RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64

Hiroshi H Horii HORII at jp.ibm.com
Fri Sep 30 10:17:05 UTC 2016


Dear David, and Dan,

Thank you for your comments.

> In hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:
> 266 the log line reads data from the forwardee even when the CAS 
> fails. I believe those reads will be unsafe without barriers after 
> the copy of the content of the object.
> hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:288 
> same problem as in line 266

Can we use o->size() or new_obj_size instead of new_obj->size()?

> If you feel that the use of new_obj->size() is potentially unsafe then 
> the fact we return new_obj means that any use of new_obj by the caller 
> may also potentially be unsafe.

In my understanding, while copying objects to a survivor space, if a 
thread creates a new_obj and sets a pointer with CAS, the other threads 
can touch the new_obj after the thread calls push_contents(new_obj) (Line: 
239). In push_contents, OrderAccess::release_store is called before 
pushing the object as a task into a deque of workstealing 
(taskqueue.inline.hpp). If the other thread reads the task, all of copy 
for new_obj is safe.

Thank you for your helps again. I may be misunderstanding or missing 
something critical. Any comments and claims are always appreciated. 

Regards,
Hiroshi
-----------------------
Hiroshi Horii, Ph.D.
IBM Research - Tokyo


David Holmes <david.holmes at oracle.com> wrote on 09/30/2016 07:16:16:

> From: David Holmes <david.holmes at oracle.com>
> To: Carsten Varming <varming at gmail.com>, Hiroshi H Horii/Japan/IBM at IBMJP
> Cc: Tim Ellison <Tim_Ellison at uk.ibm.com>, "ppc-aix-port-
> dev at openjdk.java.net" <ppc-aix-port-dev at openjdk.java.net>, "hotspot-
> runtime-dev at openjdk.java.net" <hotspot-runtime-
> dev at openjdk.java.net>, "hotspot-gc-dev at openjdk.java.net" <hotspot-
> gc-dev at openjdk.java.net>, hotspot-compiler-dev <hotspot-compiler-
> dev-bounces at openjdk.java.net>
> Date: 09/30/2016 07:17
> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and 
> copy_to_survivor for ppc64
> 
> On 30/09/2016 12:47 AM, Carsten Varming wrote:
> > Dear Hiroshi,
> >
> > In hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:266
> > the log line reads data from the forwardee even when the CAS fails. I
> > believe those reads will be unsafe without barriers after the copy of
> > the content of the object.
> 
> I find it extremely hard to reason about a barrier-less cmpxchg in 
general.
> 
> If you feel that the use of new_obj->size() is potentially unsafe then 
> the fact we return new_obj means that any use of new_obj by the caller 
> may also potentially be unsafe.
> 
> David
> -----
> 
> > hotspot/src/share/vm/gc/parallel/psPromotionManager.inline.hpp:288 
same
> > problem as in line 266
> >
> > I would argue that the logging should only happen if the thread
> > successfully copied the object and CAS failures should be logged
> > separately without reading data from the forwardee.
> >
> > BTW, unrelated to your change: It seems like the logging in line 266
> > should be guarded by something like "if (log_develop_is_enabled(Trace,
> > gc, scavenge)" like the logging in line 288.
> >
> > Carsten
> >
> > On Thu, Sep 29, 2016 at 8:00 AM, Hiroshi H Horii <HORII at jp.ibm.com
> > <mailto:HORII at jp.ibm.com>> wrote:
> >
> >     Hi all,
> >
> >     Can I please request reviews for a change for 8154736 that improve
> >     copy_to_survivor performance of ppc64 and aarch64?
> >     If possible, I would like to include this change into jdk9.
> >
> >     8154736 includes two changes, cmpxchg and copy_to_suvivor, and the
> >     former
> >     was resolved as 8155949.
> >     Now, I would like to ask a review for the remaining, 
copy_to_suvivor
> >     change.
> >
> >     webrev:
> >     
http://cr.openjdk.java.net/~mdoerr/8154736_copy_to_survivor/webrev.01/
> >     <
http://cr.openjdk.java.net/~mdoerr/8154736_copy_to_survivor/webrev.01/>
> >     JIRA: https://bugs.openjdk.java.net/browse/JDK-8154736
> >     <https://bugs.openjdk.java.net/browse/JDK-8154736>
> >
> >     I tested this change with SPECjbb2013. Also, I re-check that 
relaxed
> >     cmpxchg is available for changing forwarding pointers. However, 
because
> >     this change is sensitive, we need more reviews not only from
> >     compiler-dev,
> >     but also from gc-dev.
> >
> >     Regards,
> >     Hiroshi
> >     -----------------------
> >     Hiroshi Horii, Ph.D.
> >     IBM Research - Tokyo
> >
> >
> >
> >
> >     From:   David Holmes <david.holmes at oracle.com
> >     <mailto:david.holmes at oracle.com>>
> >     To:     "Doerr, Martin" <martin.doerr at sap.com
> >     <mailto:martin.doerr at sap.com>>, Hiroshi H
> >     Horii/Japan/IBM at IBMJP
> >     Cc:     Tim Ellison <Tim_Ellison at uk.ibm.com
> >     <mailto:Tim_Ellison at uk.ibm.com>>,
> >     "ppc-aix-port-dev at openjdk.java.net
> >     <mailto:ppc-aix-port-dev at openjdk.java.net>"
> >     <ppc-aix-port-dev at openjdk.java.net
> >     <mailto:ppc-aix-port-dev at openjdk.java.net>>,
> >     "hotspot-gc-dev at openjdk.java.net
> >     <mailto:hotspot-gc-dev at openjdk.java.net>"
> >     <hotspot-gc-dev at openjdk.java.net
> >     <mailto:hotspot-gc-dev at openjdk.java.net>>,
> >     "hotspot-runtime-dev at openjdk.java.net
> >     <mailto:hotspot-runtime-dev at openjdk.java.net>"
> >     <hotspot-runtime-dev at openjdk.java.net
> >     <mailto:hotspot-runtime-dev at openjdk.java.net>>
> >     Date:   05/10/2016 19:31
> >     Subject:        Re: RFR(M): 8154736: enhancement of cmpxchg and
> >     copy_to_survivor for ppc64
> >
> >
> >
> >     On 10/05/2016 7:41 PM, Doerr, Martin wrote:
> >     > Hi David,
> >     >
> >     > thank you very much for testing the other platforms.
> >     >
> >     > Here's an updated webrev:
> >     > 
http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.01/
> >     <http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.01/
>
> >
> >     Thanks. Second test run on its way.
> >
> >     David
> >     -----
> >
> >     > Best regards,
> >     > Martin
> >     >
> >     > -----Original Message-----
> >     > From: hotspot-runtime-dev [
> >     mailto:hotspot-runtime-dev-bounces at openjdk.java.net
> >     <mailto:hotspot-runtime-dev-bounces at openjdk.java.net>] On Behalf 
Of
> >     David
> >     Holmes
> >     > Sent: Dienstag, 10. Mai 2016 11:11
> >     > To: Hiroshi H Horii <HORII at jp.ibm.com <mailto:HORII at jp.ibm.com>>
> >     > Cc: Tim Ellison <Tim_Ellison at uk.ibm.com
> >     <mailto:Tim_Ellison at uk.ibm.com>>;
> >     ppc-aix-port-dev at openjdk.java.net
> >     <mailto:ppc-aix-port-dev at openjdk.java.net>;
> >     hotspot-gc-dev at openjdk.java.net
> >     <mailto:hotspot-gc-dev at openjdk.java.net>;
> >     hotspot-runtime-dev at openjdk.java.net
> >     <mailto:hotspot-runtime-dev at openjdk.java.net>
> >     > Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and
> >     copy_to_survivor for ppc64
> >     >
> >     > The fix seems incomplete for solaris:
> >     >
> >     > make/Main.gmk:232: recipe for target 'hotspot' failed
> >     >
> >     "/opt/jprt/T/P1/073516.daholme/s/hotspot/src/os_cpu/
> solaris_x86/vm/atomic_solaris_x86.inline.hpp",
> >     > line 124: Error: Too many arguments in call to
> >     > "_Atomic_cmpxchg_long(long, volatile long*, long)".
> >     >
> >     "/opt/jprt/T/P1/073516.daholme/s/hotspot/src/os_cpu/
> solaris_x86/vm/atomic_solaris_x86.inline.hpp",
> >     > line 128: Error: Too many arguments in call to
> >     > "_Atomic_cmpxchg_long(long, volatile long*, long)".
> >     >
> >     > David
> >     >
> >     > On 10/05/2016 5:34 PM, David Holmes wrote:
> >     >> Hi Hiroshi,
> >     >>
> >     >> On 6/05/2016 8:11 PM, Hiroshi H Horii wrote:
> >     >>> Hi David,
> >     >>>
> >     >>> Thank you for your comments.
> >     >>>
> >     >>> As Martin suggested me, I would like to separate this proposal 
to
> >     >>>   - relaxing memory order of cmpxchg
> >     >>>   - improvement of copy_to_survivior with relaxed cmpxchg
> >     >>> and discuss the former first.
> >     >>>
> >     >>> Martin thankfully created a new webrev that include a change 
of
> >     cmpxchg.
> >     >>>
> >     http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.00/
> >     <http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.00/
>
> >     >>> He has already tested it with AIX, linuxx86_64, linuxppc64le 
and
> >     >>> darwinintel64.
> >     >>> (Please tell me if I need to send a new mail for this PFR)
> >     >>
> >     >> Please do as it will be simpler to track that way.
> >     >>
> >     >>>> What I would prefer to see is an additional memory_order 
value
> >     (such
> >     as
> >     >>>> memory_order_ignored) which is the default for all methods 
declared
> >     to
> >     >>>> take a memory_order parameter.
> >     >>>
> >     >>> We added simple enum to specify memory order in atomic.hpp as
> >     follows.
> >     >>>
> >     >>> typedef enum cmpxchg_cmpxchg_memory_order {
> >     >>>   memory_order_relaxed,
> >     >>>   memory_order_conservative
> >     >>> } cmpxchg_memory_order;
> >     >>>
> >     >>> All of cmpxchg functions have an argument of 
cmpxchg_memory_order
> >     >>> with a default value memory_order_conservative that uses the 
same
> >     >>> semantics with the existing cmpxchg and requires no change for 
the
> >     >>> existing
> >     >>> callers. If you think "memory_order_ignored" is better than
> >     >>> "memory_order_conservative", I will be happy to modify this 
change.
> >     >>> (I just thought, "ignored" may resemble "relaxed" and may make
> >     >>> people who are familiar with C++11's memory semantics 
confused.
> >     >>> I would like to know thoughts of native speakers.)
> >     >>
> >     >> That is fine by me. I don't think "ignored" would be confused 
with
> >     >> "relaxed", but "conservative" is fine.
> >     >>
> >     >> I will run the patch through our internal build system while 
you
> >     prepare
> >     >> the updated RFR. My only concern is "unused argument" warnings
> >     from the
> >     >> compiler. :)
> >     >>
> >     >> We are quickly running into a hard deadline with Feature 
Complete
> >     >> however - possibly less than 24 hours - for hotspot changes. If 
this
> >     >> doesn't get in in time I will see if I can shepherd it through 
the
> >     >> approval process.
> >     >>
> >     >> Thanks,
> >     >> David
> >     >>
> >     >>
> >     >>> Regards,
> >     >>> Hiroshi
> >     >>> -----------------------
> >     >>> Hiroshi Horii, Ph.D.
> >     >>> IBM Research - Tokyo
> >     >>>
> >     >>>
> >     >>> David Holmes <david.holmes at oracle.com
> >     <mailto:david.holmes at oracle.com>> wrote on 05/04/2016 14:55:29:
> >     >>>
> >     >>>> From: David Holmes <david.holmes at oracle.com
> >     <mailto:david.holmes at oracle.com>>
> >     >>>> To: Hiroshi H Horii/Japan/IBM at IBMJP
> >     >>>> Cc: hotspot-gc-dev at openjdk.java.net
> >     <mailto:hotspot-gc-dev at openjdk.java.net>, hotspot-runtime-
> >     >>>> dev at openjdk.java.net <mailto:dev at openjdk.java.net>,
> >     ppc-aix-port-dev at openjdk.java.net
> >     <mailto:ppc-aix-port-dev at openjdk.java.net>, Tim Ellison
> >     >>>> <Tim_Ellison at uk.ibm.com <mailto:Tim_Ellison at uk.ibm.com>>,
> >     Volker Simonis <volker.simonis at gmail.com
> >     <mailto:volker.simonis at gmail.com>>,
> >     >>>> "Doerr, Martin" <martin.doerr at sap.com
> >     <mailto:martin.doerr at sap.com>>, "Lindenmaier, Goetz"
> >     >>>> <goetz.lindenmaier at sap.com <mailto:goetz.lindenmaier at sap.com
>>
> >     >>>> Date: 05/04/2016 14:57
> >     >>>> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and
> >     >>>> copy_to_survivor for ppc64
> >     >>>>
> >     >>>> Hi Hiroshi,
> >     >>>>
> >     >>>> Sorry for the delay on getting back to this.
> >     >>>>
> >     >>>> On 25/04/2016 5:09 PM, Hiroshi H Horii wrote:
> >     >>>>> Hi David,
> >     >>>>>
> >     >>>>> Thank you for your comments and questions.
> >     >>>>>
> >     >>>>>> 1. Are the current cmpxchg semantics exactly the same as
> >     >>>>>> memory_order_seq_cst?
> >     >>>>>
> >     >>>>> This is very good question..
> >     >>>>>
> >     >>>>> I guess, cmpxchg needs a more conservative constraint for 
memory
> >     >>> ordering
> >     >>>>> than C++11, to add sync after a compare-and-exchange 
operation.
> >     >>>>>
> >     >>>>> Could someone give comments or thoughts?
> >     >>>>
> >     >>>> I don't want to comment on the comparison with C++11. What I 
would
> >     >>>> prefer to see is an additional memory_order value (such as
> >     >>>> memory_order_ignored) which is the default for all methods 
declared
> >     to
> >     >>>> take a memory_order parameter. That way existing
> >     implementations are
> >     >>>> clearly ignoring the memory_order attribute and there is no
> >     potential
> >     >>>> for confusion as to whether the existing implementations 
equate to
> >     >>>> memory_order_seq_cst or not.
> >     >>>>
> >     >>>> That said, I'm not sure it makes sense to add the 
memory_order
> >     parameter
> >     >>>> to all methods with "cas" in their name, e.g.
> >     oopDesc::cas_set_mark,
> >     >>>> oopDesc::cas_forward_to, unless those methods can sensibly be
> >     called
> >     >>>> with any value for memory_order - which seems highly 
unlikely.
> >     Perhaps
> >     >>>> those methods should identify the weakest form of 
memory_order they
> >     >>>> support and that should be hard-wired into them?
> >     >>>>
> >     >>>> Thanks,
> >     >>>> David
> >     >>>>
> >     >>>>> memory_order_seq_cst is defined as
> >     >>>>>     "Any operation with this memory order is both an acquire
> >     >>> operation and
> >     >>>>>      a release operation, plus a single total order exists 
in
> >     which
> >     >>>> all
> >     >>>>> threads
> >     >>>>>      observe all modifications (see below) in the same 
order."
> >     >>>>> (http://en.cppreference.com/w/cpp/atomic/memory_order
> >     <http://en.cppreference.com/w/cpp/atomic/memory_order>)
> >     >>>>>
> >     >>>>> In my environment, g++ and xlc generate following assemblies 
on
> >     >>>> ppc64le.
> >     >>>>> (interestingly, they generates the same assemblies for any
> >     >>>> memory_order)
> >     >>>>>
> >     >>>>> g++ (4.9.2)
> >     >>>>>     100008a4:   ac 04 00 7c     sync
> >     >>>>>     100008a8:   28 50 20 7d     lwarx   r9,0,r10
> >     >>>>>     100008ac:   00 18 09 7c     cmpw    r9,r3
> >     >>>>>     100008b0:   0c 00 c2 40     bne-    100008bc
> >     >>>>>     100008b4:   2d 51 80 7c     stwcx.  r4,0,r10
> >     >>>>>     100008b8:   f0 ff c2 40     bne-    100008a8
> >     >>>>>     100008bc:   2c 01 00 4c     isync
> >     >>>>>
> >     >>>>> xlc (13.1.3)
> >     >>>>>     10000888:   ac 04 00 7c     sync
> >     >>>>>     1000088c:   28 28 c0 7c     lwarx   r6,0,r5
> >     >>>>>     10000890:   40 00 26 7c     cmpld   r6,r0
> >     >>>>>     10000894:   0c 00 82 40     bne     100008a0
> >     >>>>>     10000898:   2d 29 80 7c     stwcx.  r4,0,r5
> >     >>>>>     1000089c:   f0 ff e2 40     bne+    1000088c
> >     >>>>>     100008a0:   2c 01 00 4c     isync
> >     >>>>>
> >     >>>>> On the other hand, the current OpenJDK generates following
> >     assemblies.
> >     >>>>>
> >     >>>>>     508:   ac 04 00 7c     sync
> >     >>>>>     50c:   00 00 5c e9     ld      r10,0(r28)
> >     >>>>>     510:   00 50 3b 7c     cmpd    r27,r10
> >     >>>>>     514:   1c 00 c2 40     bne-    530
> >     >>>>>     518:   a8 40 5c 7d     ldarx   r10,r28,r8
> >     >>>>>     51c:   00 50 3b 7c     cmpd    r27,r10
> >     >>>>>     520:   10 00 c2 40     bne-    530
> >     >>>>>     524:   ad 41 3c 7d     stdcx.  r9,r28,r8
> >     >>>>>     528:   f0 ff c2 40     bne-    518
> >     >>>>>     52c:   ac 04 00 7c     sync
> >     >>>>>     530:   00 50 bb 7f     ...
> >     >>>>>
> >     >>>>> Though we can ignore 50c-514 (because they are a duplicated 
guard
> >     >>>>> condition),
> >     >>>>> the last sync instruction (52c) makes cmpxchg more strict 
than
> >     >>>>> memory_order_seq_cst.
> >     >>>>>
> >     >>>>> In some cases, the last sync is necessary when this thread 
must be
> >     >>>> able
> >     >>>>> to read
> >     >>>>> all of the changes in the other threads while executing from
> >     508 to
> >     >>>> 530
> >     >>>>> (that processes compare-and-exchange).
> >     >>>>>
> >     >>>>>> 2. Has there been a discussion already, establishing that 
the
> >     >>>> modified
> >     >>>>>> GC code can indeed use memory_order_relaxed? Otherwise who 
is
> >     >>>>>> postulating that and based on what evidence?
> >     >>>>>
> >     >>>>> Volker and his colleagues have investigated the current GC 
codes
> >     >>>>> according to this.
> >     >>>>>
> >     http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-
> >     <http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016->
> >     >>>> April/019079.html
> >     >>>>> However, I believe, we need comments of other GC expertsto 
change
> >     >>>>> the shared codes.
> >     >>>>>
> >     >>>>> Regards,
> >     >>>>> Hiroshi
> >     >>>>> -----------------------
> >     >>>>> Hiroshi Horii, Ph.D.
> >     >>>>> IBM Research - Tokyo
> >     >>>>>
> >     >>>>>
> >     >>>>> David Holmes <david.holmes at oracle.com
> >     <mailto:david.holmes at oracle.com>> wrote on 04/22/2016 21:57:07:
> >     >>>>>
> >     >>>>>> From: David Holmes <david.holmes at oracle.com
> >     <mailto:david.holmes at oracle.com>>
> >     >>>>>> To: Hiroshi H Horii/Japan/IBM at IBMJP, hotspot-runtime-
> >     >>>>>> dev at openjdk.java.net <mailto:dev at openjdk.java.net>,
> >     hotspot-gc-dev at openjdk.java.net <
mailto:hotspot-gc-dev at openjdk.java.net>
> >     >>>>>> Cc: Tim Ellison <Tim_Ellison at uk.ibm.com
> >     <mailto:Tim_Ellison at uk.ibm.com>>,
> >     >>>>> ppc-aix-port-dev at openjdk.java.net
> >     <mailto:ppc-aix-port-dev at openjdk.java.net>
> >     >>>>>> Date: 04/22/2016 21:58
> >     >>>>>> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and
> >     >>>>>> copy_to_survivor for ppc64
> >     >>>>>>
> >     >>>>>> Hi Hiroshi,
> >     >>>>>>
> >     >>>>>> Two initial questions:
> >     >>>>>>
> >     >>>>>> 1. Are the current cmpxchg semantics exactly the same as
> >     >>>>>> memory_order_seq_cst?
> >     >>>>>>
> >     >>>>>> 2. Has there been a discussion already, establishing that 
the
> >     >>>> modified
> >     >>>>>> GC code can indeed use memory_order_relaxed? Otherwise who 
is
> >     >>>>>> postulating that and based on what evidence?
> >     >>>>>>
> >     >>>>>> Missing memory barriers have caused very difficult to track 
down
> >     >>> bugs in
> >     >>>>>> the past - very rare race conditions. So any relaxation 
here has
> >     >>>> to be
> >     >>>>>> done with extreme confidence.
> >     >>>>>>
> >     >>>>>> Thanks,
> >     >>>>>> David
> >     >>>>>>
> >     >>>>>> On 22/04/2016 10:28 PM, Hiroshi H Horii wrote:
> >     >>>>>>> Dear all:
> >     >>>>>>>
> >     >>>>>>> Can I please request reviews for the following change?
> >     >>>>>>>
> >     >>>>>>> Code change:
> >     >>>>>>>
> >     >>>
> >     
http://cr.openjdk.java.net/~mdoerr/8154736_copy_to_survivor/webrev.00/
> >     <
http://cr.openjdk.java.net/~mdoerr/8154736_copy_to_survivor/webrev.00/>
> >     >>>>>>> (I initially created and Martin enhanced so much)
> >     >>>>>>>
> >     >>>>>>> This change follows the discussion started from this mail.
> >     >>>>>>>
> >     http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-
> >     <http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016->
> >     >>>>>> April/018960.html
> >     >>>>>>>
> >     >>>>>>> Description:
> >     >>>>>>> This change provides relaxed compare-and-exchange by 
introducing
> >     >>>>>>> similar semantics of C++ atomic memory operators, enum
> >     >>>> memory_order.
> >     >>>>>>> As described in atomic_linux_ppc.inline.hpp, the current
> >     >>>>> implementation of
> >     >>>>>>> cmpxchg is fence_cmpxchg_acquire. This implementation is 
useful
> >     for
> >     >>>>>>> general purposes because twice calls of sync before and 
after
> >     >>>>> cmpxchg will
> >     >>>>>>> provide strict consistency. However, they sometimes cause
> >     overheads
> >     >>>>>>> because
> >     >>>>>>> sync instructions are very expensive in the current POWER 
chip
> >     >>> design.
> >     >>>>>>> In addition, for the other platforms, such as aarch64, 
this
> >     strict
> >     >>>>>>> semantics
> >     >>>>>>> may cause some overheads (according to the Andrew's mail).
> >     >>>>>>>
> >     http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-
> >     <http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016->
> >     >>>>>> April/019073.html
> >     >>>>>>>
> >     >>>>>>> With this change, callers can explicitly specify 
constraints of
> >     >>> memory
> >     >>>>>>> ordering
> >     >>>>>>> for cmpxchg with an additional parameter, memory_order 
order.
> >     >>>>>>>
> >     >>>>>>> typedef enum memory_order {
> >     >>>>>>>    memory_order_relaxed,
> >     >>>>>>>    memory_order_consume,
> >     >>>>>>>    memory_order_acquire,
> >     >>>>>>>    memory_order_release,
> >     >>>>>>>    memory_order_acq_rel,
> >     >>>>>>>    memory_order_seq_cst
> >     >>>>>>> } memory_order;
> >     >>>>>>>
> >     >>>>>>> Because the default value of the parameter is
> >     memory_order_seq_cst,
> >     >>>>>>> existing codes can use the same semantics of cmpxchg 
without any
> >     >>>>>>> modification. The relaxed cmpxchg is implemented only on 
ppc
> >     >>>>>>> in this changeset. Therefore, the behavior on the other
> >     platforms
> >     >>> will
> >     >>>>>>> not be changed with this changeset.
> >     >>>>>>>
> >     >>>>>>> In addition, with the new parameter of cmpxchg, this 
change
> >     >>>> improves
> >     >>>>>>> performance of copy_to_survivor in the parallel GC.
> >     >>>>>>> copy_to_survivor changes forward pointers by using 
cmpxchg. This
> >     >>>>>>> operation doesn't require any sync instructions.  A 
pointer is
> >     >>> changed
> >     >>>>>>> at most once in a GC and when cmpxchg fails, the latest
> >     pointer is
> >     >>>>>>> available for the caller. cas_set_mark and cas_forward_to 
are
> >     >>> extended
> >     >>>>>>> with an additional memory_order parameter as cmpxchg and
> >     >>>>> copy_to_survivor
> >     >>>>>>> uses memory_order_relaxed to modify the forward pointers.
> >     >>>>>>>
> >     >>>>>>> Summary of source code changes:
> >     >>>>>>>
> >     >>>>>>> * src/share/vm/runtime/atomic.hpp
> >     >>>>>>>       - Defines enum memory_order and adds a parameter to
> >     cmpxchg.
> >     >>>>>>>
> >     >>>>>>> * src/share/vm/runtime/atomic.cpp
> >     >>>>>>> * src/os_cpu/bsd_x86/vm/atomic_bsd_x86.inline.hpp
> >     >>>>>>> * src/os_cpu/bsd_zero/vm/atomic_bsd_zero.inline.hpp
> >     >>>>>>> * 
src/os_cpu/linux_aarch64/vm/atomic_linux_aarch64.inline.hpp
> >     >>>>>>> * src/os_cpu/linux_sparc/vm/atomic_linux_sparc.inline.hpp
> >     >>>>>>> * src/os_cpu/linux_x86/vm/atomic_linux_x86.inline.hpp
> >     >>>>>>> * src/os_cpu/linux_zero/vm/atomic_linux_zero.inline.hpp
> >     >>>>>>> * 
src/os_cpu/solaris_sparc/vm/atomic_solaris_sparc.inline.hpp
> >     >>>>>>> * src/os_cpu/solaris_x86/vm/atomic_solaris_x86.inline.hpp
> >     >>>>>>> * src/os_cpu/windows_x86/vm/atomic_windows_x86.inline.hpp
> >     >>>>>>>       - Added a parameter for each cmpxchg function to 
follow
> >     >>>>>>>          the change of atomic.hpp. Their implementations 
are not
> >     >>>>> changed.
> >     >>>>>>>
> >     >>>>>>> * src/os_cpu/aix_ppc/vm/atomic_aix_ppc.inline.hpp
> >     >>>>>>> * src/os_cpu/linux_ppc/vm/atomic_linux_ppc.inline.hpp
> >     >>>>>>>       - Added a parameter for each cmpxchg function to 
follow
> >     >>>>>>>          the change of atomic.hpp. In addition, 
implementations
> >     >>>>>>>          are changed corresponding to the specified
> >     memory_order.
> >     >>>>>>>
> >     >>>>>>> * src/share/vm/oops/oop.hpp
> >     >>>>>>> * src/share/vm/oops/oop.inline.hpp
> >     >>>>>>>       - Add a memory_order parameter to use relaxed 
cmpxchg in
> >     >>>>>>>          cas_set_mark and cas_forward_to.
> >     >>>>>>>
> >     >>>>>>> * src/share/vm/gc/parallel/psPromotionManager.cpp
> >     >>>>>>> * src/share/vm/gc/parallel/psPromotionManager.inline.hpp
> >     >>>>>>>
> >     >>>>>>> Martin tested this changeset  on linuxx86_64, linuxppc64le 
and
> >     >>>>>>> darwinintel64.
> >     >>>>>>> Though more time is needed to test on the other platform, 
we
> >     would
> >     >>>>> like to
> >     >>>>>>> ask
> >     >>>>>>> reviews and start discussion on this changeset.
> >     >>>>>>> I also tested this changeset with SPECjbb2013 and 
confirmed that
> >     gc
> >     >>>>> pause
> >     >>>>>>> time
> >     >>>>>>> is reduced.
> >     >>>>>>>
> >     >>>>>>> Regards,
> >     >>>>>>> Hiroshi
> >     >>>>>>> -----------------------
> >     >>>>>>> Hiroshi Horii, Ph.D.
> >     >>>>>>> IBM Research - Tokyo
> >     >>>>>>>
> >     >>>>>>>
> >     >>>>>>
> >     >>>>>
> >     >>>>
> >     >>>
> >
> >
> >
> >
> >
> >
> 


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