Dear all, I update the webrev: http://cr.openjdk.java.net/~mhorie/8154736/webrev.09/ With the release barrier before the CAS, new_obj can be observed from other threads. If the CAS succeeds, the current thread can use new_obj without barriers. If the CAS fails, "o->forwardee()" is ordered with respect to CAS by accessing the same memory location "_mark", so no barriers needed. The order of (1) access to the forwardee and (2) access to forwardee's fields is preserved due to Release-Consume ordering on supported platforms. (The ordering between "new_obj = o->forwardee();" and logging or other usages is not changed.) Regarding the maintainability, the requirement is CAS(memory_order_release) as Release-Consume to be consistent with C++11. This requirement is necessary when a weaker platform like DEC Alpha is to be supported. On currently supported platforms, code change can be safe if the code meats this requirement (and the order of (1) access to the forwardee and (2) access to forwardee's fields is the natural way of coding). oop PSPromotionManager::copy_to_survivor_space(oop o) { oop new_obj = NULL; markOop test_mark = o->mark_raw(); if (!test_mark->is_marked()) { // The same test as "o->is_forwarded()" : Copy::aligned_disjoint_words((HeapWord*)o, (HeapWord*)new_obj, ...); if (o->cas_forward_to(new_obj, test_mark)) { // CAS succeeds : new_obj->push_contents(this); } else { // CAS fails : new_obj = o->forwardee(); } } else { : new_obj = o->forwardee(); } log_develop_trace(gc, scavenge)(..., new_obj->klass()->internal_name(), p2i((void *)o), p2i((void *)new_obj), new_obj->size()); return new_obj; } Best regards, -- Michihiro, IBM Research - Tokyo ----- Original message ----- From: Kim Barrett <kim.barrett@oracle.com> To: David Holmes <david.holmes@oracle.com> Cc: Michihiro Horie <HORIE@jp.ibm.com>, ppc-aix-port-dev@openjdk.java.net, hotspot-dev@openjdk.java.net, hotspot-gc-dev@openjdk.java.net, Hiroshi H Horii <HORII@jp.ibm.com> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64 Date: Fri, Apr 27, 2018 6:03 AM
On Apr 25, 2018, at 8:45 AM, David Holmes <david.holmes@oracle.com> wrote:
Hi Michihiro,
On 23/04/2018 8:33 PM, Michihiro Horie wrote:
Dear all, I would like to ask reviews on 8154736 “enhancement of cmpxchg and copy_to_survivor”. The change adds options to avoid expensive syncs with compare-and-exchange. An experiment by using SPECjbb2015 showed 6% improvement in critical-jOPS. This change focuses on ppc64 but would be potentially beneficial for aarch64. Although discussions stopped at
http://mail.openjdk.java.net/pipermail/ppc-aix-port-dev/2016-October/002718....
, I would like to restart the review by taking over Hiroshi's work if the discussion is still open.
So the very last comment there was about not implicitly assuming memory_order_consume, yet that has not been addressed in the proposal.
Further the discussion on hotspot-runtime-dev through September and October was far more illuminating. I think my post here:
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/0216...
and the closely following one from Thomas Schatzl summed up the concerns
about the proposed changes.
AFAICS the restarted proposal addresses none of those concerns but
simply takes up where the previous implementation suggestion left off.
This is a proposal to change the memory ordering semantics of part of
the shared GC code _not_ just the PPC64 implementation, but I have seen no analysis to demonstrate the correctness of such a proposal. I agree with David here. So far we've seen no such analysis. All we have seen is a series of proposed changes and non-failing test results, all of which have then been shown to have holes. (Among other things, this suggests the set of tests being applied is inadequate.) Part of the author's job is to convince reviewers that the proposed change is correct. I'm not expecting a formal proof, but I am expecting a lot more than has been provided so far. In this latest proposal, the conditional acquire doesn't look right to me. If the logging is disabled so there is no acquire, the object is then returned to the callers, who might do what with it? Is that safe? For all callers? And is it stable through future maintenance? This is not to say that I think making those acquires unconditional is sufficient.