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<div class="moz-cite-prefix">On 2015-07-02 19:43, Jon Masamitsu
wrote:<br>
</div>
<blockquote cite="mid:5595784A.9050903@oracle.com" type="cite">
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<br>
<br>
<div class="moz-cite-prefix">On 6/29/2015 2:38 AM, Stefan Karlsson
wrote:<br>
</div>
<blockquote cite="mid:5591122F.2000704@oracle.com" type="cite">
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Hi all,<br>
<br>
"8087322: Implement a Semaphore utility class" has now been
pushed, so I've updated the patch to reflect the changes.<br>
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/webrev.01.delta">http://cr.openjdk.java.net/~stefank/8087324/webrev.01.delta</a><br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/webrev.01">http://cr.openjdk.java.net/~stefank/8087324/webrev.01</a><br>
</blockquote>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.hpp.frames.html">http://cr.openjdk.java.net/~stefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.hpp.frames.html</a><br>
<br>
Are these used?<br>
<br>
<pre><span class="changed"> 194 void print_worker_started_task(AbstractGangTask* task, uint worker_id);</span>
<span class="changed"> 195 void print_worker_finished_task(AbstractGangTask* task, uint worker_id);
</span></pre>
</blockquote>
<br>
No. I'll remove them.<br>
<br>
<blockquote cite="mid:5595784A.9050903@oracle.com" type="cite">
<pre><span class="changed">
Rest looks good. Just one question (just for my education).
</span></pre>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.cpp.frames.html">http://cr.openjdk.java.net/~stefank/8087324/webrev.01/src/share/vm/gc/shared/workgroup.cpp.frames.html</a><br>
<br>
<pre><span class="changed"> 336 void GangWorker::loop() {</span>
<span class="changed"> 337 while (true) {</span>
<span class="changed"> 338 WorkData data = wait_for_task();</span>
<span class="changed"> 339 </span>
<span class="changed"> 340 run_task(data);</span>
<span class="changed"> 341 </span>
<span class="changed"> 342 signal_task_done();</span>
343 }
344 }
Does this allow the same thread to execute more than 1
task ("data" here)? Meaning if 2 threads are requested but
1 thread is not scheduled to a cpu, will the other thread
do both chunks of work?</pre>
</blockquote>
<br>
Yes, that's correct.<br>
<br>
Thanks for reviewing!<br>
StefanK<br>
<br>
<blockquote cite="mid:5595784A.9050903@oracle.com" type="cite">
<pre>
Jon
</pre>
<blockquote cite="mid:5591122F.2000704@oracle.com" type="cite"> <br>
- The IMPLEMENTS_SEMAPHORE_CLASS define was removed, since all
platforms need to provide a Semaphore implementation.<br>
<br>
- Removed the need to pass down "max number of workers" to the
Semaphore constructor.<br>
<br>
- Updated semaphore.hpp include path<br>
<br>
Thanks,<br>
StefanK<br>
<br>
<br>
<div class="moz-cite-prefix">On 2015-06-12 16:52, Stefan
Karlsson wrote:<br>
</div>
<blockquote cite="mid:557AF21B.2090102@oracle.com" type="cite">
<meta http-equiv="content-type" content="text/html;
charset=UTF-8">
Hi all,<br>
<br>
The current implementation to distribute tasks to GC worker
threads often cause long latencies (multiple milliseconds)
when the threads are started and stopped. <br>
<br>
The main reason is that the worker threads have to fight over
the Monitor lock when they are woken up from the call to
Monitor::wait. Another reason is that all worker threads call
notify_all when they finish a task and there wakes all all
sleeping worker threads, which will yet again force the worker
threads to fight over the lock. <br>
<br>
I propose that we use semaphores instead, so that the worker
threads don't have to fight over a lock when they are woken
up.<br>
<br>
<br>
The patches build upon the following patch which introduces a
Semaphore utility class. This patch will sent out for review
on the hotspot-dev, since it affects non-GC parts of the code:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087322/webrev.00/">http://cr.openjdk.java.net/~stefank/8087322/webrev.00/</a><br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://bugs.openjdk.java.net/browse/JDK-8087322">https://bugs.openjdk.java.net/browse/JDK-8087322</a><br>
<br>
<br>
The first patch that I would like to get reviewed is:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087323/webrev.00/">http://cr.openjdk.java.net/~stefank/8087323/webrev.00/</a><br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://bugs.openjdk.java.net/browse/JDK-8087323">https://bugs.openjdk.java.net/browse/JDK-8087323</a>
- Unify and split the work gang classes <br>
<br>
It prepares for JDK-8087324, by separating the generic
WorkGang implementation from the more elaborate
YieldingFlexibleWorkGang (CMS) implementation. By having this
part as a separate patch, I hope it will be easier to review
JDK-8087324. The patch changes the work gang inheritance from:<br>
<br>
AbstractWorkGang<br>
WorkGang<br>
FlexibleWorkGang<br>
YieldingFlexibleWorkGang<br>
<br>
to:<br>
<br>
AbstractWorkGang<br>
WorkGang<br>
YieldingFlexibleWorkGang<br>
<br>
Parts of the FlexibleWorkGang and WorkGang code that is going
to be used by both concrete work gang classes, has been moved
into AbstractWorkGang. I've duplicated some code in WorkGang
and YieldingFlexibleWorkGang, but that code will be removed
from WorkGang in the following patch.<br>
<br>
<br>
The second patch I'd like to get reviewed is:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/webrev.00/">http://cr.openjdk.java.net/~stefank/8087324/webrev.00/</a><br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://bugs.openjdk.java.net/browse/JDK-8087324">https://bugs.openjdk.java.net/browse/JDK-8087324</a>
- Use semaphores when starting and stopping GC task threads <br>
<br>
It first simplifies the way we distribute the tasks to the GC
worker threads. For example, the coordinator thread dispatches
a task to a specific number of workers, and then waits for all
work to be completed. There's no risk that multiple tasks will
be scheduled simultaneously, so there's no need for the
sequences number that is used in the current implementation.<br>
<br>
The patch contains two task dispatch / thread synchronization
implementations:<br>
<br>
The first implementation uses Monitors, similar to what we did
before the patch, but with a slightly lower overhead since the
code calls notify_all less often. It still suffers from the
"thundering heard" problem. When the coordinator thread
signals that the worker threads should start, they all wake up
from Monitor::wait and they all try to lock the Monitor.<br>
<br>
The second, and the more interesting, implementation uses
semaphores. When the worker threads wake up from the semaphore
wait, they don't have to serialize the execution by taking a
lock. This greatly decreases the time it takes to start and
stop the worker threads.<br>
<br>
The semaphore implementation is used on all platforms where
the Semaphore class has been implemented in JDK-8087322. So,
on some OS:es the code will revert to the Monitor-based
solution until a Semaphore class has been implemented for that
OS. So, porters might want to consider implementing the
Sempahore class.<br>
<br>
There's also a diagnostic vm option
(-XX:+/-UseSemaphoreGCThreadsSynchronization) to turn off the
Semaphore-based implementation, which can be used to debug
this new code. It's mainly targeted towards support and
sustaining engineering.<br>
<br>
<br>
The patches have been performance tested on Linux, Solaris,
OSX, and Windows.<br>
<br>
The effects of the patch can be seen by running benchmarks
with small young gen sizes, which triggers frequent and short
GCs.<br>
<br>
For example, here are runs from the SPECjvm2008 xml.transform
benchmark with:<br>
-Xmx1g -Xms1g -Xmn64m -XX:+PrintGC -XX:+UseG1GC -jar
SPECjvm2008.jar -ikv xml.transform -it 30 -wt 30<br>
<br>
I got the following GC times:<br>
<br>
<tt> Average Median 99.9 percentile Max</tt><tt><br>
</tt><tt>Baseline: </tt>
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ms 8.44 ms 25.9 ms </tt><tt>3</tt><tt>4.7 ms</tt><tt><br>
</tt><tt>Monitor:</tt><tt> 6.1</tt><tt>7 </tt><tt>ms </tt><tt>5.88
ms 26.0 ms </tt><tt>49.1 ms</tt><tt><br>
</tt><tt> </tt>
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</style><tt>Semaphore: </tt><tt>3.</tt><tt>43 ms </tt><tt>3.26 </tt><tt>ms
13.4 ms 33.4 ms</tt><tt><br>
</tt><br>
If I run an empty GC task 10 times per GC, by running the
following code:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/timedTask/">http://cr.openjdk.java.net/~stefank/8087324/timedTask/</a><br>
<br>
I get the following numbers to complete the empty GC tasks:<br>
<br>
<tt> Average Median 99.9 percentile Max</tt><tt><br>
</tt><tt>Baseline: </tt><tt>1.43 ms 0.92 ms 3.43
ms 9.30</tt><tt> ms</tt><tt><br>
Monitor</tt><tt>:</tt><tt> 0.75</tt><tt> </tt><tt>ms
</tt><tt>0.72 ms 1.74 ms 2.78</tt><tt> ms</tt><tt><br>
</tt><tt> </tt><tt>Semaphore: </tt><tt>0.</tt><tt>07 ms
</tt><tt>0.07 </tt><tt>ms 0.17 ms 0.26 ms</tt><tt><br>
</tt><br>
<br>
<br>
The code has been tested with JPRT and our nightly testing
suites. <br>
<br>
I've created a unit test to run a small test with both the
semaphore implementation and the monitor implementation:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087324/workgangTest/">http://cr.openjdk.java.net/~stefank/8087324/workgangTest/</a><br>
<br>
But since we currently don't have code to shutdown worker
threads after they have been started, I don't want to push
this test (or clean it up) until we have that in place. I
created this bug for that:<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="https://bugs.openjdk.java.net/browse/JDK-8087340">https://bugs.openjdk.java.net/browse/JDK-8087340</a><br>
<br>
Thanks,<br>
StefanK<br>
<br>
<br>
<br>
</blockquote>
<br>
</blockquote>
<br>
</blockquote>
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