Hello,

  I’ve been getting inconsistent results as well. A while ago I made a 
benchmark based on a parallel spectral norm computation. The benchmark works 
fine on Windows on most systems and uses all cores, but crashes randomly on 
other systems. I haven’t been able to figure out why. On Linux it doesn’t seem 
to use more than one core. I’d be interested to know if this is related. Here’s 
the benchmark code :

https://github.com/DexterLagan/benchmark

Dex

> On May 2, 2020, at 1:56 PM, Dominik Pantůček <dominik.pantu...@trustica.cz> 
> wrote:
> 
> Hello fellow Racketeers,
> 
> during my research into how Racket can be used as generic software
> rendering platform, I've hit some limits of Racket's (native) thread
> handling. Once I started getting SIGSEGVs, I strongly suspected I am
> doing too much unsafe operations - and to be honest, that was true.
> There was one off-by-one memory access :).
> 
> But that was easy to resolve - I just switched to safe/contracted
> versions of everything and found and fixed the bug. But I still got
> occasional SIGSEGV. So I dug even deeper (during last two months I've
> read most of the JIT inlining code) than before and noticed that the
> crashes disappear when I refrain from calling bytes-set! in parallel
> using futures.
> 
> So I started creating a minimal-crashing-example. At first, I failed
> miserably. Just filling a byte array over and over again, I was unable
> to reproduce the crash. But then I realized, that in my application,
> threads come to play and that might be the case. And suddenly, creating
> MCE was really easy:
> 
> Create new eventspace using parameterize/make-eventspace, put the actual
> code in application thread (thread ...) and make the main thread wait
> for this application thread using thread-wait. Before starting the
> application thread, I create a simple window, bitmap and a canvas, that
> I keep redrawing using refresh-now after each iteration. Funny thing is,
> now it keeps crashing even without actually modifying the bitmap in
> question. All I need to do is to mess with some byte array in 8 threads.
> Sometimes it takes a minute on my computer before it crashes, sometimes
> it needs more, but it eventually crashes pretty consistently.
> 
> And it is just 60 lines of code:
> 
> #lang racket/gui
> 
> (require racket/future racket/fixnum racket/cmdline)
> 
> (define width 800)
> (define height 600)
> 
> (define framebuffer (make-fxvector (* width height)))
> (define pixels (make-bytes (* width height 4)))
> 
> (define max-depth 0)
> 
> (command-line
> #:once-each
> (("-d" "--depth") d "Futures binary partitioning depth" (set! max-depth
> (string->number d))))
> 
> (file-stream-buffer-mode (current-output-port) 'none)
> 
> (parameterize ((current-eventspace (make-eventspace)))
>  (define win (new frame%
>                   (label "test")
>                   (width width)
>                   (height height)))
>  (define bmp (make-bitmap width height))
>  (define canvas (new canvas%
>                      (parent win)
>                      (paint-callback
>                       (λ (c dc)
>                         (send dc draw-bitmap bmp 0 0)))
>                      ))
> 
>  (define (single-run)
>    (define (do-bflip start end (depth 0))
>      (cond ((fx< depth max-depth)
>             (define cnt (fx- end start))
>             (define cnt2 (fxrshift cnt 1))
>             (define mid (fx+ start cnt2))
>             (let ((f (future
>                       (λ ()
>                         (do-bflip start mid (fx+ depth 1))))))
>               (do-bflip mid end (fx+ depth 1))
>               (touch f)))
>            (else
>             (for ((i (in-range start end)))
>               (define c (fxvector-ref framebuffer i))
>               (bytes-set! pixels (+ (* i 4) 0) #xff)
>               (bytes-set! pixels (+ (* i 4) 1) (fxand (fxrshift c 16)
> #xff))
>               (bytes-set! pixels (+ (* i 4) 2) (fxand (fxrshift c 8) #xff))
>               (bytes-set! pixels (+ (* i 4) 3) (fxand c #xff))))))
>    (do-bflip 0 (* width height))
>    (send canvas refresh-now))
> (send win show #t)
> 
>  (define appthread
>    (thread
>     (λ ()
>       (let loop ()
>         (single-run)
>         (loop)))))
>  (thread-wait appthread))
> 
> Note: the code is deliberately de-optimized to highlight the problem.
> Not even mentioning CPU cache coherence here....
> 
> Running this from command-line, I can adjust the number of threads.
> Running with 8 threads:
> 
> $ time racket crash.rkt -d 3
> SIGSEGV MAPERR si_code 1 fault on addr (nil)
> Aborted (core dumped)
> 
> real    1m18,162s
> user    7m11,936s
> sys    0m3,832s
> $ time racket crash.rkt -d 3
> SIGSEGV MAPERR si_code 1 fault on addr (nil)
> Aborted (core dumped)
> 
> real    3m44,005s
> user    20m10,920s
> sys    0m11,702s
> $ time racket crash.rkt -d 3
> SIGSEGV MAPERR si_code 1 fault on addr (nil)
> Aborted (core dumped)
> 
> real    2m1,650s
> user    10m58,392s
> sys    0m6,445s
> $ time racket crash.rkt -d 3
> SIGSEGV MAPERR si_code 1 fault on addr (nil)
> Aborted (core dumped)
> 
> real    8m8,666s
> user    45m52,359s
> sys    0m25,184s
> $
> 
> With 4 threads it didn't crash even after quite some time:
> 
> $ time racket crash.rkt -d 2
> ^Cuser break
>  context...:
>   "crash.rkt": [running body]
>   temp35_0
>   for-loop
>   run-module-instance!
>   perform-require!
> 
> real    20m18,706s
> user    61m38,546s
> sys    0m22,719s
> $
> 
> 
> I'll re-run the 4-thread test overnight.
> 
> What would be the best approach to debugging this issue? I assume I'll
> load the racket binary in gdb and see the stack traces at the moment of
> the crash, but that won't reveal the source of the problem (judging
> based on my previous experience of debugging heavily multi-threaded
> applications). Also I probably need a build with debugging symbols,
> which is my plan for this afternoon.
> 
> I am running this on:
> 
> model name    : Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
> 
> HT is enabled.
> 
> Although this is just a side project, my work (that is the paid-for
> work) relies heavily on futures and GUI, so I would really like to nail
> down and fix this problem.
> 
> Any suggestions are welcome.
> 
> 
> Cheers,
> Dominik
> 
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