[LUCENE-1308] Remove String.intern() from Field.java to increase performance and lower contention

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Duplicate
• Affects Version/s: 2.3.2
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Rene Schwietzke
• Created: 17/Jun/08 20:51
• Updated: 28/Aug/22 11:50
• Resolved: 11/Aug/09 00:30

Description

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.

Issue Links

• relates to: LUCENE-1309 Using IndexSearcher in a multithreaded context causes contention
• relates to: LUCENE-2545 improve uses of StringHelper.intern in Field,AbstractField, NumericField etc.
• relates to: LUCENE-1607 String.intern() faster alternative
• relates to: LUCENE-1600 Reduce usage of String.intern(), performance is terrible