Memory usage exploding for complex pattern matching

Simon Peyton Jones simonpj at
Thu Mar 29 17:47:01 UTC 2018

Let's make a Trac ticket for this (since there is a nice small reproducer).  Or, if it seems a dup, attach the reproducer to some existing ticket.
Maybe we should revert the (recent) patch that makes TH code behave like source code, until this pattern-match check business is solved.


| -----Original Message-----
| From: ghc-devs <ghc-devs-bounces at> On Behalf Of Ben Gamari
| Sent: 29 March 2018 17:32
| To: Victor Miraldo (UU) <v.cacciarimiraldo at>
| Cc: ghc-devs at
| Subject: Re: Memory usage exploding for complex pattern matching
| "Victor Miraldo (UU)" <v.cacciarimiraldo at> writes:
| > Hello Ben,
| >
| >
| >>> We came up with a "minimal" example that at least is self contained
| and
| >>> triggers the problem; `runghc Minimal.hs` should finish with about
| 5GB of RAM.
| >>>
| >> Hmm, with 8.2 I'm seeing a peak memory usage of 3.5 GB, with that
| peak
| >> occurring during renaming, not desugaring (when exhaustiveness
| checking
| >> happens). Are you certain that the exhaustiveness checker is the
| >> responsible party?
| >
| > Not at all! In fact, I ran some comparisons with "-ddump-phases" and
| > plotted those.
| > I'm attaching the graphs here. From base to "added E" we added 16 more
| > pattern matches,
| > then each "added" phase is an additional 16 pattern matches. The
| > simplifier stands out there.
| >
| > I couldn't run a bigger case as I had hit the ceiling of my machine.
| >
| I tried 8.4 and indeed the renamer/typechecker takes dramatically less
| memory (yay!), leaving desugar as the largest allocator (allocating
| around 3GB) while compiling the minimal example. Furthermore, disabling
| the pattern match checker (via -fmax-pmcheck-iterations=0) dramatically
| reduces desugarer allocations (to around 60MByte). It looks like you are
| right.
| Unfortunately, it's not clear what can be done to work around this short
| of
| disabling the match checker for the entire module, as suggested by
| mpickering. Ideally we would have some notion of local warning flags
| (e.g. #602), allowing us to disable this (rather expensive) warning for
| generated code. Sadly, we aren't there yet. Perhaps you would like to
| propose such a mechanism?
| Cheers,
| - Ben

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