Setting Min and Max Heap Size to the same value affects size of Young Gen

Shane Cox shane.cox at gmail.com
Fri Nov 12 11:31:53 PST 2010


This is probably well known behavior, but why does setting the Min and Max
Heap Size to the same value affect the default size of the Young
Generation?  For example:

Scenario 1:
-d64 -Xms1536m -Xmx4096m -XX:+UseConcMarkSweepGC

Young Generation is small:  18,624K

{Heap before GC invocations=0 (full 0):
 par new generation   total 18624K, used 16000K [0xfffffd7ef4e00000,
0xfffffd7ef62c0000, 0xfffffd7f05c60000)
  eden space 16000K, 100% used [0xfffffd7ef4e00000, 0xfffffd7ef5da0000,
0xfffffd7ef5da0000)
  from space 2624K,   0% used [0xfffffd7ef5da0000, 0xfffffd7ef5da0000,
0xfffffd7ef6030000)
  to   space 2624K,   0% used [0xfffffd7ef6030000, 0xfffffd7ef6030000,
0xfffffd7ef62c0000)
 concurrent mark-sweep generation total 1551616K, used 0K
[0xfffffd7f05c60000, 0xfffffd7f647a0000, 0xfffffd7ff4e00000)
 concurrent-mark-sweep perm gen total 21248K, used 7265K
[0xfffffd7ff4e00000, 0xfffffd7ff62c0000, 0xfffffd7ffa200000)
2010-11-12T14:00:16.083-0500: 0.364: [GC 0.364: [ParNew:
16000K->2150K(18624K), 0.0048839 secs] 16000K->2150K(1570240K), 0.0049538
secs] [Times: user=0.02 sys=0.01, real=0.01 secs]



Scenario 2:
-d64 -Xms1536m -Xmx1536m -XX:+UseConcMarkSweepGC

Young Generation is much larger:  172,032K

{Heap before GC invocations=0 (full 0):
 par new generation   total 172032K, used 147456K [0xfffffd7f94e00000,
0xfffffd7fa0e00000, 0xfffffd7fa0e00000)
  eden space 147456K, 100% used [0xfffffd7f94e00000, 0xfffffd7f9de00000,
0xfffffd7f9de00000)
  from space 24576K,   0% used [0xfffffd7f9de00000, 0xfffffd7f9de00000,
0xfffffd7f9f600000)
  to   space 24576K,   0% used [0xfffffd7f9f600000, 0xfffffd7f9f600000,
0xfffffd7fa0e00000)
 concurrent mark-sweep generation total 1376256K, used 0K
[0xfffffd7fa0e00000, 0xfffffd7ff4e00000, 0xfffffd7ff4e00000)
 concurrent-mark-sweep perm gen total 21248K, used 12639K
[0xfffffd7ff4e00000, 0xfffffd7ff62c0000, 0xfffffd7ffa200000)
2010-11-12T11:53:01.376-0500: 360.088: [GC 360.088: [ParNew:
147456K->7373K(172032K), 0.0093910 secs] 147456K->7373K(1548288K), 0.0094709
secs] [Times: user=0.03 sys=0.02, real=0.01 secs]



jinfo reports the value of -XX:CMSYoungGenPerWorker=16777216 in both
scenarios, as well as -XX:ParallelGCThreads=13.  So it's unclear to me why
the Young Gen would be so small when -Xms and -Xmx are different values
(Scenario 1).


java -version
java version "1.6.0_14"
Java(TM) SE Runtime Environment (build 1.6.0_14-b08)
Java HotSpot(TM) 64-Bit Server VM (build 14.0-b16, mixed mode)


Any insights would be appreciated.
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