Re: best practices for sending data from UI to RemoteIO callback thread
Re: best practices for sending data from UI to RemoteIO callback thread
- Subject: Re: best practices for sending data from UI to RemoteIO callback thread
- From: Morgan Packard <email@hidden>
- Date: Sat, 26 Mar 2011 15:59:06 -0600
Much thanks for the clear explanation Paul.
-Morgan
On Sat, Mar 26, 2011 at 3:57 PM, Paul Davis <email@hidden> wrote:
> On Sat, Mar 26, 2011 at 5:45 PM, Morgan Packard
> <email@hidden> wrote:
>> Great. Thanks. This had occurred to me as something that I should
>> maybe be doing, but had been told that it probably wasn't necessary.
>> Specifically, I'm triggering envelopes from my UI thread, which
>> involves resetting the "current value" member variable of the
>> envelope. I had been advised that a simple assignment of a float value
>> wasn't likely to result in threading problems. But perhaps this isn't
>> safe after all.
>
> there are two separate issues.
>
> 1) atomicity of variable reads/writes
> 2) ordering
>
> on most contemporary processors, reads/writes of 32 bit values are
> atomic. that is, its not possible for any thread, on any CPU, to ever
> catch the value "in the middle" of being updated. a reader will see
> either the old value, or the new value, never something in between. i
> don't know if this is true for floating point values on the processors
> used in iOS devices.
>
> ordering: you cannot control this without using write/read barriers,
> not within a thread let alone between threads. if there are negative
> side effects of one thread seeing an "out of order" value of a shared
> variable, then you must use synchronization primitives or lock free
> data structures.
>
> however ... in audio work, there are a lot of cases that look roughly like this:
>
> while (true) {
> wait_to_be_woken_by_audioengine();
> check_various_parameters ();
> synthesize_and_process_data ();
> }
>
> if you miss a parameter change on one iteration of the loop, you'll
> see it the next time around. so for quite a few things, its acceptable
> to share a variable without using mutual exclusion or lock free data
> structures. but you have to be very, very careful about this because
> there are also a lot of things for which this will not work correctly
> (even if it works most of the time).
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