Arc Forum | No problem here. I repeated the 'repeat loop 3 times. $ lsb

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5 points by palsecam 5411 days ago | link | parent

No problem here. I repeated the 'repeat loop 3 times.

  $ lsb_release -a
  [...]
  Description:	Ubuntu 9.10
  Release:	9.10
  Codename:	karmic
  $ mzscheme -v
  Welcome to MzScheme v4.2.1 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.

But bugs related to atomicity exist anyway, this is certain. The thread/atomicity stuff is a subtle mess.

Threads are never a solution. Message-passing / shared-nothing threads maybe, event-based maybe, something else maybe. But traditional "à la Java" threads have prouved to be a bad idea.

Why are threads in Arc, after all? For srv.arc. Which would do better w/ an event-based architecture.

And the GIL. Gosh. Python is learning the hard way how a GIL is painful and should probably be avoided from the start.

BTW:

  arc> (= var 0  tbl (obj var 0))
  #hash((var . 0))
  arc> (repeat 50000 (thread (++ var) (++ tbl!var)))
  nil
  arc> var
  50000
  arc> tbl!var
  50000
  ; OK, the above is normal and expected
  ; Now, let's sleep for a random time in each thread before to '++
  arc> (= var 0  tbl (obj var 0))
  #hash((var . 0))
  arc> (repeat 50000 (thread (sleep:/ (rand 40) (inc:rand 50)) (++ var) (++ tbl!var)))
  nil
  arc> var
  49817
  arc> tbl!var
  50047
  ; WTF?!

Huh I'm surprised, I knew 'assign wasn't atomic, but I thought it was OK for 'sref ('++ expands to '= which expands to a call to 'sref but in a 'atwith expression). Seems not. Is my test bogus somehow?

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Some people, when confronted with a problem, think "I know, I will use threads." Now they have two problems.

1 point by garply 5408 days ago | link

Died for me on Arch Linux too:

Also, I second the notion that shared-memory threads are a bad idea. I really like Termite's message-passing model.

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1 point by aw 5408 days ago | link

Died for me on Arch Linux too

Which test were you running, akkartik's or palsecam's?

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1 point by garply 5408 days ago | link

akkartik's code died on me with the first time I ran it, but I haven't been able to reproduce it.

This is what happens with palsecam's code:

arc> (= var 0 tbl (obj var 0))

#hash((var . 0))

arc> (repeat 50000 (thread (++ var) (++ tbl!var)))

nil

arc> var

49999

arc> tbl!var

50000

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1 point by palsecam 5408 days ago | link

  arc> var
  49999

is strange, but the code you tried is not the one demonstarted a bug. You should 'sleep in the threads (i.e: the second example in my comment).

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1 point by garply 5407 days ago | link

When I do the second example, I get results similar to yours.

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1 point by akkartik 5409 days ago | link

Did you really run that over 10 hours?

Update: I tried your test with lower sleep intervals and didn't see the error (sleep:/ (rand 40) (inc:rand 5000))

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2 points by palsecam 5409 days ago | link

> Did you really run that over 10 hours?

I never say I did.

  arc> (= var 0  tbl (obj var 0))
  #hash((var . 0))
  arc> (time:repeat 50000 (thread (sleep:/ (rand 40) (inc:rand 50)) (++ var) (++ tbl!var)))
  time: 9173 msec.  ; <-- not 10 hours...
  nil
  arc> var
  48152
  arc> tbl!var
  32039

> I tried your test with lower sleep intervals and didn't see the error

Even w/ lower intervals, it is buggy on my computer:

  arc> (= var 0  tbl (obj var 0))
  #hash((var . 0))
  arc> (time:repeat 50000 (thread (sleep:/ (rand 40) (inc:rand 5000)) (++ var) (++ tbl!var)))
  time: 5391 msec.
  nil
  arc> tbl!var
  49849
  arc> var
  49859

(Running on the Ubuntu/MzScheme combo described in previous comment, and on plain vanilla Arc 3.1 (ycombinator.com/arc/arc3.1.tar). Runned ~10 times, each time the results are different but never 50000)

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1 point by akkartik 5409 days ago | link

Yes, reproduced on ubuntu/mz4.2.4 (sorry I was making stupid mistakes last night when I tried it out). On snow leopard/mz4.2.2 the tbl!var is always at 50k, but the global var is always lower. I've seen it as low as 43866.

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1 point by akkartik 5411 days ago | link

Interesting. But there's no threads in the queue issue :(

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1 point by palsecam 5411 days ago | link

yes I know, my reply was actually more about the comment in arc.arc, sorry :-)

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