[GHC] #9696: readRawBufferPtr and writeRawBufferPtr allocate memory

16 Oct 2014

      #9696: readRawBufferPtr and writeRawBufferPtr allocate memory
-------------------------------------+-------------------------------------
       Reporter:  mergeconflict      |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  x86_64 (amd64)     |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Runtime
     Blocked By:                     |  performance bug
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 I initially filed this as a [http://stackoverflow.com/questions/26333815
 /why-do-hgetbuf-hputbuf-etc-allocate-memory question on StackOverflow],
 assuming that the behavior I'm seeing was intentional, or that I was
 misinterpreting the profiler results... but Kazu Yamamoto
 [https://twitter.com/kazu_yamamoto/status/522583112249663488 suggested] I
 should file this as an issue here. It's my first GHC ticket, so here goes:

 {{{#!hs
 main :: IO ()
 main = do
   buf <- mallocArray 1
   echo buf

 echo :: Ptr Word8 -> IO ()
 echo buf = forever $ do
   len <- readRawBufferPtr "read" stdin buf 0 1
   writeRawBufferPtr "write" stdout buf 0 (fromIntegral len)
 }}}

 I expect the only heap allocation here should be my explicit
 `mallocArray`, but profiling with `+RTS -P` indicates this isn't the case:
 both the read and write operations do appear to allocate some short-lived
 heap objects. In contrast:

 {{{#!hs
 echo :: Ptr Word8 -> IO ()
 echo buf = forever $ do
   threadWaitRead $ Fd 0
   len <- c_read 0 buf 1
   c_write 1 buf (fromIntegral len)
   yield
 }}}

 does not appear to allocate.

 I did a bit of digging (copying bits of source code from `base` into my
 own project, to get cost center annotations) and it seems like the
 allocation might be happening in `throwErrnoIfRetryMayBlock`. See
 [https://gist.github.com/anonymous/3ba3cfa118e1d05870d4 this gist] for
 more detail, including core output from `-ddump-simpl`.

 In all honesty, I don't know whether this is a bug. I was just surprised
 by it, when I first encountered it using `hGetBuf` / `hPutBuf`, so I'm
 trying to understand whether this is expected behavior and why.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9696
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