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On 2015-08-14 13:27, Stefan Johansson wrote:<br>
<blockquote cite="mid:55CDD087.3050301@oracle.com" type="cite">
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Hi Stefan,<br>
<br>
<div class="moz-cite-prefix">On 2015-07-02 17:03, Jon Masamitsu
wrote:<br>
</div>
<blockquote cite="mid:559552A9.8030008@oracle.com" type="cite">
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<br>
<br>
<div class="moz-cite-prefix">On 07/02/2015 04:48 AM, Stefan
Karlsson wrote:<br>
</div>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
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<br>
<br>
<div class="moz-cite-prefix">On 2015-07-01 18:31, Jon
Masamitsu wrote:<br>
</div>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
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<br>
<br>
<div class="moz-cite-prefix">On 6/12/2015 7:52 AM, Stefan
Karlsson wrote:<br>
</div>
<blockquote cite="mid:557AF21B.2090102@oracle.com"
type="cite">
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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>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
The changes for 8087323 looks good. I agree with Jons comments
below about removing the function is_Yielding... and the cast in
YieldingFlexibleGangTask.<br>
</blockquote>
<br>
Removed.<br>
<br>
Thanks,<br>
StefanK<br>
<br>
<blockquote cite="mid:55CDD087.3050301@oracle.com" type="cite"> <br>
Thanks,<br>
Stefan<br>
<br>
<blockquote cite="mid:559552A9.8030008@oracle.com" type="cite">
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<blockquote cite="mid:557AF21B.2090102@oracle.com"
type="cite"> </blockquote>
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.hpp.frames.html">http://cr.openjdk.java.net/~stefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.hpp.frames.html</a><br>
<br>
There seems to be only one definition of
is_YieldingFlexibleGang_task() now. Is that right? Is that
useful?<br>
<br>
<pre> 131 NOT_PRODUCT(virtual bool is_YieldingFlexibleGang_task() const {
132 return true;
133 })</pre>
</blockquote>
<br>
I agree. I don't think we need it anymore.<br>
<br>
</blockquote>
Thanks.<br>
<br>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<br>
Not a change in your patch but<br>
<br>
<pre><span class="changed"> 86 AbstractWorkGang(const char* name, uint workers, bool are_GC_task_threads, bool are_ConcurrentGC_threads) :</span>
<span class="changed"> 87 _name(name),</span>
<span class="changed"> 88 _total_workers(workers),</span>
<span class="changed"> 89 _active_workers(UseDynamicNumberOfGCThreads ? 1U : workers),</span>
<span class="changed"> 90 _are_GC_task_threads(are_GC_task_threads),</span>
<span class="changed"> 91 _are_Concurren</span></pre>
<br>
_active_workers is always calculated as >= 2 unless
_total_workers is only 1.<br>
So line 89 should be<br>
<br>
<span class="changed">_active_workers(UseDynamicNumberOfGCThreads
? </span>MIN2(2, workers) : workers)<br>
<br>
Should I file a CR for that? Or do you want to include it.<br>
</blockquote>
<br>
I'm not sure that what is proposed above is correct. I see
that AdaptiveSizePolicy::calc_active_workers returns 2 as a
minimum, but both
ConcurrentMark::calc_parallel_marking_threads and
AdaptiveSizePolicy::calc_active_conc_workers can return 1.<br>
<br>
I also don't think it should be AbstractWorkGang's
responsibility to have the knowledge about the minimum number
of worker threads that are used when
UseDynamicNumberOfGCThreads are turned on. Maybe we should set
it to 0, and let the calc_*_active_workers setup the default
value.<br>
</blockquote>
<br>
I think that at one time I had tried to set the default to 0 and
something failed. I can see the point though.<br>
<br>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<br>
I would prefer to handle any changes, to this part of the
code, as separate RFEs.<br>
</blockquote>
<br>
Fair enough. If I think it's worth doing, I'll file and RFE.
Probably something more than just setting<br>
it to 2 (maybe picking a default value with the help of
AdaptiveSizePolicy).<br>
<br>
<br>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
Have you considered (maybe for a later patch) changing
YieldingFlexibleWorkGang to<br>
simply YieldingWorkGang? The "Flexible" attribute of
YieldingFlexibleWorkGang having<br>
been moved into AbstractWorkGang.<br>
</blockquote>
<br>
I thought about it, but didn't think it was important enough
to warrant that change in this patch. I wouldn't mind if a RFE
was created to change the name.<br>
</blockquote>
<br>
I'll file the RFE if you agree with the point that "Flexible"
describes what AbstractWorkGang does now.<br>
<br>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://cr.openjdk.java.net/%7Estefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.cpp.frames.html">http://cr.openjdk.java.net/~stefank/8087323/webrev.00/src/share/vm/gc/cms/yieldingWorkgroup.cpp.frames.html</a><br>
<br>
Is the cast at 53 necessary? I see it in the original code
too.<br>
<br>
<pre><span class="changed"> 50 AbstractGangWorker* YieldingFlexibleWorkGang::allocate_worker(uint which) {</span>
51 YieldingFlexibleGangWorker* new_member =
52 new YieldingFlexibleGangWorker(this, which);
53 return (YieldingFlexibleGangWorker*) new_member;
54 }</pre>
</blockquote>
<br>
Yes, this is unnecessary.<br>
</blockquote>
<br>
Thanks.<br>
<br>
Jon<br>
<br>
<blockquote cite="mid:55952520.6030609@oracle.com" type="cite">
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<br>
The rest looks good.<br>
</blockquote>
<br>
Thanks.<br>
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<br>
I'll do the second patch next.<br>
</blockquote>
<br>
Great.<br>
<br>
StefanK<br>
<br>
<blockquote cite="mid:559415DD.7030604@oracle.com" type="cite">
<br>
Jon<br>
<br>
<blockquote cite="mid:557AF21B.2090102@oracle.com"
type="cite"> <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>
<br>
</blockquote>
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