String.intern() would seem to provide better coverage considering that some users may not use the G1 collector.
On Mon, Feb 8, 2021 at 3:19 PM Keith Turner <ke...@deenlo.com> wrote: > Recently while running some large map reduce jobs I learned that > Hadoop uses String.intern() in its RPC code (below is a link to an > example on one place where Hadoop does this). I learned this because > when I ran jstack on NN, RM, and/or AM that were under distress > sometimes I kept seeing RPC server threads that were in > String.intern(). I never was quite sure if it was a problem though. > Not saying String.intern() is bad or good, just sharing something I > observed that I was uncertain about. > > May make sense to create some sort of stress test that could simulate > the usage pattern of the TabletLocator and try the different options > and see what happens. If any long pauses or problems happen in the > simulation, they may happen in the real environment. > > > https://github.com/apache/hadoop/blob/ba631c436b806728f8ec2f54ab1e289526c90579/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/TaskStatus.java#L481 > > https://github.com/apache/hadoop/blob/ba631c436b806728f8ec2f54ab1e289526c90579/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/StringInterner.java#L67 > > On Mon, Feb 1, 2021 at 9:55 PM Christopher <ctubb...@apache.org> wrote: > > > > While code reviewing, I saw that > > core/src/main/java/org/apache/accumulo/core/clientImpl/TabletLocator.java > > was using a WeakHashMap to deduplicate some strings. > > > > This code can probably be removed in favor of one of the following two > options: > > > > 1. Just explicitly use String.intern() - As of Java 7, there is no > > longer a separate, fixed-size PermGen space, so intern'd strings will > > be in the main heap, no longer constrained to a limited size pool. > > These strings are still subject to garbage collection. It is > > implemented as a HashMap internally (native implementation), with a > > default bucket size of more than 60K, plenty big enough for the > > interning that TabletLocator is doing... but this is configurable by > > the user with JVM flags if it's not. Interning will use less memory as > > WeakHashMap and similar performance, as long as the bucket size is big > > enough. > > > > 2. Just use -XX:+UseStringDeduplication JVM flag - as of Java 9, G1 is > > the new default Java garbage collector. This garbage collector has the > > option to automatically attempt to deduplicate all strings behind the > > scenes, by swapping out their underlying char arrays (so, it likely > > won't affect == equality because the String object references > > themselves won't change, unlike option 1). This is more passive than > > option 1, but would apply to the entire JVM. G1GC also implements some > > heuristics to prevent too much overhead. > > > > With both options, it's possible to output statistics. > > > > If I remove the WeakHashMap for the string deduplication in > > TabletLocator, does anybody have an opinion on which option I should > > replace it with? I'm leaning towards option 2 (adding it to > > assemble/conf/accumulo-env.sh as one of the default flags). >
