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9750 :: Use multithreading without channel and threadpool

• c4UlMu (orginal) [2022-12-22T20:41:37+01:00] view original

  I noticed that I cant't use ref variable in thread even with orc (it said that with orc we have shared memory) For example

  var workers: array[10, Thread[void]]
  
  var name = "Hey"
  
  
  proc printer() {.gcsafe.} =
      echo name
  
  for i in 0..high(workers):
      createThread(workers[i], printer)
  
  joinThreads(workers)
  
  echo "Hello, World!"
  
  Run

  Compile like this nim c -r --threads:on --mm:orc main.nim

  And get such an error Error: 'printer' is not GC-safe as it accesses 'name' which is a global using GC'ed memory

  I can fix it like this

  proc printer() =
      {.cast(gcsafe).}:
          withLock myLock:
              echo name
  Run

  My question If I use {.cast(gcsafe).} and use lock around shared variable will I get safe code ? Or I still can mess reference counter of a shared variable?

  And Is it valid way of accessing shared variables? For example if I want to use hash table as cache between threads

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• ElegantBeef (orginal) [2022-12-22T21:37:11+01:00] view original

  Yes the code will be threadsafe, though there is a caveat. If you create a variable that points to a reference type you will likely want to do let myVar {.cursor.} = myRef this creates a 'weak' reference to the reference that does not cause any GC on this thread. As this thread likely does not own that it's a good thing to do.

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• c4UlMu (orginal) [2022-12-24T18:22:03+01:00] view original

  Thank you

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