[linux-audio-dev] futex vs lock-free

[Simon Jenkins]

Tom Schouten wrote:

>hi all,
>
>i'm considering to add lock free queue support to packet forth, just to
>follow the hype, and maybe also because i like the concept. but i'm
>wondering if it would make much sense to go through all the trouble with
>fast mutexes being available in 2.6
>
>in other words, how much better (apart from being more elegant) are lock
>free structures wrt a mutex approach where there is a minimal system
>penalty? (not many collisions) did anyone look into this? or have i not
>been lurking properly? ;)
>
OK, futexes are fast in the uncontested case so, with sufficiently rare 
collisions,
you expect performance to approach that of a lock-free implementation.

And in terms of overall throughput, it probably does. But in terms of 
worst case
delay when there *is* a collision things are no better. The point of the 
lock-free
approach is to minimise this worst case delay in a real-time system, so 
if this is
important to you then lock-free is the way to go (but see below).

I'm not sure if/when they'll become mainstream, but fuqueues might also be
worth a quick look. (Yes: It is an unfortunate name. Enough said.) They 
tackle
the priority inversion problems that can occur with standard 
m(/f)utexes, and
are intended for real-time use.

Simon Jenkins
(Bristol, UK)