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Hi Moh,<br>
<br>
<div class="moz-cite-prefix">On 06/01/2015 04:42 AM, Rezaei,
Mohammad A. wrote:<br>
</div>
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<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The
problems start with the ReferenceQueue object and the heavy
synchronization in it. Consider structures like
j.u.WeakHashMap that need to expunge entries. If such
structures are made somewhat concurrent, the lock in
ReferenceQueue starts to become a serious problem:<o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span
style="mso-list:Ignore">-<span style="font:7.0pt
"Times New Roman""> </span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In
structures that are concurrent at the entry level (like jdk
8’s ConcurrentHashMap), if methods like get() or put() try
to expunge, the lock in ReferenceQueue renders the
structures non-concurrent.</span></p>
</div>
</blockquote>
<br>
The presented prototype changes the implementation of
ReferenceQueue. It doesn't use a lock any more for enqueu-ing when
there are no waiters and never for poll-ing. ReferenceQueue.poll()
is a single volatile read when queue is empty and a read followed by
CAS when it is not (with retries when contended). If there is a
desire and new API could be extended, the method like the following:<br>
<br>
Iterator<Reference<? extends T>> pollChunk(int
maxChunkSize);<br>
<br>
...could return a chunk of enqueued references (or an empty
iterator) so that the reduced number of CAS instructions per
de-queued reference could be traded for the greater probability of
retries because of contention.<br>
<br>
<blockquote
cite="mid:6882C9A35DFB9B4FA2779F7BF5B9757D20761169E3@GSCMAMP06EX.firmwide.corp.gs.com"
type="cite">
<div class="WordSection1">
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span
style="mso-list:Ignore">-<span style="font:7.0pt
"Times New Roman""> </span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In
structures that are multi-reader-single-writer locked, read
methods cannot expunge (because they have to promote to a
writer), but they can’t even check the queue, because that
turns the multi-reader structure into a synchronized one.</span></p>
</div>
</blockquote>
<br>
By checking you mean poll() which also de-queues a reference if
available? What do you do when this happens. Would you need a peek()
method maybe?<br>
<br>
<blockquote
cite="mid:6882C9A35DFB9B4FA2779F7BF5B9757D20761169E3@GSCMAMP06EX.firmwide.corp.gs.com"
type="cite">
<div class="WordSection1">
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span
style="mso-list:Ignore">-<span style="font:7.0pt
"Times New Roman""> </span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">In
addition to expunge calls contending on the ReferenceQueue
lock, ReferenceHandler thread can also contend on the same
lock.</span></p>
</div>
</blockquote>
<br>
That's right. And in the presented prototype, this is minimized by
allowing ReferenceHandler thread to enqueue a chunk of pre-prepared
references in one go, minimizing the need to frequently notify any
possible waiters via a lock.notifyAll().<br>
<br>
<blockquote
cite="mid:6882C9A35DFB9B4FA2779F7BF5B9757D20761169E3@GSCMAMP06EX.firmwide.corp.gs.com"
type="cite">
<div class="WordSection1">
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><!--[if !supportLists]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><span
style="mso-list:Ignore">-<span style="font:7.0pt
"Times New Roman""> </span></span></span><!--[endif]--><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">There
is no fast clear() method on ReferenceQueue. That would be
quite useful on a resize event.</span></p>
</div>
</blockquote>
<br>
This would be easy to implement if new API could be added.<br>
<br>
<blockquote
cite="mid:6882C9A35DFB9B4FA2779F7BF5B9757D20761169E3@GSCMAMP06EX.firmwide.corp.gs.com"
type="cite">
<div class="WordSection1">
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo1"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The
above issues forced us to have a dedicated thread that does
periodic expunging of References. This works ok under light
load, but can fall behind under heavy load.</span></p>
</div>
</blockquote>
<br>
Because of the overhead/bottleneck of the reference processing I
assume. It would be great if you could check whether the prototype
in webrev.02 improves your situation. It should be simple. Just
compile the sources and prepend the resulting classes to the
bootclasspath of the JDK.<br>
<br>
Regards, Peter<br>
<br>
<blockquote
cite="mid:6882C9A35DFB9B4FA2779F7BF5B9757D20761169E3@GSCMAMP06EX.firmwide.corp.gs.com"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Moh<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0in
0in 0in 4.0pt">
<p class="MsoNormal" style="margin-left:5.25pt">@Moh<br>
<br>
Can you elaborate some more on what twists were necessary or
what problems you had?<br>
<br>
<br>
<span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span
style="color:#1F497D"><o:p> </o:p></span></p>
</div>
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</blockquote>
<br>
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