[GHC] #14929: Program compiled with -O2 exhibits much worse performance
#14929: Program compiled with -O2 exhibits much worse performance -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- A user on reddit reports that compiling the program with `-O2` makes his program use a lot more memory. https://www.reddit.com/r/haskell/comments/84qovv/an_example_of_code_where_gh... It runs in constant memory without `-O2` and leaks memory with `-O2. It seems worth investigating as the example is quite small (< 1000 lines) and self-contained. https://github.com/luispedro/TestingNGLESS -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14929 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14929: Program compiled with -O2 exhibits much worse performance -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * priority: normal => high * failure: None/Unknown => Runtime performance bug -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14929#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14929: Program compiled with -O2 exhibits much worse performance -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by George): According to https://www.reddit.com/r/haskell/comments/84qovv/an_example_of_code_where_gh... it's now down to 60 lines but I don't see any way to get those 60 lines: https://www.reddit.com/r/haskell/comments/84qovv/an_example_of_code_where_gh... Also according to that page, compiling with -fno-full-laziness is a workaround -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14929#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14929: Program compiled with -O2 exhibits much worse performance -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by sgraf): Retainer profiling shows mentions this CAF: {{{ -- RHS size: {terms: 2, types: 3, coercions: 0, joins: 0/0} lvl24_r7ue :: conduit-1.2.13:Data.Conduit.Internal.Pipe.Pipe B.ByteString B.ByteString Data.Void.Void () (ResourceT IO) () [GblId] lvl24_r7ue = scc<interpretTop> scc<parseFile> scc<interpretTop> scc<parseFile> scc<>>=> scc<>>=.\> scc<>>=.\.\> scc<>>=> scc<>>=.\> scc<>>=.\.\> scc<interpretTop> scc<parseFile> scc<>>=> scc<>>=.\> scc<>>=.\.\> tick<>>=> scc<>>=> scctick<>>=.\> countC_r7tY @ Data.Void.Void @ B.ByteString @ () lvl23_r7ud }}} `-fno-full-laziness` is a work-around, but maybe we should have a more granular way to influence float out. We probably still want to float lambdas, for example. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14929#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14929: Program compiled with -O2 exhibits much worse performance -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I have not looked at the code, but full laziness is definitely capable of increasing space usage. Consider even {{{ f xs = sum [x+n | n <- [1..], x <- xs] }}} Full laziness will turn this into {{{ ns = [1..] f xs = sum [x+n | n<-ns, x<-xs] }}} which will retain a top-level CAF whose length is the longest list ever passed to `f`. In this case it is probably better to re-generate the list `[1..]` on every call, but it's not so clear if it is `map expensive [1..]`. The OP doesn't say if the same effect happens with -O. Is there something -O2 specific going on, I wonder? Regardless * Extracting the test case and uploading it here with repro instructions would be good * More specific insight into exactly what is happening would be good -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14929#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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