Kristofer Karlsson created LUCENE-4755:
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Summary: Unmap MMapIndexInput's in a delayed way, and avoid
WeakReference usage.
Key: LUCENE-4755
URL: https://issues.apache.org/jira/browse/LUCENE-4755
Project: Lucene - Core
Issue Type: Improvement
Components: core/store
Reporter: Kristofer Karlsson
(Most of this is shamelessly borrowed from Uwe Schindler)
It would be nice to move away from using WeakReference to clean up clones.
Instead, the clones could depend on the master by using a shared boolean
closed-flag.
In order to ensure visibility of this value, or at least make it less likely to
crash, we could delay the unmapping operation.
Rough suggestion of changes:
<code>
public class ByteBufferUnmapper {
/**
* <code>true</code>, if this platform supports unmapping mmapped files.
*/
public static final boolean UNMAP_SUPPORTED;
private static final ScheduledExecutorService SCHEDULED_EXECUTOR_SERVICE =
Executors.newSingleThreadScheduledExecutor(new NamedThreadFactory("unmapper-"));
static {
boolean v = false;
try {
Class.forName("sun.misc.Cleaner");
Class.forName("java.nio.DirectByteBuffer")
.getMethod("cleaner");
v = true;
} catch (Exception e) {
// Do nothing
} finally {
UNMAP_SUPPORTED = v;
}
}
public static void unmap(final ByteBuffer buffer) throws IOException {
try {
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
Method getCleanerMethod = buffer.getClass().getMethod("cleaner");
getCleanerMethod.setAccessible(true);
final Object cleaner = getCleanerMethod.invoke(buffer);
if (cleaner != null) {
cleaner.getClass().getMethod("clean").invoke(cleaner);
}
return null;
}
});
} catch (PrivilegedActionException e) {
final IOException ioe = new IOException("unable to unmap the mapped
buffer");
ioe.initCause(e.getCause());
throw ioe;
}
}
public static void unmapLater(final ByteBuffer buffer, long delay, TimeUnit
unit) {
SCHEDULED_EXECUTOR_SERVICE.schedule(new Runnable() {
public void run() {
try {
unmap(buffer);
} catch (IOException e) {
e.printStackTrace();
}
}
}, delay, unit);
}
}
</code>
<code>
// MMapDirectory
final void cleanMapping(final ByteBuffer buffer) throws IOException {
if (useUnmapHack) {
ByteBufferUnmapper.unmapLater(buffer, 10, TimeUnit.SECONDS);
}
}
</code>
<code>
// MMapIndexInput
@Override
public short readShort() throws IOException {
if (closed[0]) {
throw new AlreadyClosedException("MMapIndexInput already closed: " +
this);
}
</code>
<code>
@Override
public void close() throws IOException {
try {
if (isClone || buffers == null) return;
closed[0] = true;
// make local copy, then un-set early
final ByteBuffer[] bufs = buffers;
for (final ByteBuffer b : bufs) {
cleanMapping(b);
}
} finally {
unsetBuffers();
}
}
</code>
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