[GHC] #8835: 7.6.3 vs 7.8-RC performance regression

2 Mar 2014

      #8835: 7.6.3 vs 7.8-RC performance regression
------------------------------+--------------------------------------------
       Reporter:  cbm80       |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Compiler    |           Version:  7.8.1-rc2
       Keywords:              |  Operating System:  Linux
   Architecture:  x86_64      |   Type of failure:  Runtime performance bug
  (amd64)                     |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
------------------------------+--------------------------------------------
 '''NOTE: code for this is on github [https://github.com/cbm80/Minimal]'''

 I have a program that uses xml-conduit to parse nzb files, a file format
 commonly found on Usenet.

 Running the program

 `./mini demo1.nzb +RTS -sstderr`

 compiled with GHC 7.6.3 yields:

 {{{
     37,827,736 bytes allocated in the heap
        703,392 bytes copied during GC
        172,040 bytes maximum residency (2 sample(s))
         35,024 bytes maximum slop
              2 MB total memory in use (0 MB lost due to fragmentation)

                                   Tot time (elapsed)  Avg pause  Max pause
     Gen  0        71 colls,     0 par    0.00s    0.00s     0.0000s
 0.0001s
     Gen  1         2 colls,     0 par    0.00s    0.00s     0.0003s
 0.0005s

     TASKS: 3 (1 bound, 2 peak workers (2 total), using -N1)

     SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

     INIT    time    0.00s  (  0.00s elapsed)
     MUT     time    0.01s  (  0.01s elapsed)
     GC      time    0.00s  (  0.00s elapsed)
     EXIT    time    0.00s  (  0.00s elapsed)
     Total   time    0.02s  (  0.02s elapsed)

     Alloc rate    2,603,976,578 bytes per MUT second

     Productivity  84.1% of total user, 86.9% of total elapsed
 }}}

 The same program, same libraries but compiled with `GHC 7.8.0.20140228`
 gives:

 {{{
     11,217,541,448 bytes allocated in the heap
         31,184,656 bytes copied during GC
          6,540,288 bytes maximum residency (178 sample(s))
            597,248 bytes maximum slop
                 19 MB total memory in use (5 MB lost due to fragmentation)

                                       Tot time (elapsed)  Avg pause  Max
 pause
     Gen  0     20623 colls,     0 par    0.50s    0.41s     0.0000s
 0.0008s
     Gen  1       178 colls,     0 par    0.04s    0.05s     0.0003s
 0.0009s'''

     TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1)

     SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

     INIT    time    0.00s  (  0.00s elapsed)
     MUT     time    3.41s  (  3.50s elapsed)
     GC      time    0.55s  (  0.45s elapsed)
     EXIT    time    0.00s  (  0.00s elapsed)
     Total   time    3.96s  (  3.96s elapsed)

     Alloc rate    3,293,236,224 bytes per MUT second

     Productivity  86.1% of total user, 86.1% of total elapsed
 }}}

 While total memory usage always stays around 2 to 3 mb when compiled with
 GHC 7.6.3, even if using bigger nzb files (try demo2.nzb), the 7.8-RC
 compiled program eats huge amounts of memory and becomes very slow. Using
 +RTS -S shows even more worrying GC differences.

 Unfortunately I can't tell whether the culprit is in one of the libraries
 or in the compiler.

 GHC 7.8-RC used was `ghc-7.8.0.20140228-x86_64-unknown-linux-deb7.tar.xz`
 from [http://www.haskell.org/ghc/dist/7.8.1-rc2]

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8835
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