691731 - Use mmap, not vm_allocate, on Mac

Status: RESOLVED WORKSFORME

(Core :: JavaScript Engine, defect)

Description

[Igor Bukanov]

Attachment: test C program

Currently the GC chunk allocator uses vm_allocate, not mmap, on Mac OS X, to allocate GC chunks. However on OSX 10.7 those functions are significantly slower that mmap. The attached test C program tests that via repeatedly allocating a series of 1MB chunks and then releasing them using either mmap or vm_allocate.

On Mac Mini with OS X 10.7I get for a typical run using vm_allocate:

@users-Mac-mini~/s> time ./a.out vm

real    0m0.045s
user    0m0.004s
sys     0m0.041s

With mmap numbers become:

@users-Mac-mini~/s> time ./a.out mmap

real    0m0.026s
user    0m0.003s
sys     0m0.016s

That is, the program runs 2 times faster.

If I change the chunk size from 1MB to 64K, then the number becomes:

@users-Mac-mini~/s> time ./a.out vm

real    0m3.134s
user    0m0.065s
sys     0m3.069s

@users-Mac-mini~/s> time ./a.out mmap

real    0m2.767s
user    0m0.043s
sys     0m2.717s

Which is still 13% faster.

Given this results I suggest to use mmap for chunk allocation on MAC OSX.

Comment 1

[Justin Lebar (not reading bugmail)]

Do you have any indication that the functions are the same speed on 10.{5,6}?

Comment 2

[Igor Bukanov]

(In reply to Justin Lebar [:jlebar] from comment #1)
> Do you have any indication that the functions are the same speed on 10.{5,6}?

I do not know about older MACs. I plan to prepare a patch and see how it affects Talos try server runs.

Comment 3

[Igor Bukanov]

Attachment: v1

The patch removes Mac-specific code that uses vm-allocate so now Mac uses the chunk allocation based on mmap code. In the same time the patch optimizes that to take advantage of munmap ability to release a part of existing mmap alocation. Also on Windows the code use VirtualAlloc without a commit when searching for unallocated address space where the aligned chunk can be placed.