Remove String.intern() from Field.java to increase performance and lower
contention
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Key: LUCENE-1308
URL: https://issues.apache.org/jira/browse/LUCENE-1308
Project: Lucene - Java
Issue Type: Improvement
Affects Versions: 2.3.2
Reporter: Rene S
Right now, *document.Field is interning all field names. While this makes sense
because it lowers the overall memory consumption, the method intern() of String
is know to be difficult to handle.
1) it is a native call and therefore slower than anything on the Java level
2) the String pool is part of the perm space and not of the general heap, so
it's size is more restricted and needs extra VM params to be managed
3) Some VMs show GC problems with strings in the string pool
Suggested solution is a WeakHashMap instead, that takes care of unifying the
String instances and at the same time keeping the pool in the heap space and
releasing the String when it is not longer needed. For extra performance in a
concurrent environment, a ConcurrentHashMap-like implementation of a weak
hashmap is recommended, because we mostly read from the pool.
We saw a 10% improvement in throughout and response time of our application and
the application is not only doing searches (we read a lot of documents from the
result). So a single measurement test case could show even more improvement in
single and concurrent usage.
****** The Cache:
/** Cache to replace the expensive String.intern() call with the java version
*/
private final static Map<String, WeakReference<String>> unifiedStringsCache =
Collections.synchronizedMap(new WeakHashMap<String,
WeakReference<String>>(109));
****** The access to it, instead of this.name = name.intern;
// unify the strings, but do not use the expensive String.intern() version
// which is not "weak enough", uses the perm space and is a native call
String unifiedName = null;
WeakReference<String> ref = unifiedStringsCache.get(name);
if (ref != null)
{
unifiedName = ref.get();
}
if (unifiedName == null)
{
unifiedStringsCache.put(name, new WeakReference(name));
unifiedName = name;
}
this.name = unifiedName;
I guess it is sufficient to have mostly all fields names interned, so I skipped
the additional synchronization around the access and take the risk that only
99.99% :) of all field names are interned.
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