mikeash.com pyblog/friday-qa-2012-02-03-ring-buffers-and-mirrored-memory-part-i.html comments

a3f - 2017-03-01 15:56:29

Wed, 01 Mar 2017 15:56:29 GMT

As Some Guy said, the code doesn't have to be racy. Here's an example of using VM_FLAGS_OVERWRITE:

https://github.com/a3f/libvas/blob/master/mach/ringbuf.c

Some Guy - 2014-10-06 14:38:48

Mon, 06 Oct 2014 14:38:48 GMT

actually vm_remap has acces to VM_FLAGS_OVERWRITE and will do what you want, I just tested it does the right thing (in the 'flags' arguments) and is non racy.

Magnus Reftel - 2012-02-06 06:42:21

Mon, 06 Feb 2012 06:42:21 GMT

Instead, you either have to complicate things with multiple reads, or read into a separate buffer and then copy the memory over.

Well, you could of course also set up a scatter/gather operation via readv. Still, it's nowhere near as cool as the mirroring trick.

mikeash - 2012-02-04 02:15:01

Sat, 04 Feb 2012 02:15:01 GMT

Jean-Daniel: Nifty. It seems unfortunate that vm_remap doesn't have access to those flags. I'm not sure whether your version or mine is better or less complicated. Good to know other ways of doing it, anyway.

Rich Pollock: Having a coffee API in mach would simplify some things, certainly. Fixed the typo, thanks.

Rich Pollock - 2012-02-03 23:28:45

Fri, 03 Feb 2012 23:28:45 GMT

$article ~= s/locatino/location/

Otherwise, it sounds like passing 0 as the fifth argument of vm_remap forces it to use a fancy (if fictitious) coffee rather than choosing its own address.

Jean-Daniel - 2012-02-03 20:43:08

Fri, 03 Feb 2012 20:43:08 GMT

@Mike: You're right about mmap implemented above mach API. I dislike too having to use the filesystem for such task, and before your comment, I didn't realized mmap was based on mach.

But knowing that, I just managed to write a working mach based version using the following functions:
- vm_allocate(capacity*2, anywhere) to create a large block.
- vm_allocate(capacity, VM_FLAGS_FIXED | VM_FLAGS_OVERWRITE) to create a region in the first half of the block.
- make_memory_entry() to get an port representing this region, and vm_map(VM_FLAGS_FIXED | VM_FLAGS_OVERWRITE) to remap it in the second half of the block.

John McLaughlin - 2012-02-03 20:06:46

Fri, 03 Feb 2012 20:06:46 GMT

Thanks Mike,

As usual a very nicely done article on an interesting problem.

-John

mikeash - 2012-02-03 17:05:07

Fri, 03 Feb 2012 17:05:07 GMT

That's interesting, I'll have to see how mmap is implemented that lets it atomically replace an existing mapping, since as far as I know it's implemented on top of the mach APIs. I dislike how the POSIX version requires hitting the filesystem, but obviously that's a better choice if you can't assume the existence of mach.

Jean-Daniel - 2012-02-03 16:57:34

Fri, 03 Feb 2012 16:57:34 GMT

While using mach API is fun, it's also good to know that this trick can also be written in a more portable way using only POSIX functions (for instance mmap), and moreover it does not suffer from the race condition you have using the mach API.

An implementation using mmap can be found here:

http://en.wikipedia.org/wiki/Circular_buffer#Optimized_POSIX_Implementation