On 30/08/2012 12:29, Joachim Breitner wrote:

Hi,

I am preparing a talk about the details of how data and programs look in memory in Haskell (well, GHC). When explaining the memory consumption of a large String, I wanted to show the effect of short-int-replacement that happens in http://hackage.haskell.org/trac/ghc/browser/rts/sm/Evac.c#L550

I use my ghc-heap-view-package to observe the heap. This programs shows the effect:

import GHC.HeapView import System.Mem

main = do let hallo = "hallo" mapM_ (\x -> putStrLn $ show x ++ ": " ++ show (asBox x)) hallo performGC mapM_ (\x -> putStrLn $ show x ++ ": " ++ show (asBox x)) hallo

gives, as expected:

$ ./SmallChar 'h': 0x00007f2811e042a8/1 'a': 0x00007f2811e08128/1 'l': 0x00007f2811e09ef0/1 'l': 0x00007f2811e0bcd8/1 'o': 0x00007f2811e0db10/1 'h': 0x00000000006d9bd0/1 'a': 0x00000000006d9b60/1 'l': 0x00000000006d9c10/1 'l': 0x00000000006d9c10/1 'o': 0x00000000006d9c40/1

but in GHCi, it does not work:

$ runhaskell SmallChar.hs 'h': 0x00007f5334623d58/1 'a': 0x00007f5334626208/1 'l': 0x00007f5334627fc0/1 'l': 0x00007f5334629dc0/1 'o': 0x00007f533462bba8/1 'h': 0x00007f533381a1c8/1 'a': 0x00007f5333672e30/1 'l': 0x00007f533381a408/1 'l': 0x00007f533381a6b8/1 'o': 0x00007f533389c5d0/1

Note that the GC does evacuate the closures, as the pointers change. Why are these not replaced by the static ones here?

Probably because GHCi has a dynamically loaded copy of the base package, so the pointer comparisons that the GC is doing do not match the dynamically-loaded I# and C# constructors. Cheers, Simon