mikemccand commented on PR #13581:
URL: https://github.com/apache/lucene/pull/13581#issuecomment-2260766983
I was curious about the HEAP cost of `Lock` and wrote a silly little program
that keeps making and saving `Lock` into an `ArrayList` and catches OOME and
prints how far it got:
```java
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.concurrent.locks.Lock;
import java.util.ArrayList;
import java.util.List;
import java.util.Locale;
import java.util.Random;
public class LockHeap {
public static void main(String[] args) {
final List<ReentrantReadWriteLock> locks = new ArrayList<>();
Thread[] threads = new Thread[16];
//Thread[] threads = new Thread[0];
for(int i=0;i<threads.length;i++) {
threads[i] = new Thread() {
@Override
public void run() {
Random random = new Random();
while (true) {
int size = locks.size();
if (size == 0) {
continue;
}
ReentrantReadWriteLock lock = locks.get(random.nextInt(size));
Lock lock2;
if (random.nextBoolean()) {
lock2 = lock.readLock();
} else {
lock2 = lock.writeLock();
}
lock2.lock();
lock2.unlock();
}
}
};
threads[i].start();
}
int count = 0;
while (true) {
try {
ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
//lock.readLock();
//lock.writeLock();
locks.add(lock);
count++;
if (count % 10000 == 0) {
System.out.println(String.format(Locale.ROOT, "%.1fM locks", count
/ 1_000_000.));
}
} catch (OutOfMemoryError oome) {
locks.clear();
System.out.println(String.format(Locale.ROOT, "OOME at %.1fM locks",
count / 1_000_000.));
throw oome;
}
}
}
}
```
When I run with `-Xmx2g` I get:
```
17.2M locks
17.2M locks
OOME at 17.2M locks
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:241)
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:231)
at LockHeap.main(LockHeap.java:41)
```
and `-Xmx4g`:
```
34.2M locks
OOME at 34.2M locks
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:241)
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:231)
at LockHeap.main(LockHeap.java:41)
```
and `-Xmx6g`:
```
51.3M locks
51.3M locks
OOME at 51.3M locks
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock$Sync.<init>(ReentrantReadWriteLock.java:339)
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync.<init>(ReentrantReadWriteLock.java:680)
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:241)
at
java.base/java.util.concurrent.locks.ReentrantReadWriteLock.<init>(ReentrantReadWriteLock.java:231)
at LockHeap.main(LockHeap.java:41)
```
So it seems to be a linear function :) And it works out to ~125 bytes per
`Lock`:
```
raptorlake:~$ python3
Python 3.12.4 (main, Jun 7 2024, 06:33:07) [GCC 14.1.1 20240522] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> (6 * 1024 * 1024 * 1024) / 51300000
125.58383906432749
```
[Turning on the threads randomly acquiring/releasing the read and write lock
didn't seem to change the heap usage]
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