Hi, I have a program which runs, then calls performGC multiple times. I'd expect all (or nearly all) of the memory to be freed by that point. But it isn't. Given this program: import Language.Haskell.Exts.Annotated -- from haskell-src-exts import System.Mem main :: IO () main = do let src = fromParseResult $ parseFileContents $ "data C = C {a :: F {- " ++ replicate 400000 'd' ++ " -} }" putStrLn $ "Total length is: " ++ show (length $ show $ src) performGC performGC performGC I run with: ghc --make Temp.hs -rtsopts && Temp.exe +RTS -G1 -S (the use of 1 generation is to make sure all GC's clear up everything, the leak still happens without it). I am using GHC 7.0.1 I get the output: Alloc Copied Live GC GC TOT TOT Page Flts bytes bytes bytes user elap user elap ... lots of values ... 16187336 9598808 9600428 0.05 0.06 0.47 0.66 0 0 (Gen: 0) 19513288 11571104 11572724 0.06 0.07 0.55 0.77 0 0 (Gen: 0) 23519176 13948292 13949912 0.09 0.08 0.67 0.89 0 0 (Gen: 0) Total length is: 4226 24645752 548 2091128 0.00 0.00 0.75 0.98 0 0 (Gen: 0) 20 540 2091120 0.00 0.00 0.75 0.99 0 0 (Gen: 0) 0 540 2091120 0.00 0.00 0.75 1.00 0 0 (Gen: 0) 1288 844 3028 0.00 0.00 0.75 1.01 0 0 (Gen: 0) 0 0.00 0.00 162,592,312 bytes allocated in the heap 81,004,756 bytes copied during GC 13,949,912 bytes maximum residency (30 sample(s)) 226,344 bytes maximum slop 63 MB total memory in use (6 MB lost due to fragmentation) Generation 0: 30 collections, 0 parallel, 0.48s, 0.51s elapsed INIT time 0.02s ( 0.02s elapsed) MUT time 0.25s ( 0.47s elapsed) GC time 0.48s ( 0.51s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.75s ( 1.01s elapsed) %GC time 64.6% (50.6% elapsed) Alloc rate 613,093,182 bytes per MUT second Productivity 33.3% of total user, 24.7% of total elapsed QUESTION 1: My reading of this report is that performGC leaves 2091120 of live data. However, the program seems to run a GC on termination, when only 3028 bytes of data are left. Why is all the data not released on performGC? QUESTION 2: It seems that the haskell-src-exts parser has some kind of space leak in "data C = C {a :: F {- ddddd -} }", but not in "data C = C {a :: F} {- ddddd -}". That's fair enough, but is it because it's a space leak that performGC doesn't get the data? If I change to the second form then the in use memory after performGC is about 3Kb. I found that if I did the parseFileContents/print bit once I get 2Mb of leak, if I do it twice I get 2Mb, and if I do it three times I get 1Mb of leak, which really confused me. Thanks, Neil