[erlang-questions] '--' BIF to be used with caution?

Mikael Pettersson <>
Tue Jan 13 14:31:05 CET 2009


Malcolm Dowse writes:
 > Hello,
 > 
 > I ran into an issue with Erlang the other day. I'm not sure if it's a bug,
 > but I thought I should warn others.
 > 
 > I was using '--' to get the difference between two large lists. As expected
 > it was very slow, but, to my surprise, ERTS became completely unresponsive
 > for a long time. (Only one CPU was maxed out, and there was still plenty of
 > memory available.)
 > 
 > To reproduce (on Ubuntu 7.x, R12B-x), open an Erlang shell and type:
 > 
 > spawn(fun() -> lists:seq(1,1000000) -- lists:seq(1,1000000,2) end).
 > 
 > The unresponsiveness seems to be caused by '--' being a BIF. Is there a good
 > reason for making it a BIF if it's not always an improvement?

1) The time complexity for '--'/2 is quadratic, so you shouldn't
   expect it to be fast.
2) The unresponsiveness appears to be an implementation bug/limitation.
   BIFs have a mechanism for "yielding" the CPU when "enough work"
   has been done, in order to allow other processes to make progress,
   but the implementation of '--'/2 does not make use of that
   mechanism so it has to run to completion.



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