<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Perhaps if you're really, really ... really squeeze on available heap space and wanted stuff cleaned from old asap, then InitiatiingHeapOccupancyPercent=0 could be justified?<div><br></div><div>Btw, I thought about your question you asked at J1 on "why use survivor spaces with G1?" ... I'll offer an answer, John Cu or Bengt along with Monica are free to offer their thoughts as well.</div><div><br></div><div>By using survivor spaces, you should, (and I'd expect that to be the case) reduce the amount of concurrent cycles you'll do. And, you will likely more frequently visit more long live objects if you didn't have survivor spaces as a result of doing more concurrent cycles. In addition, the total number of different regions you evacuate may be more without survivor spaces, and you may evacuate the same (live) objects more times than without survivor spaces. In short, I would expect in most cases you end evacuating fewer times per object and you end doing fewer concurrent cycles, all of which saves you CPU cycles for application threads. Of course, I'm sure we can write an application where it would be advantageous to not have a survivor spaces in G1. But, we could also write would that could never have the need for a concurrent cycle in a G1 heap that has survivor spaces.</div><div><br></div><div>Thanks again for the question!</div><div><br></div><div>charlie ...</div><div><br><div><div>On Oct 19, 2012, at 2:16 PM, Kirk Pepperdine wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><meta http-equiv="Content-Type" content="text/html charset=us-ascii"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Thanks Charlie,<div><br></div><div>I only had a cursor look at the source and found the initial calculation but stopped there figuring someone here would know off the top of their heads. Didn't expect someone to splunk through the code so a big thanks for that.<div><br></div><div>Again, I'm struggling to think of a use case for this behaviour.</div><div><br></div><div>Regards,</div><div>Kirk</div><div><br><div><div>On 2012-10-19, at 8:56 PM, Charlie Hunt <<a href="mailto:chunt@salesforce.com">chunt@salesforce.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Don't mean to jump in front of Monica. :-/ But, she can confirm. ;-)<div><br></div><div>A quick look at the G1 source code suggests that if InitiatingHeapOccupancyPercent=0, the following will happen:</div><div>- the first minor GC will initiate a concurrent cycle implying that you'll see a young GC with an initial-mark in the GC log w/ +PrintGCDetails</div><div>- every minor GC there after, as long as there is not an active concurrent cycle, will initiate the start of a concurrent cycle</div><div>* So, in other words, concurrent cycles will run back to back. Remember that there needs to be a minor GC to initiate the concurrent cycle, i.e. the initial-mark. There is at least one caveat to that which I'll explain next. So, once a concurrent cycle complete, the next concurrent cycle will not start, until the next minor GC, or a humongous allocation occurs as described next.</div><div>- If there is a humongous object allocation, a concurrent cycle will be initiated (if InitiattingHeapOccupancyPercent=0). This is done before the humongous allocation is done.</div><div><br></div><div>charlie ...</div><div><br><div><div><div>On Oct 19, 2012, at 12:58 PM, Kirk Pepperdine wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><meta http-equiv="Content-Type" content="text/html charset=us-ascii"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Hi Monica,<div><br></div><div>Can you comment on what a value of 0 means?</div><div><br></div><div>Regards,</div><div>Kirk</div><div><br><div><div>On 2012-10-19, at 2:55 PM, Monica Beckwith <<a href="mailto:monica.beckwith@oracle.com">monica.beckwith@oracle.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
<div bgcolor="#FFFFFF" text="#000000">
Couple of quick observations and questions - <br>
<ol>
<li>G1 is officially supported in 7u4. (There are numerous
performance improvements that I recommend updating to the latest
jdk7 update, if possible)</li>
<li>What do you mean by old gen collection? Are you talking about
MixedGCs?</li>
<li>Instead of setting InitiatingHeapOccupancyPercent to zero,
have you tried resizing your young generation?</li>
<ol>
<li>I see the NewRatio, but that fixes the nursery to 640,
instead have you tried with a lower (than the min default) of
nursery using the NewSize option?</li>
</ol>
</ol><p>-Monica<br>
</p>
<br>
<br>
On 10/19/2012 12:13 AM, csewhiz wrote:
<blockquote cite="mid:13a7770622f.3368361147554026191.2919233442948217281@zoho.com" type="cite">
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
<div style="font-size:10pt;font-family:Verdana,Arial,Helvetica,sans-serif;">
<div>Hello All,</div>
<div> Sorry for posting this question in this mailing list. I
am unable to find any answer for this. I am trying to tune our
application for G1GC as we need very small pauses Below
500msec. </div>
<div> But the problem is when we are runing with G1GC (under
jdk 6_u37) Old generation of garbage collection only
happening when it is reaching the Max GC size I noticed on jdk
6U 37 if max heap size is 1GB then it is close to 1sec 2GB
close to 2 sec pauses. </div>
<div> </div>
<div> Is there any parameter to force the old gc happening
regularly. </div>
<div> </div>
<div>I am trying following setting, </div>
<div> </div>
<div>-Xms1280M -Xmx1280M -XX:+UseG1GC
-XX:MaxTenuringThreshold=15 -XX:SurvivorRatio=8 -XX:NewRatio=1
-XX:GCPauseIntervalMillis=7500 -XX:MaxGCPauseMillis=500
-XX:InitiatingHeapOccupancyPercent=0 -XX:ParallelGCThreads=7
-XX:ConcGCThreads=7 </div>
<div> </div>
<div>If anyone can give insight on how full GC is triggred
internals will be of great help.</div>
<div> </div>
<div>PS: I have tried without any option for G1 but not of much
help hence .. this one trying to be agressive ? but of not
much help.</div>
<div> </div>
<div> </div>
<div>Regards,</div>
<div>Soumit<br>
<br>
</div>
</div>
</blockquote>
<br>
<div class="moz-signature">-- <br>
<a href="http://www.oracle.com/" target="_blank"><span><oracle_sig_logo.gif></span></a><br>
<font color="#666666" face="Verdana, Arial, Helvetica, sans-serif" size="2">Monica Beckwith | Java Performance Engineer<br>
VOIP: <a href="tel:+1%20512%20401%201274" moz-do-not-send="true">+1 512 401 1274</a>
<br>
Texas </font>
<br>
<a href="http://www.oracle.com/commitment" target="_blank"><span><green-for-email-sig_0.gif></span></a>
<font color="#4B7D42" face="Verdana, Arial, Helvetica, sans-serif" size="1">Oracle is committed to developing practices and
products that help protect the environment</font>
<!-- This signature was generated by the MyDesktop Oracle Business Signature utility version 3.9 -->
</div>
</div>
</blockquote></div><br></div></div></blockquote></div><br></div></div></div></blockquote></div><br></div></div></div></blockquote></div><br></div></body></html>