Possible bug in vector-cas!

[Dominik Pantůček]

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

[Gustavo Massaccesi]

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.

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