Re: Allocating memory above 4GB
- From: Mike Pall <mike-1308@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 21 Aug 2013 12:31:53 +0200
Geoff Leyland wrote:
> So far, asking mmap for memory at 4GB seems to give the next
> free space above 4GB, which is nice, but the docs suggest that's
> luck rather than subject to a guarantee.
Actually, since you're on OSX, the built-in LuaJIT allocator for
OSX/x64 depends on mmap() doing a linear search starting at the
given address (to allocate 32 bit memory). So you might as well
use it to always allocate memory above 4GB.
Which in turn explains why malloc() goes for the low 4GB when
pagezero is shrunk: mmap() with a NULL address starts looking for
free space at address zero. Duh.
[Linux mmap() starts searching at (random) higher addresses if
given NULL. That would solve your problem. Unfortunately, it does
that too when the requested address is not free -- a simple
probing approach to get 32 bit memory wouldn't work there. Ok, so
let's parse /proc/self/maps and handle race conditions ... Yay.]
--Mike
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