I just noticed that I/O performane on Windows without -threaded is
pretty terrible. The following program:
main = getConetnts >>= putStr
tested with a 1MB file:
$ rm cat.exe; ghc-7.0.1 --make -rtsopts cat.hs
$ ./cat <1M >/dev/null +RTS -s
<snip>
Total time 0.28s ( 0.49s elapsed)
<snip>
But if we add -threaded:
$ rm cat.exe; ghc-7.0.1 --make -rtsopts cat.hs -threaded
$ ./cat <1M >/dev/null +RTS -s
<snip>
Total time 0.05s ( 0.07s elapsed)
<snip>
Several times better. I just made a tweak in my working tree to
increase the I/O buffer sizes, and this helps quite a lot:
$ rm cat.exe; ../ghc-testing/inplace/bin/ghc-stage2.exe --make -rtsopts
cat.hs
$ ./cat <1M >/dev/null +RTS -s
<snip>
Total time 0.06s ( 0.05s elapsed)
<snip>
$ rm cat.exe; ../ghc-testing/inplace/bin/ghc-stage2.exe --make -rtsopts
cat.hs -threaded
$ ./cat <1M >/dev/null +RTS -s
<snip>
Total time 0.05s ( 0.05s elapsed)
<snip>
Why is this? I'm not entirely sure, but I suspect it is the
implementation of asynchronous I/O in the RTS that is only used on
Windows without -threaded (rts/win32/AsyncIO.c etc.). Every I/O
operation in the non-threaded RTS on Windows involves a couple of
OS-thread switches.
I'll get the buffer size fixes into 7.0.2.
Cheers,
Simon
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