Does your computer have exactly 8 cores (or subcores or whatever is the
official name of that hardware feaure)?
What happens if you use 7 futures instead of 8?
What happens if you use 9 (or 100) futures instead of 8?
Gustavo
PS: I don't know the low level details of the implementation of the
futures, but my guess is that in your example there is a main thread(?) and
8 futures, so you are running 9 things in 8 cores, and there is a race
condition and if you are unlucky it can cause a the program to block.
On Sat, May 30, 2020 at 1:12 PM Dominik Pantůček <
dominik.pantu...@trustica.cz> wrote:
> Hello Racketeers,
>
> it didn't take long before I hit another strange behavior when
> extensively using futures. The setup is relatively simple: I am
> processing a large fxvector/flvector and I am processing it on a
> line-by-line basis (the vectors themselves are width*height in nature).
> To avoid races I setup a (make-vector height #f) to serve as a vector of
> locks. Locking is simple:
>
> (let lloop ()
> (when (not (vector-cas! locks y #f #t))
> (lloop)))
>
> .. do something ...
>
> (vector-set! locks y #f)
>
> Of course, the code in question is in 8 futures on my CPU and they are
> scheduled in parallel pretty consistently.
>
> Most of the time it works fine, but sometimes it just hangs with
> multiple CPU cores being utilized at 100% (I assume they are in the
> busy-wait "lloop").
>
> The problem is I cannot isolate a minimal working example from that. The
> code in question is about 3000 LoC and the issue expresses itself only
> in about 1 in 30 times just after the program has started. The GUI
> thread is running and there are lots of flonum operations and lots of
> thrown away flonum results (meaning GC gets triggered quite often).
>
> Also I cannot trigger the same issue with unsafe-vector*-cas! - I'll
> look into the differences in the source.
>
> I see this with latest Racket 3m.
>
> Any suggestions how to approach finding the source of the problem are -
> as always - very welcome.
>
>
> Cheers,
> Dominik
>
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