Slower Compilation on GHC 7.8.4 (vs. 7.6.3)

Simon Peyton Jones simonpj at
Mon Apr 6 07:47:40 UTC 2015

Oz, and others,

Thanks for this data point.  Two things:

·         Oz: is there any chance you could boil out a test case that highlights the regression most starkly?  It is sad to know that GHC’s compilation time is increasing, but hard to tackle.  A test case, preferably one that doesn’t rely on zillions of other libraries, increases the chances of something getting done by a factor of 10 or 100.

Do make a ticket for it as well.  Otherwise it just gets lost.

·         Everyone.  It’s really hard to keep focus on keeping GHC’s compilation time and space down.  Everyone is usually focused on bugs and features.  It would be incredibly helpful if someone, or a small group, could build a profiled version of GHC 7.8, 7.10, and HEAD, and track down what is happening.  Usually there are multiple causes, but a factor of 7.5 ought not to be hard to nail down.




From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Ozgun Ataman
Sent: 05 April 2015 19:01
To: ghc-devs at
Subject: Slower Compilation on GHC 7.8.4 (vs. 7.6.3)


Apologies upfront for the long post below, but I thought our experience on this particular point might be worth sharing. I present some observations first and then mention the particular problem around "cabal repl" facilitated development workflow. Any ideas/feedback on the latter point would be much appreciated and please let me know if there are other metrics, info or output I can produce here for further investigation/clarification.

Recently at work, we upgraded a decent size project from GHC 7.6.3 to GHC 7.8.4 and as a result observed a very significant slowdown in compile times. Code changes were very minimal and should not be a factor. Any mistakes below are mine, but I can fairly confidently say that there is a visible-to-the-eye slowdown in compile times.

We made a few measurements to quantify the issue and here is the data:

  *   This project has 220 direct modules (non-dependency) that are compiled with each "cabal build" and we always use sandboxes
  *   Tests were performed on OS X, though we saw similar results on Linux
  *   Single threaded (-j=1) wall-clock compile time has gone up by around 30% for the overall project
  *   With O1 and -j=N, the overall wall-clock time is approximately the same (around 8 min), but the CPU time spent is a staggering 7.5X higher (!)
  *   With O0 and -j=N the overall wall-clock time is 40% higher, but the CPU time spent is a staggering 7.5X higher (!)
  *   With -O0, "cabal repl" load times have gone up by a staggering 2.5X (!)

     *   7.8.4: 3 min. 35 seconds
     *   7.6.3: 1 min. 2 seconds

  *   Measuring compile times for individual modules, we see that those that are heavily loaded with lots of types, TH-facilitated type and typeclass derivations and those that contain large "blobs" of values directly at the top level now take much longer to compile:

     *   We have 10 modules that each take over 10 seconds
     *   We have 3 modules that each take over 35 seconds

  *   Sidenote observation: In general, parallel builds with -j appear to cause a very significant overhead under the "system" part of timing:

     *   Example with 7.6.3, O0: cabal build  140.57s user 13.25s system 100% cpu 2:33.70 total
     *   Example with 7.8.4, O0: cabal build  507.83s user 655.43s system 549% cpu 3:31.59 total

The main hurt here has been the infeasibility of using "cabal repl / ghci"  fast-feedback style development (via emacs, vim, command-line, etc.), since:

  *   Unlike "cabal build", "cabal repl" re-compiles from scratch with every invocation ":l App.Foo.Bar", which itself is the necessary first step when starting to hack on a module (we can then use :r to somewhat help the situation)
  *   The slowdown is about 2.5X as stated above: What used to take a minute now takes 3.5 minutes.
  *   cabal repl does not seem to benefit from -j at all
  *   cabal repl 7.8.4 appears to be hurt particularly by the heavier modules
  *   The heavier modules are often at the top of the compile tree (types, derivations, etc) and are practically loaded on the critical path all the time
Let me know if I'm missing anything here and any/all feedback is much appreciated!

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