Re: [LAD] Jack midi sequencer, threading and shared state

Hi Tim,

thanks, that was very helpful. Unfortunately, I am only now finally
getting back to looking into this again.

On Tue, Aug 27, 2013 at 11:50 AM, Tim Goetze <tim@email-addr-hidden> wrote:
> It's been a while since I did some research: IIrc on i386, 32-bit read
> or write access is atomic if the variable address is 32-bit aligned,
> while on ARM only byte-size read and write operations are atomic,
> regardless of alignment. (On a sidenote, it is not realistic to
> expect gcc to honour alignment instructions.)

I take from this that it's generally not safe to assume that any read
or write operation is atomic, because I can't reliably control
alignment. Indeed, this is the general impression I get from reading
and searching on the web (mostly StackOverflow).

I stumbled upon this superb talk by Herb Sutter [1], which really made
things a lot clearer to me. My understanding now is that: 1. Atomicity
(meaning atomic read and write) in and by itself is not enough. I also
need to ensure that memory access does not get reordered across my
atomic reads and writes (so that, for example, I don't happen to
increase the write counter before the data is actually written to the
buffer). 2. To ensure atomicity and to enforce these memory barriers I
always need some (cpu/architecture specific) special instructions.

It looks like C++11 actually now has support for atomic operations
which fulfill the above two points, so I'll likely try to go down that
route. Does anybody have experience with the new C++ atomic header?

> For passing data between threads locklessly in a deterministic manner,
> I'd recommend a FIFO, which you can build with byte-sized counters.

I am using two queues, one to pass messages from the gui to the
engine, and one the other way around. Still, for largish data
structures it does not seem practical to copy them back and forth
between the threads so I need to be careful how I update these data
structures.

Cheers,
Burkhard

[1] http://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-2012-Herb-Sutter-atomic-Weapons-1-of-2
_______________________________________________
Linux-audio-dev mailing list
Linux-audio-dev@email-addr-hidden
http://lists.linuxaudio.org/listinfo/linux-audio-dev
Received on Tue Sep 17 12:15:02 2013