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

[Shane Cox]

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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