<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=us-ascii"><meta name=Generator content="Microsoft Word 12 (filtered medium)"><style><!--
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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I did not look at your code, but given your description of the symptoms, and that you have 2 cores working on 3 sublists, and that you observe one of those cores pegging for a few seconds after the other one completes, one hypothesis worth testing is that only 2 of the 3 sublists are complete and that the third is still processing (on one core).<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> erlang-questions-bounces@erlang.org [mailto:erlang-questions-bounces@erlang.org] <b>On Behalf Of </b>Ashley Holman<br><b>Sent:</b> Thursday, August 09, 2012 8:33 AM<br><b>To:</b> erlang-questions@erlang.org<br><b>Subject:</b> [erlang-questions] Execution delays<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Hi,<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>(code attached)<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I've just began learning Erlang over the last few days and so far a really enjoyed programming in it. As a learning exercise, I decided to implement the classic "factorial" function: fac(N) = N*fac(N-1); F(0) = 1. For calculating large factorials, this takes a long time (fac(200000) takes 42 secs on my laptop).<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>So, I made some optimisations (using condense_terms/1 and shuffle/1 - but you can basically ignore those). However, it still was only using 100% of 1 cpu core, and I have 2 cores, so this was a good opportunity to learn an important feature of Erlang, spawning processes! I partitioned the list into 3 sublists and spawned a process to multiply each sublist. eg. [pseudo code] Proc1([1,2,3]) * Proc2([4, 5, 6]) * Proc3(7, 8, 9) should give me the result of fac(9) using 3 processes.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>This appears to be working but I'm completely confused about certain delays in execution I'm seeing - I'm not sure where it is spending some of its time in the code. I put in some debugging, and it appears that the child processes (fac_multi_actor) are finishing their calculations and returning the results, but the receiver (fac_multi_collect) doesn't return for several seconds later. <o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>The whole point of this was to try to optimise it, so I would like to know where these few seconds are going (~30% of execution time).<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>fac:fac(200000) is taking ~9.5s using the three processes, but based on the debugging it looks like it should be completing in 6. Something else hangs around using 100% of one core for several seconds later. I've attached the code in case anyone is able to take a look at this behaviour.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>PS. I realise it will take some effort for someone to actually bother reading the code and trying to reproduce my problem, so no problem if I don't get any answers. In case anyone feels like helping though it would be much appreciated because I'm a bit confused at the moment!<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks<o:p></o:p></p></div><div><p class=MsoNormal>Ash<o:p></o:p></p></div></div></div></div></body></html>