[erlang-questions] atomic ets access sequences

Ulf Wiger (TN/EAB) <>
Mon Jun 4 22:37:35 CEST 2007


Hi Thomas,

Thanks for the response.

I will admit that I also favoured the idea of 
extending the match spec processing, but I 
discarded it because I couldn't easily come to
grips with PAM (which I understood to stand for
"Patrik's Abstract Machine", Patrik being Patrik
Nyblom.) It was a lot easier to tackle the 
erl_db.c module, but I of course ran into the very
problems you mention - feature creep and also
less-than-stellar performance.

So I couldn't agree more, including the syntax
issue. I've previously wished for the addition 
of 'let' and 'subterm' to the match specs to 
allow for controlled recursion through lists 
or tuple structures (or binaries perhaps) in
a match spec, but I got stuck on the same hurdle 
(PAM) then.

You, being a die-hard compiler writer, could perhaps
sacrifice the Summer holidays and make ms_transform
more powerful? ;-) It would be a truly good deed.

BR,
Ulf W

> -----Original Message-----
> From: Thomas Lindgren [mailto:] 
> Sent: den 4 juni 2007 19:48
> To: Ulf Wiger (TN/EAB); 
> Subject: Re: [erlang-questions] atomic ets access sequences
> 
> 
> This was something I intended to answer in detail, but for 
> some reason it got stuck in the TODO-list. Anyway, here goes 
> a short comment:
> 
> I have found that I usually want to do a short sequence of 
> operations to a single key, for example "lookup and delete if 
> exists, returning the deleted tuple(s)". This is much less 
> general than a transaction, but it still should be atomic.
> 
> It's difficult to specify what operations should be permitted 
> beforehand, so the solution should be fairly general. The 
> main problem with a general approach is, I suppose, feature 
> creep: if we permit, say, ets:atomic(Fun, Key), then we 
> should restrict Fun to something that cannot hog the Key 
> forever (among other things).
> 
> So your proposal really makes me think not of a new set of C 
> BIFs, but of extending the existing mini-language of match 
> specifications to also permit some set of atomic operations 
> inside them. I'm not sure how far that can or should be 
> taken. (Would the end result even look like a match spec?)
> 
> (By the way, one big candidate for cleanup in an "Erlang 
> Redux" would surely be to get rid of the awful pseudo-syntax 
> trees for match specifications and make fun2ms or the 
> equivalent the default.)
> 
> Best,
> Thomas
> 
> --- "Ulf Wiger (TN/EAB)" <>
> wrote:
> 
> > 
> > After bashing ets, I will now venture to try to make it 
> even dirtier 
> > than it already is. (:
> > 
> > I've put together a BIF in erl_db.c (the ets "module"), 
> called seq/3. 
> > Rather than posting the C code (which is certainly 
> horrific, as I am 
> > no C programmer; it is also pretty bulky), I've written a 
> reasonably 
> > equivalent Erlang function (attached).
> > 
> > The main difference between the two, except for the C version being 
> > faster, is that the C version is atomic. It allows for some 
> > rudimentary lookups and selects, followed by updates (insert  
> > <<eets.erl>> and delete), provided that the preconditions are met.
> > 
> > 
> > I've so far only written one simple example: 
> > increment(Tab,Key,Incr),
> > which increments a counter object if it exists, or creates 
> it with a 
> > reasonable default (Incr), if it doesn't. This is what it 
> looks like - 
> > ugly, but I think the main ugliness lies in the unavoidable select 
> > patterns:
> > 
> > increment(Tab, Key, Incr) ->
> >   Pat = [{ {Key, '$1'}, [],
> >          [{{ Key, {'+','$1',Incr} }}] }],
> >   seq(Tab, [{1, bind, Pat}],
> >       [{foreach, 1, insert},
> >        {if_nil,  1, [{insert, {Key, Incr}}]}]).
> > 
> > (Explanation: The call is seq(Tab, Preconditions,
> > Actions)
> > The {1, bind, Pat} calls ets:select(Tab, Pat) and saves the 
> result in 
> > a numbered slot (there are 10 of them).
> > The foreach instruction takes each object in slot 1 and - 
> in this case 
> > - calls ets:insert(Tab, Obj); the
> > 
> > if_nil instruction is run if slot 1 contains []. It then 
> initializes 
> > the counter object.)
> > 
> > 
> > Here's where you get to provide feedback, shout heresy!!
> > or perhaps ask to get the C code an help whip it into 
> shape. Some of 
> > you may have good suggestions on how to make it more 
> intuitive. I've 
> > tried to limit myself to actions that are unlikely to 
> crash, once the 
> > preconditions are met.
> > 
> > The biggest gripe of my own is that the function cannot be made to 
> > return anything other than true or false, but I've 
> saturated my quota 
> > for prototyping in C for now.
> > 
> > One use I had in mind is the extended process registry, which is 
> > currently dependent on sending all registrations to a gen_server. 
> > Using this function, registering a unique name could look like this:
> > 
> > reg(Name) ->
> >   Pat = [{{{Name,rn,'_'}}, [], [true]}]}]
> >   seq(?REG, 
> >       [{0, select_count, Pat],
> >       [{insert, [{{Name,rn,self()}},
> >                  {{self(),n,Name}}]).
> > 
> > (The select_count pattern returns 1 if there is a reverse 
> mapping of 
> > the name Name to any pid.
> > We perform the insert action only if the select_count result is 0.)
> > 
> > In a simple test, this turned out to be less than twice as 
> fast as my 
> > Jungerl version (proc). This is a bit disappointing, but latency 
> > should be much improved, and a registry basically needs to be as 
> > lightweight as it can possibly be.
> > 
> > Another obvious use would be a sort of transaction memory, where an 
> > object is first read, then modified, and later committed, assuming 
> > that the stored object hasn't been changed by someone else:
> > 
> > commit(Tab, NewObj, OldObj) ->
> >   Pat = [{OldObj,[],[true]}],
> >   seq(Tab, [{0,select_count,Pat}],
> >       [{insert, NewObj}]).
> > 
> > Anyway, there it is. Comments?
> > 
> > BR,
> > Ulf W
> > > _______________________________________________
> > erlang-questions mailing list
> > 
> >
> http://www.erlang.org/mailman/listinfo/erlang-questions
> > 
> 
> 
> 
>        
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