src/spec/doc/performance-guide.adoc

//////////////////////////////////////////

  Licensed to the Apache Software Foundation (ASF) under one
  or more contributor license agreements.  See the NOTICE file
  distributed with this work for additional information
  regarding copyright ownership.  The ASF licenses this file
  to you under the Apache License, Version 2.0 (the
  "License"); you may not use this file except in compliance
  with the License.  You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing,
  software distributed under the License is distributed on an
  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  KIND, either express or implied.  See the License for the
  specific language governing permissions and limitations
  under the License.

//////////////////////////////////////////

= Tune parsing performance of Parrot parser

The Parrot parser is based on antlr4 and introduced since Groovy 3.0.0. It provides the following options to tune parsing performance:

[cols="<,<,<,<,<",options="header"]
|=======================================================================
| Option | Description | Default | Version | Example
| groovy.parallel.parse | Parsing groovy source files in parallel. | `false` util Groovy 4.0.0 | 3.0.5+ | -Dgroovy.parallel.parse=true
| groovy.antlr4.cache.threshold | antlr4 relies on DFA cache heavily for better performance, so antlr4 will not clear DFA cache, thus OutOfMemoryError will probably occur. Groovy trades off parsing performance and memory usage, when the count of Groovy source files parsed hits the cache threshold, the DFA cache will be cleared. *Note:* `0` means managing the DFA cache automatically since Groovy 5.0.0, `-1` means never clearing DFA cache, so requiring bigger JVM heap size. Or set a greater value, e.g. 200 to clear DFA cache if threshold hits. **Note: ** the threshold specified is the count of groovy source files | `64`; `0` since Groovy 5.0.0 | 3.0.5+ | -Dgroovy.antlr4.cache.threshold=200
| groovy.antlr4.sll.threshold | Parrot parser will try SLL mode and then try LL mode if SLL failed. But the more tokens to parse, the more likely SLL will fail. If SLL threshold hits, SLL will be skipped. Setting the threshold to `0` means never trying SLL mode, which is not recommended at most cases because SLL is the fastest mode though SLL is less powerful than LL. **Note: ** the threshold specified is the token count | `-1` (disabled by default) | 3.0.9+ | -Dgroovy.antlr4.sll.threshold=1000
| groovy.antlr4.clear.lexer.dfa.cache | Clear the DFA cache for lexer. The DFA cache for lexer is always small and important for parsing performance, so it's strongly recommended to leave it as it is until OutOfMemoryError will truly occur | `false`| 3.0.9+ | -Dgroovy.antlr4.clear.lexer.dfa.cache=true
|=======================================================================