Name: jl125535 Date: 05/12/2004


FULL PRODUCT VERSION :
J2SE v1.5beta, 1.4.2_04

ADDITIONAL OS VERSION INFORMATION :
Windows, Linux, Solaris

A DESCRIPTION OF THE PROBLEM :
I think the implemetation of java.nio.Bits#reserveMemory is not good.
Here is a part of java.nio.Bits.java source code for v1.5beta.

 1. static synchronized void reserveMemory(long size) {
     ...
 6. if (reservedMemory + size > maxMemory) {
 7. System.gc();
 8. try {
 9. Thread.sleep(100);
10. } catch (InterruptedException x) {
11. // Restore interrupt status
12. Thread.currentThread().interrupt();
13. }
14. if (reservedMemory + size > maxMemory)
15. throw new OutOfMemoryError("Direct buffer memory");
16. }
17. reservedMemory += size;
18. }

I propose changing sleep(100) into Object.wait(100) or Object.wait() at line 9.

[Reason]

The reserveMemory method is "static synchronized".

It seems like the "sleep(100)" at line 9 is called so that that the
reservedMemory field used at line 14 can wait for the ReferenceHandler
thread to call java.nio.Bits#unreserveMemory(long).

1. static synchronized void unreserveMemory(long size) {
2. if (reservedMemory > 0) {
3. reservedMemory -= size;
4. assert (reservedMemory > -1);
5. }
6. }

However, because the sleep does not lock a Bits.class object,
and java.nio.Bits#unreserveMemory is also "static synchronied",
java.nio.Bits#unreserveMemory can not run while the
java.nio.Bits#reserveMemory is sleeping.

Here is a proposed implementaion of these methods.
 - change sleep(100) into wait(100) in reserveMemory, and
 - add notifyAll() as last statement in unreserveMemory

 1. static synchronized void reserveMemory(long size) {
    ...
 6. if (reservedMemory + size > maxMemory) {
 7. System.gc();
 8. try {
 9. Bits.class.wait(100);
10. } catch (InterruptedException x) {
11. // Restore interrupt status
12. Thread.currentThread().interrupt();
13. }
14. if (reservedMemory + size > maxMemory)
15. throw new OutOfMemoryError("Direct buffer memory");
16. }
17. reservedMemory += size;
18. }

1. static synchronized void unreserveMemory(long size) {
2. if (reservedMemory > 0) {
3. reservedMemory -= size;
4. assert (reservedMemory > -1);
5. }
6. Bits.class.notifyAll();
7. }

I evaluate these implementations by using the same sample
source code as those of 4879883.
Even with -XX:MexDirectMemorySize=8k, new implementation work fine.

----
import java.nio.ByteBuffer;
public class ByteBufferTest {

  public static void main(String[] args) {
    for (;;) {
      ByteBuffer buf = ByteBuffer.allocateDirect(8192);
    }
  }
}
----

I hope both of v1.4.2_05 and v1.5 release has these implementations,
or more good implemetations.


REPRODUCIBILITY :
This bug can be reproduced always.
(Incident Review ID: 265100)
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