from space and to space size is different and varies

[Charlie Hunt]

Hi Thomas,

It is common for Parallel GC, and Parallel Old GC to adjust survivor sizes when -XX:+UseAdaptiveSizePolicy is enabled.  And, -XX:+UseAdaptiveSizePolicy is enabled by default with either -XX:+UseParallelGC and -XX:+UseParallelOldGC, (iirc, the latter is not available in a Java 5 HotSpot VM).

If you disable adaptive size policy, via -XX:-UseAdaptiveSizePolicy, survivor sizes should remain the same size.

hths,

charlie ...

On Oct 25, 2012, at 3:47 AM, Thomas Rohde wrote:

> Hi Folks,
> 
> up to yesterday I always thought, that from-space and to-space have 
> always the same size. In a GC log of a colleague I saw the following and 
> was wondering about it:
> 
> 22.10.2012 00:01:59 {Heap before gc invocations=590:
> 22.10.2012 00:01:59  PSYoungGen      total 72768K, used 70928K 
> [0xcdc00000, 0xd3800000, 0xf8800000)
> 22.10.2012 00:01:59   eden space 54528K, 100% used 
> [0xcdc00000,0xd1140000,0xd1140000)
> 22.10.2012 00:01:59   from space 18240K, 89% used 
> [0xd1460000,0xd2464168,0xd2630000)
> 22.10.2012 00:01:59   to   space 18048K, 0% used 
> [0xd2660000,0xd2660000,0xd3800000)
> 22.10.2012 00:01:59  PSOldGen        total 1400832K, used 334889K 
> [0x78400000, 0xcdc00000, 0xcdc00000)
> 22.10.2012 00:01:59   object space 1400832K, 23% used 
> [0x78400000,0x8cb0a778,0xcdc00000)
> 22.10.2012 00:01:59  PSPermGen       total 20480K, used 19997K 
> [0x74400000, 0x75800000, 0x78400000)
> 22.10.2012 00:01:59   object space 20480K, 97% used 
> [0x74400000,0x75787568,0x75800000)
> 22.10.2012 00:01:59 299113.923: [GC [PSYoungGen: 70928K->15357K(72512K)] 
> 405818K->350331K(1473344K), 0.0717791 secs]
> 22.10.2012 00:01:59 Heap after gc invocations=590:
> 22.10.2012 00:01:59  PSYoungGen      total 72512K, used 15357K 
> [0xcdc00000, 0xd3800000, 0xf8800000)
> 22.10.2012 00:01:59   eden space 54464K, 0% used 
> [0xcdc00000,0xcdc00000,0xd1130000)
> 22.10.2012 00:01:59   from space 18048K, 85% used 
> [0xd2660000,0xd355f7d8,0xd3800000)
> 22.10.2012 00:01:59   to   space 18112K, 0% used 
> [0xd14a0000,0xd14a0000,0xd2650000)
> 22.10.2012 00:01:59  PSOldGen        total 1400832K, used 334973K 
> [0x78400000, 0xcdc00000, 0xcdc00000)
> 22.10.2012 00:01:59   object space 1400832K, 23% used 
> [0x78400000,0x8cb1f778,0xcdc00000)
> 22.10.2012 00:01:59  PSPermGen       total 20480K, used 19997K 
> [0x74400000, 0x75800000, 0x78400000)
> 22.10.2012 00:01:59   object space 20480K, 97% used 
> [0x74400000,0x75787568,0x75800000)
> 22.10.2012 00:01:59 }
> 
> Before GC from space has 18240K and to space has 18048K.
> After GC from space has 18048K and to space has 18112K.
> 
> java version "1.5.0_20"
> Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_20-b02)
> Java HotSpot(TM) Server VM (build 1.5.0_20-b02, mixed mode)
> 
> 1. Why is the size of from space and to space not equal?
> 2. Why is the size always changing?
> 
> Bye,
> Thomas
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