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Re: starvation in pthread_once?
- From: Gottlob Frege <gottlobfrege at gmail dot com>
- To: Ross Johnson <rpj at callisto dot canberra dot edu dot au>, pthreads-win32 at sources dot redhat dot com
- Date: Wed, 16 Sep 2009 11:38:22 -0400
- Subject: Re: starvation in pthread_once?
- References: <OF322CFBF5.EC73CEFF-ONC1256FBE.003646C6-C1256FBE.003671FF@de.ibm.com> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
Blast from the past - whatever happened to changing call_once() to not
create the named mutex when it wasn't needed?
On Tue, Mar 22, 2005 at 1:26 PM, Gottlob Frege <firstname.lastname@example.org> wrote:
> If it comes down to it, I might vote for daisy-chaining over
> busy-looping (assuming the busy-looping is endless). ?Remember, this
> all started because the original implementation was polling/sleeping
> on 'initted' - and if the busy-looping thread is high-priority, then
> we are locked forever...
> On Tue, 22 Mar 2005 15:14:07 +1100, Ross Johnson
> <email@example.com> wrote:
>> On Mon, 2005-03-21 at 11:07 -0500, Gottlob Frege wrote:
>> > So, it doesn't seem to be getting any easier! ?*Almost* to the point
>> > where a big named mutex becomes tempting - there is a lot to be said
>> > for simplicity. ?However, my/the goal is still to at least minimize
>> > the non-contention simple init case...
>> I'm less and less tempted to use a named mutex. Perhaps there's a
>> standard technique, but AFAICS it's impossible to guarantee that the
>> name is unique across the system (and all Windows variants).
>> And I agree, minimum overhead for the uncontended case is the top
>> priority (after correct behaviour). I'm not concerned at all about speed
>> in the cancellation case.
>> > And the event is still an auto-reset, although I no longer think it
>> > really matters - I really haven't had the tenacity to think this stuff
>> > through. ?If it doesn't matter, manual-reset would be better, I think
>> > - I don't like having 1 thread relying on another thread waking it up,
>> > - for cases where the thread is killed, or strange thread priorities,
>> > etc.
>> It all looks to me like it will work. I don't recall, in the version
>> that's in pthreads-win32 now, why I included eventUsers (++/--) in what
>> you have as the __lock() sections. Maybe to save additional Atomic calls
>> (bus locks). But now I realise [in that version - not yours] that waking
>> threads can block unnecessarily when leaving the wait section.
>> It probably doesn't matter if cancel_event is auto or manual. I think
>> there will be at most one thread waiting on it. And, for 'event', like
>> you I'm uncomfortable with daisy-chaining SetEvent() calls.
>> The only problem with the alternative of using a manual-reset event is
>> that some thread/s may busy-loop for a bit until an explicit reset
>> occurs. It seems untidy, but it's probably more robust than daisy-
>> chained SetEvents given the issues you've identified above.
>> So I'm tempted to leave both events as manual-reset events. I'm also
>> guessing that this busy-looping will be extremely rare - perhaps only
>> when a new thread sneaks in to become initter, then suspends just inside
>> while the first waiter is waking and heading back to the loop start.
>> I'll run your design and let you know the results.