How does JVM decide the tenuringThreshold?

[Jun Zhuang]

Hi,


 
I am trying to gain some knowledge regarding how GC works byrunning a performance test against a java web service running on JBOSS andadjusting various java startup parameters. One interesting thing I noticed wasthe tenuring threshold, I thought it would change with size of the survivorspace, but I could never make it happen, can someone provide some clue? Followingis what I did:


 
JAVA startupparameters:

-Xms2g -Xmx2g -XX:MaxPermSize=256m -XX:NewSize=128m-XX:MaxNewSize=128m -XX:SurvivorRatio=126


 
I then changed the SurvivorRatio to 30, 6, and 1, but the “newthreshold” in the GC log always ends up with 1, adding -XX:TargetSurvivorRatio=90did not seem to make any difference:


 
4.504: [GC

Desired survivor size 16777216 bytes, new threshold 7 (max15)

 [PSYoungGen:98304K->7553K(114688K)] 98304K->7553K(2080768K), 0.0199500 secs] [Times: user=0.04sys=0.01, real=0.02 secs]

12.784: [GC

Desired survivor size 16777216 bytes, new threshold 7 (max15)

 [PSYoungGen:105857K->8979K(114688K)] 105857K->8979K(2080768K), 0.0177280 secs][Times: user=0.04 sys=0.01, real=0.01 secs]

18.935: [GC

Desired survivor size 16777216 bytes, new threshold 7 (max15)

 [PSYoungGen:107283K->13322K(114688K)] 107283K->13322K(2080768K), 0.0282660 secs][Times: user=0.06 sys=0.00, real=0.02 secs]

22.944: [GC

Desired survivor size 16777216 bytes, new threshold 7 (max15)

 [PSYoungGen:111626K->11304K(114688K)] 111626K->11377K(2080768K), 0.0255770 secs][Times: user=0.08 sys=0.00, real=0.03 secs]

24.089: [GC

Desired survivor size 16777216 bytes, new threshold 6 (max15)

 [PSYoungGen:109608K->16377K(114688K)] 109681K->17678K(2080768K), 0.0373650 secs][Times: user=0.10 sys=0.01, real=0.04 secs]

24.801: [GC

Desired survivor size 16777216 bytes, new threshold 5 (max15)

 [PSYoungGen:114681K->16358K(114688K)] 115982K->21621K(2080768K), 0.0627800 secs][Times: user=0.21 sys=0.01, real=0.06 secs]

25.500: [GC

Desired survivor size 16777216 bytes, new threshold 4 (max15)

 [PSYoungGen:114662K->16359K(114688K)] 119925K->30181K(2080768K), 0.0577540 secs][Times: user=0.17 sys=0.01, real=0.06 secs]

26.280: [GC

Desired survivor size 16777216 bytes, new threshold 3 (max15)

 [PSYoungGen:114663K->16354K(114688K)] 128485K->37506K(2080768K), 0.0539360 secs][Times: user=0.15 sys=0.01, real=0.05 secs]

27.330: [GC

Desired survivor size 16777216 bytes, new threshold 2 (max15)

 [PSYoungGen:114658K->14928K(114688K)] 135810K->42581K(2080768K), 0.0588730 secs][Times: user=0.20 sys=0.01, real=0.06 secs]

28.108: [GC

Desired survivor size 16777216 bytes, new threshold 1 (max15)

 [PSYoungGen:113232K->16363K(114688K)] 140972K->59269K(2080768K), 0.0586460 secs][Times: user=0.15 sys=0.01, real=0.06 secs]

. . .

1538.726: [GC

Desired survivor size 3932160 bytes, new threshold 1 (max15)

 [PSYoungGen:129280K->1792K(125184K)] 433305K->308417K(2091264K), 0.2403020 secs][Times: user=0.83 sys=0.01, real=0.24 secs]

1543.537: [GC

Desired survivor size 4063232 bytes, new threshold 1 (max15)

 [PSYoungGen:125184K->2592K(126016K)] 431809K->310233K(2092096K), 0.2318620 secs][Times: user=0.83 sys=0.01, real=0.23 secs]

1548.268: [GC

Desired survivor size 3932160 bytes, new threshold 1 (max15)

 [PSYoungGen:125984K->1344K(127232K)] 433625K->310657K(2093312K), 0.2294920 secs][Times: user=0.83 sys=0.01, real=0.23 secs]


 
If the threshold is really 1, then nothing should bepromoted to the old gen, but that definitely was not the case.


 
-------------------------------------------------------------------------------------------------------


 
I also experimented with -XX:-UsePSAdaptiveSurvivorSizePolicy,the “new threshold” always stays at 7 despite the SurvivorRatio value.


 
1643.408: [GC

Desired survivor size 4194304 bytes, new threshold 7 (max15)

 [PSYoungGen:519729K->4032K(520192K)] 935797K->420443K(2093056K), 0.2605620 secs][Times: user=0.94 sys=0.01, real=0.26 secs]

1662.092: [GC

Desired survivor size 4194304 bytes, new threshold 7 (max15)

 [PSYoungGen:520128K->4096K(520192K)] 936539K->421235K(2093056K), 0.2588890 secs][Times: user=0.95 sys=0.01, real=0.26 secs]

1680.882: [GC

Desired survivor size 4194304 bytes, new threshold 7 (max15)

 [PSYoungGen:520192K->4096K(520192K)] 937331K->422347K(2093056K), 0.2661180 secs][Times: user=0.97 sys=0.00, real=0.27 secs]

1698.975: [GC

Desired survivor size 4194304 bytes, new threshold 7 (max15)

 [PSYoungGen:520192K->4096K(520192K)] 938443K->423246K(2093056K), 0.2626090 secs][Times: user=0.95 sys=0.01, real=0.26 secs]


 

 
Appreciate any input,


Jun

 
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