On Wed, 24 Apr 2024 10:08:42 GMT, Claes Redestad <redes...@openjdk.org> wrote:
>> This patch suggests a workaround to an issue with huge SCF MH expression >> trees taking excessive JIT compilation resources by reviving (part of) the >> simple bytecode-generating strategy that was originally available as an >> all-or-nothing strategy choice. >> >> Instead of reintroducing a binary strategy choice I propose a threshold >> parameter, controlled by >> `-Djava.lang.invoke.StringConcat.highArityThreshold=<val>`: For expressions >> below or at this threshold there's no change, for expressions with an arity >> above it we use the `StringBuilder`-chain bytecode generator. >> >> There are a few trade-offs at play here which influence the choice of >> threshold. The simple high arity strategy will for example not see any reuse >> of LambdaForms but strictly always generate a class per indy callsite, which >> means we might end up with a higher total number of classes generated and >> loaded in applications if we set this value too low. It may also produce >> worse performance on average. On the other hand there is the observed >> increase in C2 resource usage as expressions grow unwieldy. On the other >> other hand high arity expressions are likely rare to begin with, with less >> opportunities for sharing than the more common low-arity expressions. >> >> I turned the submitted test case into a few JMH benchmarks and did some >> experiments with `-XX:CompileCommand=MemStat,StringConcat::concat,print`: >> >> Baseline strategy: >> 13 args: 6.3M >> 23 args: 18M >> 123 args: 868M >> >> `-Djava.lang.invoke.StringConcat.highArityThreshold=0`: >> 13 args: 2.11M >> 23 args: 3.67M >> 123 args: 4.75M >> >> For 123 args the memory overhead of the baseline strategy is 180x, but for >> 23 args we're down to a 5x memory overhead, and down to a 3x overhead for 13 >> args. Since the absolute overhead for 23 is borderline acceptable (+15Mb) >> I've conservatively chosen a threshold at arity 20. This keeps C2 resource >> pressure at a reasonable level (< 18M) while avoiding perturbing performance >> at the vast majority of call sites. >> >> I was asked to use the new class file API for mainline. There's a version of >> this patch implemented using ASM in 7c52a9f which might be a reasonable >> basis for a backport. > > Claes Redestad has updated the pull request incrementally with one additional > commit since the last revision: > > Make Set.of(STRONG) a constant, fix compilation, minor code improvements Thanks for reviewing! I'll split out and PR an ASM version as a subtask and rebase this PR on top of that. ------------- PR Comment: https://git.openjdk.org/jdk/pull/18690#issuecomment-2075654943
