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Hi Martin,<br>
<br>
An enqueued card let's the refinment threads know that the oops
spanned by that card need to be walked but we're only interested in
the latest contents of the fields in those oops. IOW the oop in (3')
doesn't need to be the oop stored in (1). If there's a subsequent
store (3) to the same location then we want the load at (3') to see
the lastest contents. For example suppose we have:<br>
<br>
x.f = a;<br>
x.f = b;<br>
<br>
If the application thread sees the card spanning x.f is dirty at the
second store then we won't enqueue the card after the second store.
As long as the refinement thread sees 'b' when the card is 'refined'
then we're OK since we no longer need to add an entry into the RSet
for the region containing a - we do need an entry in the RSet for
the region containing b.<br>
<br>
If the application thread sees the card as clean at the second store
before the refinement thread loads x.f we have just needlessly
enqueued the card again.<br>
<br>
It is only if the application thread sees the card as dirty but the
refinement thread reads 'a' then there could be a problem. We have a
missing RSet entry for 'b'.<br>
<br>
JohnC<br>
<br>
<div class="moz-cite-prefix">On 5/17/2013 1:29 AM, Doerr, Martin
wrote:<br>
</div>
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<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">Hi
all,<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">we
have a question about the interaction between G1 post
barriers and the refinement thread's concurrent dirty card
cleaning.<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">The
case in which the G1 post barrier sees a clean card is
obviously not problematic, because it will add an entry in a
dirty card queue.<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">However,
in case in which the Java thread (mutator thread) sees the
card already dirtied, it won’t enqueue the card again. Which
is safe as long as its stored oop (1) is seen and processed
(3’) by the parallel refinement after having cleaned the
card (1’):<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">Java
Thread (mutator) Refinement Thread
(G1RemSet::concurrentRefineOneCard_impl calls
oops_on_card_seq_iterate_careful)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">(1)
store(oop)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">(
StoreLoad required here ?)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">(2)
load(card==dirty)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">
(1’) store(card==clean)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">
(2’) StoreLoad barrier<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">
(3’) load(oop)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">So
the refinement thread seems to rely on getting the oop which
was written BEFORE the (2) load(card==dirty) was observed.<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">We
wonder how this ordering is guaranteed? There are no
StoreLoad barriers in the Java Thread's path. (StoreLoad
ordering needs explicit barriers even on TSO platforms.)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"" lang="EN-US">Kind
regards,<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span
style="font-family:"Courier New"">Martin<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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