[GHC] #12848: Reduce long-term memory usage of GHCi

[GHC]

#12848: Reduce long-term memory usage of GHCi
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           Reporter:  arybczak         |             Owner:
               Type:  feature request  |            Status:  new
           Priority:  normal           |         Milestone:
          Component:  GHCi             |           Version:  8.0.1
           Keywords:                   |  Operating System:  Linux
       Architecture:  x86_64 (amd64)   |   Type of failure:  Other
          Test Case:                   |        Blocked By:
           Blocking:                   |   Related Tickets:
Differential Rev(s):                   |         Wiki Page:
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 If you take moderately sized Haskell project (let's use lens as an
 example) and decide to work with it using `cabal repl`, memory usage of
 GHCi becomes a bit problematic after a few code reloads.

 I compared 7.10.3 and 8.0.1 using the following steps:

 1. Start GHCi with `cabal repl`.
 2. Wait for all modules to load, check memory usage with `htop`.
 3. Issue `find . -name '*.hs' -exec touch {} +` in src directory.
 4. Issue `:r` in GHCi so it reloads all modules.
 5. Repeat (3) and (4) four more times.

 Results:

 '''GHC 7.10.3'''

 * After `cabal repl`: 874MB
 * After reloading all modules (1st): 1304MB
 * After reloading all modules (2nd): 1763MB
 * After reloading all modules (3rd): 2216MB
 * After reloading all modules (4th): 2672MB
 * After reloading all modules (5th): 3121MB

 Basically, every time modules are reloaded memory usage increases by
 ~450MB, which is definitely not fun.

 '''GHC 8.0.1'''

 * After `cabal repl`: 1322MB
 * After reloading all modules (1st): 2144MB
 * After reloading all modules (2nd): 2201MB
 * After reloading all modules (3rd): 2208MB
 * After reloading all modules (4th): 2212MB
 * After reloading all modules (5th): 2213MB

 Here the initial memory usage is much higher as with 7.10.3 and there is a
 sharp increase after first full reload, but then it more or less plateaus.

 I presume this change is the result of
 6ae616f5be1db6da8bc0c5e36736a76cfea46844 - upfront memory usage is
 probably higher due to more things being forced (and the fact that memory
 usage doesn't increase indefinitely is great).

 However, I'm still concerned about the spike after the first reload -
 would it be possible to get rid of (or at least decrease) it?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12848>
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