[Openmcl-devel] Decreasing the size of the reserved heap

[Tim Bradshaw]

On 8 Jan 2014, at 19:30, Gary Byers wrote:

> On most 64-bit platforms, "reserving" 512GB of contiguous address space
> costs essentially the same as reserving 1GB.  That "reserved address space"
> isn't (yet) readable or writable; there's no physical of virtual memory
> associated with it.

How does CCL do this reservation?  I understand that allocating memory which you don't touch is really free, but there is usually a question of OS book-keeping, which can matter.  In particular are you assuming that the OS allows overcommit, or doing some other kind of reservation (I think even platforms which don't allow overcommit often have mechanisms of doing soft allocations of some kind which avoid the nasties associated with overcommit)?

Sorry if this is an idiot question!