Some results from profiling

[jrudolph]

Hi @olafurpg,

I spent a bit of time looking into the performance of scalafmt. See the
flame-graph below. I found a few things:

VirtualFile.value

That one was a mystery to me first, it amounts to 8% of total time. I guess the problem here is that Input is used as a key to many of the Maps. For some reason, scalafmt creates VirtualFiles in Scalafmt.format which means that every Map access with the VirtualFile as the key has to hash the complete code again?

TreeOps.getOwners

Removing the recursion seems to help and using a java.util.HashMap instead of a mutable.Map builder.

FormatOps.iter

Basically the same as TreeOps.getOwners.

getSplits

That red "Interpreter" frame is getSplits. It seems to mean that the JIT-compiler gave up on that method completely... It may be as you say:

scalafmt/scalafmt-core/shared/src/main/scala/org/scalafmt/internal/Router.scala

Line 59 in 8111bb2

* NOTE(olafurpg). The pattern match in this file has gotten out of hand. It's

Not sure how the pattern match could be restructured to make it easier for the JIT and for performance.

I'll open a PR for ideas about to improve the first three things. That will probably already improve performance somewhat.

[olafurpg]

@jrudolph Thank you so much for this thorough investigation. I believe the Scalamt codebase is ripe for microoptimizations 👍 The code was written when I was just starting Scala a few years ago and hasn't changed so much since then 😅

I have also investigated several low-hanging opportunities to speed up Scalameta

• delay setting Tree.parent until after parser completes, this seems to speed up parsing ~35% Add option to disable Tree.parent scalameta#1130 (comment)
• change Tree from trait into abstract class, I haven't measured the difference but I suspect we're paying a lot for virtual interface calls.

I personally don't have much time to pursue these ideas but if somebody is interesting in making a contribution I will be happy to provide feedback and review.

[kitbellew]

@jrudolph can you share the steps how you generated this flame graph (and how you read it)?

[olafurpg]

You can generate flame graphs with https://github.com/jvm-profiling-tools/async-profiler