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<div class="moz-cite-prefix">On 11/22/13 6:48 PM, Jon Masamitsu
wrote:<br>
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<div class="moz-cite-prefix">On 11/22/2013 4:33 AM, Bengt Rutisson
wrote:<br>
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<div> Do we need the flag AdjustGCThreadsToCores? It
seems like this is a good change and it is always
nice to reduce the number of flags we have. If you
feel unsure about the change it may be good to be
able to disable it, but in general I think it
would be better to decide that this is the default
behavior and go with it.<br>
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<div>I have no strong opinion. If you guys can confirm
the improvements and feel like you don't need the
flag, that would be ideal.</div>
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Yes, this is an interesting dilemma. Personally I don't think I
have the cycles right now to do any performance testing of this
fix. But I assume that you don't have access to enough machines
and benchmarks to test this properly on all supported platforms.
I'll bring this up internally to see if there is any process for
handling this type of situation...<br>
<br>
Since your change is for Linux only and you have done the
measurements there, I'm fine with your results. I would prefer
skipping the AdjustGCThreadsToCores flag based on the data you
provided. Especially since we can still set ParallelGCThreads
explicitly on the command line if we need to work around a
regression somewhere.<br>
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I fully expect to need the AdjustGCThreadsToCores flag on Sparc so
I would prefer<br>
it be left in. <br>
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Can you explain why this will be particularly needed on Sparc? I
assume you mean Linux/Sparc since the Solaris implementation is not
changed by this patch. If we think this is not a good change for
Sparc, then maybe we should not do it there...<br>
<br>
<blockquote cite="mid:528F98EF.1090407@oracle.com" type="cite"> Make
it an experimental flag to make it easier to remove later<br>
<pre><span class="new">1405 <font color="#ff0000">experimental</font>(bool, AdjustGCThreadsToCores, true, \</span>
<span class="new">1406 "Create GC worker threads with respect to the number of physical "\</span>
<span class="new">1407 "cores available from the CPU mask. Ignores hardware threads") \</span>
<span class="new">1408 </span></pre>
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Yes, this is a bit better, but I still don't fully understand why we
want to keep the old behavior under a flag. More comments on that
below.<br>
<br>
<blockquote cite="mid:528F98EF.1090407@oracle.com" type="cite"> We
do have the ParallelGCThreads flag to workaround regressions but
users don't<br>
know what value is currently set by the ergonomics so won't know
what value<br>
to use for ParallelGCThreads to workaround a regression. <br>
</blockquote>
<br>
It is not too hard to come up with the "old" default value for
ParallelGCThreads. We know what the logic was so if just get
processor information from the machine we can figure it out and in
worst case we can ask a customer to run with an older version of the
JDK and do "PrintFlagsFinal | grep ParallelGCThreads".<br>
<br>
I'm not going to insist that we remove the AdjustGCThreadsToCores
flag. We can keep it if you feel it is important. Personally I just
think the cost of having it is higher than the benefit. We have
extra code to support, more complex logic to take into account when
debugging and we most likely will not test with this flag enabled so
bugs may creep in without us noticing.<br>
<br>
Bengt<br>
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<blockquote cite="mid:528F98EF.1090407@oracle.com" type="cite"> <br>
Jon<br>
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