[GHC] #10557: Use `+RTS -G1` for more stable residency measurements

[GHC]

#10557: Use `+RTS -G1` for more stable residency measurements
-------------------------------------+-------------------------------------
              Reporter:  thomie      |             Owner:
                  Type:  task        |            Status:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Test Suite  |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Tests that measure `peak_megabytes_allocated` and `max_bytes_used` can be
 very sensitive to small changes in, well, just about anything: order of
 command line flags, what should be irrelevant compiler patches, minor
 changes in the test file to compile.

 From `Note [residency]` in `testsuite/tests/perf/compiler/all.T`:
 {{{
 # Residency (peak_megabytes_allocated and max_bytes_used) is sensitive
 # to when the major GC runs, which makes it inherently inaccurate.
 # Sometime an innocuous change somewhere can shift things around such
 # that the samples occur at a different time, and the residency
 # appears to change (up or down) when the underlying profile hasn't
 # really changed.
 }}}

 I propose setting `+RTS -G1` for these tests. This seems to make them more
 robust to small changes, as can be seen in the following two graphs,
 adapted from the test for #9675:

 https://ghc.haskell.org/trac/ghc/attachment/ticket/9675/T9675-peak_megabytes_allocated.png
 https://ghc.haskell.org/trac/ghc/attachment/ticket/9675/T9675-max_bytes_used.png

 This shows the `peak_megabytes_allocated` and `max_bytes_used` as a
 function of the number fields when compiling a single record:
 {{{
 data Foo = Foo
    { field1 :: Int -> Int
    , field2 :: Int -> Int
    ...
 }}}

 The default `GHC -O` line is wiggling up and down, while the `GHC -O +RTS
 -G1` line is nice and smooth, which is what we want.

 I don't know if making this change defeats the purpose of some tests,
 which is why I'm opening this ticket.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10557>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler