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Subject: Re: [boost] [asio] Bug: Handlers execute on the wrong strand (Gavin Lambert).
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2013-10-30 21:12:03

On 31 Oct 2013 at 12:12, Gavin Lambert wrote:

> > ASIO is, once you compile it with optimisation, really a thin wrapper
> > doing a lot of mallocs and frees around Win IO completion ports. Any
> > latency spikes are surely due to either IOCP or the memory allocator
> > causing a critical section to exceed its spin count, and therefore go
> > to kernel sleep?
> No, the latency was very definitely coming from *some* instance of
> either boost::asio::detail::mutex or boost::asio::detail::static_mutex.
> I didn't trace it down any further than that.

>From what I can tell ASIO uses Win32 critical sections which come
with spin counts. If you keep every operation under the count, all is
good, else you get spikes as it drops to kernel wait.

> (As far as the memory allocator goes, I'm actually using nedmalloc --
> which I know isn't lock-free but it's pretty decent at avoiding lock
> contention. And I had that instrumented too and there were no memory
> allocation locks around the time when the latency occurred.)

I might know something about nedmalloc :). Mmm, I was just about to
suggest that nedmalloc might be doing a free space consolidation run
and that might be the cause of the spike, but if it isn't then okay.

> My application was using an io_service with 6-8 threads, and a large
> number of serial_ports each with their own io_strand and deadline_timer.
> The deadline_timer is cancelled and restarted most of the time rather
> than being allowed to expire -- but this shouldn't be unusual behaviour
> for a timeout.

It just goes to show how complex and unpredictable modern C++ has
become :( At the bleeding edge we're actually all complex systems
engineers now, not software or computer science engineers. I wish how
we as an industry interview job hire candidates reflected that (or I
wouldn't be having the problems I have had!).


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