New blog post: A First Look at Info Table Profiling

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#1

@mpickering and David Eichmann have written a post for the Well-Typed group blog on an experimental new profiling mode for GHC, as part of a bunch of improvements to profiling tools:

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#2

I tried to use this way to profile my app, but the resulting profile is… uninformative despite the app being compiled with -finfo-table-map and -fdistinct-constructor-tables compile time flags.

JOB "test-app-please-ignore +RTS -hi"
DATE "Tue Sep 12 14:07 2023"
SAMPLE_UNIT "seconds"
VALUE_UNIT "bytes"
BEGIN_SAMPLE 0.000000
END_SAMPLE 0.000000
BEGIN_SAMPLE 0.117509
0x512ee90       80
0x5139ee0       32
0x51294e0       32
0x4febfe0       16
0x4febba8       16
0x3542bb0       24
0x34f3440       16
0x2f8a068       16
0x2f8b2b0       72
0x2f8b380       216
0x34ebdb8       16
0x3287290       40
0x512a128       72
0x4f06ca0       80
0x5138cf0       72
0x5131748       24
...

Also, eventlog sometimes comes out empty when using -hi. -hc usually works fine.

#3

Which platform / GHC version? Did you recompile all library dependencies with the flags, as well as the app itself?

Even if so, you may still see a few info tables with missing provenance information. Mostly these come from libraries distributed with GHC (e.g. base) for which there isn’t yet a good story about how to distribute the IPE data (#22972: Providing IPE-enabled bindists in the release pipeline · Issues · Glasgow Haskell Compiler / GHC · GitLab). Or they may come from corner cases where GHC doesn’t emit IPE information yet (#23901: Add infotable location info for thunks introduced by tag inference. · Issues · Glasgow Haskell Compiler / GHC · GitLab). But in my experience these cases usually affect only a small proportion of the data so the Detailed tab generated by eventlog2html is still useful.

Also, eventlog sometimes comes out empty when using -hi . -hc usually works fine.

I’ve not seen this. Is it possible that the app is being killed rather than shutting down normally? (In that case the eventlog buffer might not be flushed to disk quickly enough.)

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