[erlang-questions] Multiple ETS lookups

[Ulf Wiger]

I posted a slightly more general suggestion a while back:

http://www.erlang.org/pipermail/erlang-questions/2007-May/026440.html
http://www.erlang.org/pipermail/erlang-questions/2007-June/026998.html
http://www.erlang.org/pipermail/erlang-questions/2007-June/027004.html

The main reason for doing this, I think, would be atomicity, without
having to go either through a serializing process or mnesia.

Conceptually, using a process is a lot cleaner. The problem
(apart from forcing you to write more code) is that you double the
amount of copying needed...

BR,
Ulf W

2008/10/1 Colm Dougan <colm.dougan@REDACTED>:
> Hi list,
>
> What do people think of the idea of adding a multiple ETS lookup
> function, analogous to the SQL 'IN' operator, where you can pass a
> list of keys and you get a list of matches?
>
> Something like :
>
> 1> T0 = ets:new(testtable, [set]).
> 15
> 2> ets:insert(T0, [{foo, 10}, {bar, 20}, {baz, 30}]).
> true
> 3> ets:multi_lookup(T0, [foo, bar, baz]).
> [{foo, 10},{bar,20},{baz,30}]
> 4> ets:multi_lookup(T0, [dog, foo, cat]).
> [{foo,10}]
>
> My motivation for this suggestion is mainly performance but I think it
> is also a common enough idiom that the interface would be generally
> useful.
>
> I realize that this problem has been solved in a more general way with
> match specs but unless I'm missing something using a match spec (even
> a compiled one) to do (say) 3 lookups is slower than doing 3
> individual ets:lookup calls which defeats the point (in that case).
> Am I missing something here?  I also find the match spec interface a
> bit unwieldy for such a simple operation (although I realize it is
> designed for flexibility).
>
> In my case the multiple ets lookup would help work around the
> performance dip I got in critical sections doing intensive lookups
> when I moved from non-SMP mode to SMP mode.  I realize that relatively
> speaking the SMP locking is not that expensive unless there are lock
> conflicts but there is still a cost and when doing intensive lookups
> you definitely notice it.   Being able to "unroll" my lookups to do
> (say) 4 or 8 at once and only incur the locking overheads a quarter of
> the time would be a big win.
>
> Yeah I know people will then potentially be able to do nasty things
> like try to do too many lookups at once, but then again they could
> just not do that.
>
> Anyway, just throwing the idea out there to be shot at.
>
> Thanks,
> Colm
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