[erlang-questions] performance vs msg queue length

Matthew Sackman <>
Wed Apr 21 00:42:54 CEST 2010


On Tue, Apr 20, 2010 at 03:25:44PM -0700, John Erickson wrote:
> These results are clearly better, but still I'm a bit surprised there is a
> noticable slowdown, especially from 100-10k.  Does anyone know the reason
> for this?

I don't *know*, but my guesses are:

1. The VM deschedules processes sending messages to processes with full
mailboxes. I don't know if this extends to a process sending a message
to itself - I would hope it didn't extend to that, but then again,
sending messages to yourself is a reasonably rare thing to do, so it's
possible they've not bothered optimising that case.

2. Allocation of a bigger mailbox. I would expect that it would probably
be a standard "everytime we run out of space, double the size of the
mailbox", but there are probably a few sensible policies for this.
malloc won't be free.

3. You are constructing ever longer lists of 'ok' atoms. Doing a
recursive loop with counter would avoid this, and so you would get down
to timings of doing the actual receives.

This list is probably in reverse order of liklihood.

Matthew


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