2.16 Isomalloc

It is occasionally useful to allocate memory at a globally unique virtual address. This is trivial on a shared memory machine (where every address is globally unique); but more difficult on a distributed memory machine (where each node has its own separate data at address 0x80000000). Isomalloc provides a uniform interface for allocating globally unique virtual addresses.

Isomalloc can thus be thought of as a software distributed shared memory implementation; except data movement between processors is explicit (by making a subroutine call), not on demand (by taking a page fault).

Isomalloc is useful when moving highly interlinked data structures from one processor to another, because internal pointers will still point to the correct locations, even on a new processor. This is especially useful when the format of the data structure is complex or unknown, as with thread stacks.

void *CmiIsomalloc(int size)
Allocate size bytes at a unique virtual address. Returns a pointer to the allocated region.

CmiIsomalloc makes allocations with page granularity (typically several kilobytes); so it is not recommended for small allocations.

void CmiIsomallocFree(void *doomedBlock)
Release the given block, which must have been previously returned by CmiIsomalloc. Also releases the used virtual address range, which the system may subsequently reuse.

After a CmiIsomallocFree, references to that block will likely result in a segmentation violation. It is illegal to call CmiIsomallocFree more than once on the same block.

void CmiIsomallocPup(pup_er p,void **block)
Pack/Unpack the given block. This routine can be used to move blocks across processors, save blocks to disk, or checkpoint blocks.

After unpacking, the pointer is guaranteed to have the same value that it did before packing.

int CmiIsomallocLength(void *block);
Return the length, in bytes, of this isomalloc'd block.

int CmiIsomallocInRange(void *address)
Return 1 if the given address may have been previously allocated to this processor using Isomalloc; 0 otherwise. CmiIsomallocInRange(malloc(size)) is guaranteed to be zero; CmiIsomallocInRange(CmiIsomalloc(size)) is guaranteed to be one.