Re: RFR: 8327247: C2 uses up to 2GB of RAM to compile complex string concat in extreme cases [v9]
On Wed, 24 Apr 2024 19:13:43 GMT, Claes Redestad 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=`: 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: > > Nits, clean up constant, introduce missing constant MethodTypeDesc for > toString It's a little confusing for JDK-8327247 to have 2 PRs but it's okay. We can add a note in JDK-8327247 to clarify. I assume you plan to use PR to convert to use ClassFile API after you pull from JDK-8327247 once integrated? - PR Comment: https://git.openjdk.org/jdk/pull/18690#issuecomment-2078512445
Re: RFR: 8327247: C2 uses up to 2GB of RAM to compile complex string concat in extreme cases [v9]
> 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=`: 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: Nits, clean up constant, introduce missing constant MethodTypeDesc for toString - Changes: - all: https://git.openjdk.org/jdk/pull/18690/files - new: https://git.openjdk.org/jdk/pull/18690/files/9742f074..0ac8f24b Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=18690&range=08 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18690&range=07-08 Stats: 13 lines in 1 file changed: 2 ins; 5 del; 6 mod Patch: https://git.openjdk.org/jdk/pull/18690.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/18690/head:pull/18690 PR: https://git.openjdk.org/jdk/pull/18690
