Hey, For what it's worth, the Guava team added that bit about "String.intern() has some well-known performance limitations, and should generally be avoided" was added on Oct 13, 2020.
Thanks. On Mon, Feb 8, 2021 at 5:06 PM Christopher <[email protected]> wrote: > Guava's argument in the linked comment appears to be based on > pre-Java8, before the PermGen space was consolidated with the main > heap and had a fixed size. > > In response to the other observations: a stress test here seems > particularly difficult to compare between String.intern and G1GC > deduplication, since the latter will deduplicate across the JVM, and > not just the TabletLocator stuff. So, we wouldn't be able to get a > good direct comparison between the overall impact between the two. > > As for running a stress test between WeakHashMap and String.intern, > just for TabletLocator, I'm not going to bother because others have > already done that work and determined them to be similar in > performance, given an adequate string table size (one is at > http://java-performance.info/string-intern-in-java-6-7-8/) > > So, what I will do is create a PR to replace the non-tunable > WeakHashMap with the tunable String.intern for TabletLocator, on the > basis that they have comparable performance, but the latter is > user-tunable if they need to, uses less memory, and involves less > code. I will not do anything with the G1GC settings, leaving that up > to users to experiment with and tune on their own, if they wish. > > On Mon, Feb 8, 2021 at 4:31 PM David <[email protected]> wrote: > > > > Guava argues for the use of a weak hashmap. > > > > > https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/Interner.java#L28-L30 > > > > On Mon, Feb 8, 2021 at 3:57 PM Brian Loss <[email protected]> wrote: > > > > > It might make sense to do both approaches. > > > > > > It seems there are limits to when -XX:+UseStringDeduplication takes > > > effect. By default, it only interns objects that have survived 3 GC > cycles, > > > although that number can be changed. If the objects in question are > > > short-lived, then it wouldn’t make sense to call String.intern on them > > > either. However, if we know based on the usage pattern that we’d get a > lot > > > of deduplication on long-lived strings, then String.intern is better > > > because it will happen right away and will also save more memory since > the > > > String object itself is de-duped (vs just the internal char array for > the > > > automatic de-duplication). It wasn’t completely clear from my reading, > but > > > if I understood correctly the other potential downside to > > > UseStringDeduplication is that it happens after GC if there’s time. On > a > > > heavily loaded system that doesn’t time left in the pause time goal > window > > > after completing GC, the string de-duplication might not happen at all. > > > > > > Adding -XX:+UseStringDeduplication wouldn’t hurt and could potentially > > > provide some benefit, so I’d be in favor of adding it. For > TabletLocator > > > specifically, if we know that’s an area where string de-duplication > will > > > help, then we should probably use String.intern there. As Keith > suggested, > > > a stress test might help determine whether it makes sense. In the > absence > > > of that, if we assume the previous WeakHashMap was there to solve a > > > specific problem (vs an uninformed attempt to save memory) then > > > String.intern sounds to me like the way to go as well. > > > > > > > On Feb 8, 2021, at 3:34 PM, Dave Marion <[email protected]> wrote: > > > > > > > > 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 <[email protected]> > 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 <[email protected]> > 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). > > > >> > > > > > > >
