Home » Linux » Any way to reserve but not commit memory in linux?

Any way to reserve but not commit memory in linux?

Posted by: admin November 29, 2017 Leave a comment

Questions:

Windows has VirtualAlloc, which allows you to reserve a contiguous region of address space, but not actually use any physical memory. Later when you want to use it (or part of it) you call VirtualAlloc again to commit the region of previously reserved pages.

This is actually really useful, but I want to eventually port my application to linux – so I don’t want to use it if I can’t port it later. Does linux have a way to do this?

EDIT – Use Case

I’m thinking of allocating 4 GB or some such of virtual address space, but only committing it 64K at a time. This would give me a zero-copy way to grow an array up to 4 GB. Which is important, because the typical double the array size and copy introduces seemingly random unacceptable latency for very large arrays.

Answers:

mmap a special file, like /dev/zero (or use MAP_ANONYMOUS) as PROT_NONE, later use mprotect to commit.

Questions:
Answers:

You can turn this functionality on system-wide by using kernel overcommit. This is usually default setting on many distributions.

Here is the explanation http://www.mjmwired.net/kernel/Documentation/vm/overcommit-accounting

Questions:
Answers:

The Linux equivalent of VirtualAlloc() is mmap(), which provides the same behaviours. However as a commenter points out, reservation of contiguous memory is the behaviour of calls to malloc() as long as the memory is not initialized (such as by calloc(), or user code).

Questions:
Answers:

“seemingly random unacceptable latency
for very large arrays

You could also consider mlock() or mmap() + MAP_LOCKED to mitigate the impact of paging. Many CPUs support huge (aka large) pages, pages larger than 4kb. These larger pages can mitigate the impact of the TLB on streaming reads/writes.