[project @ 2005-07-12 11:38:44 by simonmar]

Add some comments on why we don't use posix_memalign() right now.

diff --git a/ghc/rts/MBlock.c b/ghc/rts/MBlock.c
index 0ce1e76..331982d 100644 (file)
--- a/ghc/rts/MBlock.c
+++ b/ghc/rts/MBlock.c
@@ -134,6 +134,16 @@ getMBlock(void)
    chunk, on the grounds that this is aligned and likely to be free.
    If it turns out that we were wrong, we have to munmap() and try
    again using the general method.
    chunk, on the grounds that this is aligned and likely to be free.
    If it turns out that we were wrong, we have to munmap() and try
    again using the general method.

+
+   Note on posix_memalign(): this interface is available on recent
+   systems and appears to provide exactly what we want.  However, it
+   turns out not to be as good as our mmap() implementation, because
+   it wastes extra space (using double the address space, in a test on
+   x86_64/Linux).  The problem seems to be that posix_memalign()
+   returns memory that can be free()'d, so the library must store
+   extra information along with the allocated block, thus messing up
+   the alignment.  Hence, we don't use posix_memalign() for now.
+

    -------------------------------------------------------------------------- */
 
 #if !defined(mingw32_HOST_OS) && !defined(cygwin32_HOST_OS)
    -------------------------------------------------------------------------- */
 
 #if !defined(mingw32_HOST_OS) && !defined(cygwin32_HOST_OS)