test/hotspot/jtreg/vmTestbase/gc/memory/Churn/Churn.README

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The Churn tests are a series of small programs designed to
simply churn memory over, mixing in calls to GC and Finalization.
They are all basically the same program with permutations.
Each program contains:

LocalRandom class -- used to isloate calls to random generator
        for reproducibility

MemoryObject class -- simply allocated and reclaimed. contains
        a byte array

Churn class -- entry point. Allocates threads and counts objects.
        When loop is done, sets Semaphore to signal threads to cease

ThreadObject class -- allocates and deallocates (by assignment)
        MemoryObjects

SemaphoreObject class -- static class for signalling threads when
        to stop running. Perhap poorly named.

CommandLine class -- parses command line args. Static class.
--------------------------
Usage:

java Churn [-l numberOfLoops] [-t numberOfThreads] [-a sizeOfArray] [-m multiplier] [-f <run forever>] [-s seed]

NOTE: the options can be passed in any order


seed is the seed for the LocalRandom pseudo-random generator

numberOfLoops is the number of iterations a test will go through.
This defaults to 2000

numberOfThreads is the number of threads activating and deactivating
MemoryObjects. This deaults to 2.

sizeOfArray is the number of MemoryObjects each thread manipulates.
This defaults to 1024.

multiplier is a value used to determine how much memory each MemoryObject
holds. It defaults to 10.

The tests can be configured to run forever by passing -f on the
command line. This is useful for continued stress testing outside
of a test harness.
-----------------------------
The total amount of needed memory is approximately
numberOfThreads * (sizeOfArray * sizeOfArray) * (multiplier / 2) bytes

The default is about 10 Megs needed, not couting garbage collection
overhead.  Garbage collection doubles the memory requirements for
programs that run out of heap space, so the the true default size
is 20 Megs.


Testname        What it does
-------------   ---------------------------------------------------------
Churn1          Simply creates new MemObjects. No finalizers. No user
                calls to System.gc(). Count of objects simply grows
                since no method ever decreases the count -- object count
                equals total objects ever allocated. Memory used is
                approx = (object count) * (array size) * (multiplier) / 2

Churn2          Same as Churn1, but adds a call to System.gc() every loop.
                Object Count should simply increase. About half the time,
                this test will hang before the 100th loop. About 10% of
                the time this test will SEG FAULT with either a BUS ERROR
                or ACCESS VIOLATION

Churn3          Same as Churn1, but the MemoryObject class has a
                finalize() method on it. The method is empty. The objecty
                count should continue to rise. However, this program
                runs out of memory after around 35 loops. The threads
                dies from OutOfMemoryErrors. It looks as if the
                finalizers are not called fast enough during GC, so
                enough memory cannot be freed.
                Additionally, about 10% of the time, this program SEGFAULTS

Churn3a         Same as Churn3, but with an explicit call to
                System.runFinalization() in each loop.
                Occasionally, this test will run out of memory
                early on for one or more of the threads.
                Also, this test sometimes hangs, even if one or
                all of the ThreadObjects have died.
                Even if no threads die, and it doesn't hang, this
                program will still die with:
                        java.lang.NullPointerException
                                at java.lang.String.indexOf(String.java)
                                at Churn3a.main(Churn3a.java:71)
                which contains only a call to join()



Churn4          Same as Churn3, but with code in the finalize method
                which would decrement a *static* field in the class.
                This test runs out of memory much more quickly, something
                like after 3 or 4 loops.