RFR(M): Memory ordering in taskqueue.hpp
>>
>>
>> Hi Goetz,
>>
>> If I understand correctly the problem is that bottom is loaded before age via compiler or arch reordering.  But why is a full fence needed? Is it not sufficient to have just a load fence to load age and relaxed load (just compiler barrier) for bottom?
>>
>> Thanks
>>
>> Sent from my phone
>> On Mar 8, 2013 9:06 AM, "Lindenmaier, Goetz" <goetz.lindenmaier at sap.com<mailto:goetz.lindenmaier at sap.com>> wrote:
>> Hi,
>>
>> we have been, and still are, doing research on this issue.
>> We want to keep you up to date on this, and propose on the
>> further proceeding.
>>
>> You asked explicit memory ordering operations and a rationale
>> why we added them.
>>
>> Axel found a paper describing the task queue algorithm and the
>> needed ordering on arm and power:
>> Correct and Efficient Work-Stealing for Weak Memory Models;
>> Lê, Pop, Cohen, Nardelli; PPoPP'13;
>> http://www.di.ens.fr/~zappa/readings/ppopp13.pdf
>>
>> According to this paper we need to add one fence and one load_acquire
>> to your implementation of the task queue.  You find this fence in this small
>> webrev:  http://cr.openjdk.java.net/~goetz/webrevs/8006971-2/
>>
>> With this fence, the algorithm works on Linux for our openjdk ppc
>> port, and also for our SAP JVM .
>>
>> Actually, the fence fixes a problem we discovered with the concurrency
>> torture test suite.  The problem occurs with four or more GC threads.
>> If three threads are stealing from the queue of the fourth, two of
>> them can pop the same element. Without a fence between the access
>> to age and bottom in pop_global(), bottom can be older than age.
>>
>>
>> Unfortunately, the OpenJDK implementation with the added fence
>> does not work on AIX.  Axel already detected one place where the xlC
>> compiler optimization removed load instruction that is required for
>> the correctness of the algorithm.  If we use our access routines with load_acquire
>> (see original webrev below) everything works, unclear why.
>>
>> Now, we think C++ might allow that this load is removed and xlC does
>> the correct, but unexpected thing.  On the other hand it might also be
>> a compiler bug.
>> We are currently discussing this with the IBM xlC developers.
>>
>> Best regards,
>>    Axel and Goetz.
>>
>>
>> PS: The webrev we proposed originally:
>> http://cr.openjdk.java.net/~goetz/webrevs/webrev-taskqueue/
>>
>>
>>
>>
>>
>>