The Glasgow Haskell Users Mailing List

Joachim Breitner | 30 Aug 2012 13:29

Small Int and Char closures in GHCi

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

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Simon Marlow | 31 Aug 2012 14:14

Re: Small Int and Char closures in GHCi

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

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Joachim Breitner | 31 Aug 2012 16:10

Re: Small Int and Char closures in GHCi

Hi,

Am Freitag, den 31.08.2012, 13:14 +0100 schrieb Simon Marlow:
> > 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.

could be; I could not find a way to verify it. I tried comparing the
info pointers of a "1::Int" that I generated in ghci and one that comes
from a statically compiled module loaded into ghci, but that is probably
also linked against GHHi’s dynamically loaded base.

But if it is really the case that the RTS has different copies of base,
wouldn’t code like this then break?

          if (i == &stg_TSO_info
              || i == &stg_WHITEHOLE_info 
              || i == &stg_BLOCKING_QUEUE_CLEAN_info
              || i == &stg_BLOCKING_QUEUE_DIRTY_info) {
              copy(p,info,q,sizeofW(StgInd),gen_no);
              return;
          }
        (http://hackage.haskell.org/trac/ghc/browser/rts/sm/Evac.c#L635)

Or is it, by accident or careful design, the case that all the instances
where the RTS compares info pointers with well known pointers are not
relevant when running in GHCi?

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