GHC's great space profiling tools don't appear to be much help when your
leaked memory is stored in references (IORefs, StablePtrs, etc). I had
a real-life case where the allocation profile showed me where the leaked
data came from, and I could guess that it was being held by some
reference, but I couldn't tell which one. Retainer set profiling showed
a big suspicious entry for "SYSTEM", but I couldn't find any way to
pinpoint the problem. (It ended up being a missing freeStablePtr in
hsgnutls, found by code inspection.)
Here's a contrived example that just allocates a bunch of IORefs:
import Control.Monad
import Data.IORef
main = repeatM (newIORef [1,2,3])
repeatM io = liftM2 (:) io (repeatM io)
Retainer set profiling shows everything in "SYSTEM". None of the other
profiling methods say anything interesting either. What I'd like to
get, I think, is (1) your memory is being held in IORefs (2) allocated
by this cost center and (3) being retained by this cost center. I guess
I'm looking for something like a memory profiler for a traditional
language. But I haven't really thought it all out (and I don't even
understand everything the existing profiles tell me).
Are there some tricks I've missed for this sort of debugging? And
perchance would this be an interesting GHC hackathon subject?
Andrew
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