#7561: Unnecessary Heap Allocations - Slow Performance
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Reporter: wurmli | Owner:
Type: bug | Status: new
Priority: normal | Component: Build System
Version: 7.6.1 | Keywords:
Os: Linux | Architecture: x86_64 (amd64)
Failure: Runtime performance bug | Blockedby:
Blocking: | Related:
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Using the vector library operations that should in principle take place
locally and fast, are slow and build a large heap. While trying to analyse
what is going on a strange effect showed. Compiling the attached small
program with heap profiling support produced an executable that runs fast
and uses the heap as expected, whereas built without profiling support it
is slow. The effect shows on linux architectures amd64 and i386, using ghc
7.6.1 and 7.4.1, respectively.
1) With profiling support
ghc --make -rtsopts -threaded -O2 -prof -fprof-auto heapAllocVec2.hs
./heapAllocVec2 +RTS -s -RTS 3628800
produces
fromList [3628800]
667,829,536 bytes allocated in the heap
125,768 bytes copied during GC
65,560 bytes maximum residency (2 sample(s))
20,096 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
...
Total time 0.34s ( 0.35s elapsed)
2) Without profiling support
ghc --make -rtsopts -threaded -O2 heapAllocVec2.hs
./heapAllocVec2 +RTS -s -RTS 3628800
fromList [3628800]
26,098,406,816 bytes allocated in the heap
22,674,848 bytes copied during GC
47,184 bytes maximum residency (2 sample(s))
22,448 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
...
Total time 10.99s ( 11.06s elapsed)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7561>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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