memory - Why does my garbage collection log show 3.8GB as the max available heap size while I have allocated 4GB as the max heap size?

Question

I have a 64-bit hotspot JDK version 1.7.0 installed on a 64-bit RHEL 6 machine. I use the following JVM options for my tomcat application.

CATALINA_OPTS="${CATALINA_OPTS} -Dfile.encoding=UTF8 -Dorg.apache.catalina.loader.WebappClassLoader.ENABLE_CLEAR_REFERENCES=false -Duser.timezone=EST5EDT"

# General Heap sizing
CATALINA_OPTS="${CATALINA_OPTS} -Xms4096m -Xmx4096m -XX:NewSize=2048m -XX:MaxNewSize=2048m -XX:PermSize=512m -XX:MaxPermSize=512m -XX:+UseCompressedOops -XX:+DisableExplicitGC"

# Enable the CMS GC policy
CATALINA_OPTS="${CATALINA_OPTS} -XX:+UseConcMarkSweepGC -XX:CMSWaitDuration=15000 -XX:+CMSParallelRemarkEnabled -XX:+CMSCompactWhenClearAllSoftRefs -XX:+CMSConcurrentMTEnabled -XX:+CMSScavengeBeforeRemark -XX:+CMSClassUnloadingEnabled"

# Verbose Garbage Collection Logging
CURRENT_DATE=`date +%Y%m%d%H%M%S`
CATALINA_OPTS="${CATALINA_OPTS} -verbose:gc -XX:+PrintGCDetails -Xloggc:${CATALINA_BASE}/logs/gc-${CURRENT_DATE}.log -XX:+PrintGCDateStamps -XX:+PrintTenuringDistribution"

When I have a Garbage Collection analysis, the GC logs show a maximum available heap of only 3.8GB instead of 4GB allocated to the JVM. Why is that?

Answer 1

New Generation (2048M) consists of 80% Eden (1638.4M) and two Survivor Spaces (10% or 204.8M each):

Heap
 par new generation   total 1887488K, used 134226K [0x00000006fae00000, 0x000000077ae00000, 0x000000077ae00000)
  eden space 1677824K,   8% used [0x00000006fae00000, 0x00000007031148e0, 0x0000000761480000)
  from space 209664K,   0% used [0x0000000761480000, 0x0000000761480000, 0x000000076e140000)
  to   space 209664K,   0% used [0x000000076e140000, 0x000000076e140000, 0x000000077ae00000)
 concurrent mark-sweep generation total 2097152K, used 242K [0x000000077ae00000, 0x00000007fae00000, 0x00000007fae00000)

At any time one of survivor spaces is empty (see Generations).
So, the useful heap size is 1638.4 + 204.8 + 2048 = 3891.2 MB