I added notes about this to http://haskell.org/haskellwiki/Performance/GHC S | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] | On Behalf Of Simon Marlow | Sent: 27 September 2006 13:44 | To: Duncan Coutts | Cc: glasgow-haskell-users@haskell.org | Subject: Re: bug in mallocForeignPtrBytes (both 6.4 and 6.6) | | Duncan Coutts wrote: | > On Sun, 2006-09-24 at 01:28 +0400, Bulat Ziganshin wrote: | > | >>Hello Duncan, | >> | >>Sunday, September 24, 2006, 12:22:38 AM, you wrote: | >> | >> | >>>>after program prints "40 mb allocated" look at Task Manager indication | >>>>- it shows that two times more memory actually in use. it seems that | >>>>problem is only with allocating memory buffers whose sizes are powers | >>>>of 2 or very close - bug shows for bufsize = 4kb, 1mb or 4095, but not | >>>>not for 4000 or 10^6 | >> | >>>The reason for this is that for larger objects ghc has an allocation | >>>granularity of 4k. That is it always uses a multiple of 4k bytes. | >> | >>with 512k blocks it also allocates two times more data than requested. | > | > Oh, sorry I missed that point. Something fishy is going on then. | | Duncan's explanation works for allocating 4k, but there's another explanation | for larger blocks. GHC allocates memory from the OS in units of a "megablock" | (see http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage), currently | 1Mb. So if you allocate a 1Mb array, the storage manager has to allocate 1Mb + | overhead, which will cause it to allocate a 2Mb megablock. The surplus will be | returned to the system in the form of free blocks, but if all you do is allocate | lots of 1Mb arrays, you'll waste about half the sapce because there's never | enough contiguous free space to contain another 1Mb array. Similar problem for | 512k arrays: the storage manager allocates a 1Mb block, and returns slightly | less than half of it as free blocks, so each 512k allocation takes a whole new | 1Mb block. | | Cheers, | Simon | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users