[libdispatch-dev] testing tree now ported/automaked and committed

Robert Watson robert at fledge.watson.org
Thu Nov 5 02:24:11 PST 2009

On Thu, 5 Nov 2009, Paolo Bonzini wrote:

>> There are arguments for/against both approaches, but I was pondering adding 
>> a non-portable POSIX semaphore extension to FreeBSD supporting relative 
>> timeouts to avoid additional clock reads to convert between the two bases.
> Computing relative timeouts from absolute timeouts is problematic because 
> you never know if the process can be scheduled between the computation of 
> the delay and the actual invocation of the wait. Computing absolute timeouts 
> from relative timeouts, instead, never has this problem.
> Since time-fetching syscalls usually have a fastpath on modern OSes, it 
> would be nice if libdispatch provided a way to specify an absolute timeout.

The libdispatch API and time representation seem to get this fairly right -- 
dispatch_time_t is able to internally represent the difference between a 
relative time and an absolute time.

Robert N M Watson
Computer Laboratory
University of Cambridge

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