[erlang-questions] ETS & SMP was: Parallelism, SMP, and multicore question

[Edwin Fine]

Well, now I know that at least *one* person read my question :)

Aren't concerns about when SMP is and isn't beneficial at least
thought-provoking? Or do I need to write and publish actual code showing the
differences?

2008/9/3 Valentin Micic <valentin@REDACTED>

>  Is ETS utilizing the same locking policy for all table types (namely:
> public, protected or private), and if so, would it be possible to relax
> locking for protected and private access?
>
> We've noticed that if more than one process requires an access to the same
> ets table (in SMP environment), the system slows down considerably due to
> the locking mechanism. It is quite possible to optimize this by fronting
> such a table with a dedicated process for request serialization -- works
> better as there is always only one proccess requesting a lock. Actually... as much as this works well for one table, not so sure how would
> such an "optimization" work for a large number of tables. By relaxing (or
> not having) a locking policy for (at least) tables with a private access,
> there would be no questions about it.
>
> V.
>
>
> ----- Original Message -----
> *From:* Edwin Fine <erlang-questions_efine@REDACTED>
> *To:* erlang-questions@REDACTED
> *Sent:* Tuesday, September 02, 2008 7:09 PM
> *Subject:* [erlang-questions] Parallelism, SMP, and multicore question
>
> Since the cognoscenti are talking about parallelism, maybe someone can
> answer something that has been perplexing me.
>
> Sometimes it has been measurably faster on a multicore system to run one VM
> per core, each with SMP disabled (or +S 1), than it is to run one or more
> SMP VMs. This is true even when running code that is innately parallel. On
> the other hand, I have seen benchmarks that show near-linear speedups using
> SMP.
>
> I don't have code to support this; it's just something I, along with a
> couple of other people -- for whom I don't want to speak -- have noticed in
> passing.
>
> Does anyone have any insight into the internals of SMP who can tell me when
> SMP might be an asset, and when a liability? I know this is a "how long is a
> piece of string" question, but even though it's pretty general, someone
> somewhere who really knows Erlang inside out (i.e. not me) must surely have
> encountered this phenomenon.
>
> Are there conditions where contention between the threads of the SMP
> processes (e.g. mutexes) becomes significant enough that multiple separate
> non-SMP VMs perform better? If so, what would those conditions be?
>
> Could it have anything to do with the mix of CPU-bound processes and
> inter-process messaging or network I/O? If so, please could someone explain
> the mechanism?
>
> Anyone?
>
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