On Sat, 2006-09-23 at 13:58 +0400, Bulat Ziganshin wrote:
Hello glasgow-haskell-users,
the attached program show up the bug in mallocForeignPtrBytes (and newPinnedByteArray#) implementation - it allocates two times more memory as requested. The bug seen both on 6.6rc and june 6.4 windows builds, namely:
http://www.haskell.org/ghc/dist/current/dist/ghc-6.5.20060901-i386-unknown-m... http://www.haskell.org/ghc/dist/stable/dist/ghc-6.4.2.20060609-i386-unknown-...
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. However a byte array has some overhead: it needs one word for the heap cell header and another for the length. So if you allocate a 4k byte array then it uses 8k. So the trick is to allocate 4k - overhead. This is what the Data.ByteString library does. Duncan