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Thanks! Simone. :) Aaron - Please look at the J1 presentation that
Simone has linked here (and feel free to ask questions/ provide
comments). And like Simone mentioned, please send in the GC logs for
the case where G1 does poorly with "-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps"
enabled on the command line. Looking at the GC log(s), we can
hopefully identify the cause of that major GC (which I am assuming
to be a Full GC). We should also be able to work on improving (or
at-least explaining) your response times, but we would like to know
a) what is your goal, b) what are the important factors (e.g. sizing
limitations, etc) and c) if you are working off some comparison
(e.g. a previously tuned garbage collector).<br>
<br>
Thank you for trying G1!<br>
<br>
-Monica<br>
<br>
<br>
On 10/16/2012 8:19 AM, Simone Bordet wrote:
<blockquote
cite="mid:CAFWmRJ17cbctmWbPPqMGXO_i+Yi8-xf0f4bc-_WMPnRY-_2T3Q@mail.gmail.com"
type="cite">
<pre wrap="">Hi,
On Tue, Oct 16, 2012 at 2:54 PM, Aaron Daubman <a class="moz-txt-link-rfc2396E" href="mailto:daubman@gmail.com"><daubman@gmail.com></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Greetings,
I recently switched to trying G1GC for several webapps and am trying
now to make sense of the GC behavior.
I have a few hopefully not-too-newbie questions:
1) Is there any up-to-date documentation on tuning G1GC? (many
articles about it seem to be pre-7u4 where there were apparently more
experimental flags available for tuning). Also, is this currently the
full set of flags available:
<a class="moz-txt-link-freetext" href="http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html#G1Options">http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html#G1Options</a>
</pre>
</blockquote>
<pre wrap="">
Perhaps most up to date information is from the JavaOne sessions just held.
See in particular:
<a class="moz-txt-link-freetext" href="https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=7236">https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=7236</a>
<a class="moz-txt-link-freetext" href="https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=6583">https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=6583</a>
</pre>
<blockquote type="cite">
<pre wrap="">4) In one webapp, the constant reason for frequent GCs is listed by
VisualVM as "G1: humongous allocation attempt". However, searching to
try and figure out what this means results in only a very few bug
entries (e.g. 7018286) that do not explain at all what this means, or
what the threshold for being declared 'humongous' is - were can I find
out more about this reason for minor GC?
</pre>
</blockquote>
<pre wrap="">
Humongous allocation means an allocation for a large object (a
"humongous object").
Normally these are large byte[], or objects that contain large byte[].
In G1, I believe that "humongous" is when the size is greater than
half a region, which by default are 1 MiB, but not 100% sure.
</pre>
<blockquote type="cite">
<pre wrap="">5) I think I am perhaps just missing something given the lack of
capability of the current suite of tools, however this one really
bothers/intrigues me.
One webapp where I switched to G1GC does quite poorly (somewhat
inexplicably) for 12 or so hours (in terms of response times). This
app has a med/large (16G) heap size, and typically runs at 1/2 to 1/4
heap utilization. On occasion, even though overall heap utilization is
below 40%, there will occur a (I've only ever seen single) major GC,
and, without explanation, the application runs with order-of-magnitude
better performance after this (heap utilization remains about 40%
after major GC... eden and old commit size go up a small amount,
nothing else changes that I can see). What would cause this markedly
better performance after a major GC, and how can I dig in to this
better?
(I am frustrated, since it seems I should be able to figure out what
JVM flags to use that would cause performance to be always as good as
it is after the mysterious full GC, but I haven't been able to figure
anything out so far).
</pre>
</blockquote>
<pre wrap="">
Please record logs with the flags suggested in the 2 presentations I
linked above.
In particular, I use:
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintAdaptiveSizePolicy
Once you have the log, attach it here, and hopefully someone will look at it.
Keep in mind that G1 is still "young" and while it will be the future,
today it may not be up to par with the other collectors.
Simon
</pre>
</blockquote>
<br>
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