[ The Java FAQ Daily Tips, weekly publication ]
Foreword: Excuse me for
possible mistakes. English is not native language for me.
In this issue:
Hello dear friends!
Question: What are additional types of GC do exist in the J2SE
platform, version 1.4.2?
1. The throughput collector: this collector uses a parallel version of the young
generation collector. It is used if the -XX:+UseParallelGC option is passed on
the command line. The tenured generation collector is the same as the default
2. The concurrent low
pause collector: this collector is used if the -XX:+UseConcMarkSweepGC is passed
on the command line. The concurrent collector is used to collect the tenured
generation and does most of the collection concurrently with the execution of
the application. The application is paused for short periods during the
collection. A parallel version of the young generation copying collector is used
with the concurrent collector if the combination -XX:+UseConcMarkSweepGC
-XX:+UseParNewGC is passed on the command line.
This kind is also known as concurrent GC.
3. The incremental
(sometimes called train) low pause collector: this collector is used only if
-Xincgc is passed on the command line. By careful bookkeeping, the incremental
garbage collector collects just a portion of the tenured generation at each
minor collection, trying to spread the large pause of a major collection over
many minor collections. However, it is even slower than the default tenured
generation collector when considering overall throughput.
-XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC. The argument
parsing in the J2SE platform, version 1.4.2 should only allow legal combinations
of command line options for garbage collectors, but earlier releases may not
detect all illegal combinations and the results for illegal combinations are
Also important to
remember: always try default GC before using additional GCs!
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Question: With 1.3.1 we got 4GB heaps on Solaris, why can't I get
this to work on Windows?
Answer: It's due to fragmentation of the address space. After Windows
has loaded all its stuff, plus the java dlls, the largest available continuous
chunk of addresses for the heap is about 1.4-1.6G on Win/NT. Might be smaller on
If you really need more
space they can try rebasing java.exe and the JDK dlls higher. But that won't buy
you much (maybe a couple hundred meg).
from the Java Hotspot Virtual Machine FAQ:
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