[erlang-questions] Slow when using chained gen_server:call's, redesign or optimize?

God Dang <>
Sun Jan 29 09:38:47 CET 2012

Thank you all for taking time to help me out. Some clarification, I've come to the conclusion that gen_server(or more precisely my design) is to blame after running multiple sessions with fprof, eprof and alot of timer:now_diff and timer:tc. For example, if I put load on the system and time my handle_call({long_operation,Data},From,State) I see that the execution time begins rise from <10ms to around ~2000ms and stabilizing there under my load. If I remove the last gen_server and instead implement it as a simple function it stabilizes around ~1000ms and removing yet another one gives me yet lower execution times.
A word on my design, a lot of the time the only reason we use a gen_server is for keeping a state and more precisely a connection. For instance a generic db backend which keeps the pid of the db connection, a cache backend which keeps the memcache connection pid, my own server that keep state. and within my handle_call({long_operation,Data},From,State) I could be doing a couple of gen_server:call's and within the database gen_server I could be doing additional gen_server call's to the cache backend. Why we've constructed it as separate gen_server's is also to be able to call it from different parts of the system like db:do_something() or cache:get().
On paper this looked like a clean and nice design but under load it starts to behave poorly.
To answer all in order:# Gianfranco Alongi 2012-01-28:> You could use the synchronized serialization to generate push-back
> behaviour from your system,
> so that you do not handle a new request before it's possible - maybe
> you are already doing this, or not.
I don' understand what this mean. Can you please clarify.
# Matthew Evans 2012-01-28> The obvious, and maybe non-OTP, answer is to hold some of this state information in a public or protected named ETS table> that your clients read from directly. A single gen_server can still own and write to that ETS table.Sounds like a smart approach.
> Another obvious answer is to provide back-pressure of some kind to prevent clients from requesting data when it is under load.I don't understand this fully.

> You might find that a particular infrequent  gen_server:call operation is taking a long time to complete causing a message queue> to suddenly grow. You might want to change such an operation from:I've done that on the first most handle_call. That's what I meant with deferring gen_server responses. I saw some speedups on the first handle_call but doing this on short lived handle_call's did see slowdowns.
# Jesper Louis Andersen 2012-01-28> This would be my first idea. Create an ETS table being protected. Writes to the table goes through the gen_server,
I like this approach, not as "beautiful" as a pure OTP-approach, but if does the trick. Hey.
# Jachym Holecek 2012-01-29> You're probably treating asynchronous things as synchronous somewhere along the path, inflicting collateral damage to concurrent users?it's basically a get_data() function where we need to do roundtrips to db, other stuff to be able to return the value. Can I do this in a asynchronous fashion? I tend to think of get's as handle_calls and sets as hanlde_cast.
Big thank you all.
> Date: Sat, 28 Jan 2012 18:13:25 -0500
> From: 
> To: 
> CC: 
> Subject: Re: [erlang-questions] Slow when using chained gen_server:call's, redesign or optimize?
> Hi,
> [Replying to multiple replies at once, all quoted text reformatted for
> readability (seems people these days can't be bothered!?).]
> [Warning: excessively nitpicky content ahead, here and there.]
> # God Dang 2012-01-28:
> > I'm creating a system where I've ended up with alot of gen_servers that
> > provides a clean interface. When I run this under load I see that the
> > gen_server:call's is becoming a bottleneck.
> You're probably treating asynchronous things as synchronous somewhere along
> the path, inflicting collateral damage to concurrent users? Synchronous
> high-level API is fine, but if you know some operation is expensive or
> depends on response from the outside world you should record request
> context in ETS (row looking something like {Req_id, Timeout, From[, ...]}
> and process requests out-of-order OR offload the actual processing to
> short lived worker processes like Matthew Evans says OR a combination
> of both OR somesuch.
> My point being that gen_server:call/N, by itself, is *very* fast in
> practice, so chances are you're doing something wrong elsewhere.
> Other (unlikely) thing: you're not sending very large data structures in
> messages, are you? That could hurt, but there are ways to address that
> too if needed.
> # Matthew Evans 2012-01-28:
> > Another obvious answer is to provide back-pressure of some kind to prevent
> > clients from requesting data when it is under load.
> On external interfaces (or for global resource usage of some sort): yes, a
> fine idea (a clear "must have", actually!); but doing this internally would
> seem excessively defensive to me, unless further justification was given.
> > You might want to change such an operation from:
> >
> > handle_call({long_operation,Data},From,State) ->
> >     Rsp = do_lengthy_operation(Data),
> >     {reply, Rsp, State};
> >
> > to:
> >
> > handle_call({long_operation,Data},From,State) ->
> >     spawn(fun() -> Rsp = do_lengthy_operation(Data), gen_server:reply(Rsp,From) end),
> >     {noreply, State};
>   1. Why do people bother introducing "one-shot" variables for trivial
>      expressions they could have inlined? Means less context to maintain
>      when reading the code...
>   2. Surely you meant proc_lib:spawn_link/X there, didn't you? SASL logs
>      and fault propagation are the reason. While there are exceptions to
>      this, they're extremely rare.
>   3. The order of arguments to gen_server:reply/2 is wrong.
> Regarding the general approach: yes, a fine idea too. Depending on what
> do_lengthy_operation/1 does putting these workers under supervisor might
> be called for.
> # Jesper Louis Andersen 2012-01-28:
> > This would be my first idea. Create an ETS table being protected.
> > Writes to the table goes through the gen_server,
> Yes, a fine idea too -- ETS is one of the less obvious cornerstones
> of Erlang programming (but don't tell "purity" fascists )... One
> detail: almost all of my ETS tables are public even when many of
> them are really treated as private or protected, reason is to keep
> high degree of runtime tweakability just in case (this might be a
> bit superstitious I admit).
> > -export([write/1, read/1]).
> > 
> > write(Obj) ->
> >   call({write, Obj}).
> > 
> > call(M) ->
> >   gen_server:call(?SERVER, M, infinity).
>   1. Abstracting trivial functionality such as call/1 above only
>      obfuscates code for precisely zero gain.
>   2. Same goes for typing "?SERVER" instead of the actual server
>      name. Using "?MODULE" is however alright, as long as it's
>      only referred to from current module (as it should).
>   3. No infinite timeouts without very good justification! You're
>      sacrificing a good default protective measure for no good
>      reason...
> > but reads happen in the calling process of the API and does not go
> > through the gen_server at all,
> > 
> > read(Key) ->
> >   case ets:lookup(?TAB, Key) of
> >     [] -> not_found;
> >     [_|_] = Objects -> {ok, Objects}
> >   end.
> (2) from above also applies to "?TAB" here. More to the point, it's
> sometimes perfectly OK to do table writes directly from caller's
> context too, like:
>   write(Item) ->
>       true = ets:insert_new(actual_table_name, Item).
> It can of course be very tricky business and needs good thinking first.
> I bet you're aware of this, mentioning it just because it's a handy
> trick that doesn't seem to be widely known.
> > Creating the table with {read_concurrency, true} as the option will
> > probably speed up reads by quite a bit as well. It is probably going
> > to be a lot faster than having all caching reads going through that
> > single point of contention. Chances are that just breaking parts of
> > the chain is enough to improve the performance of the system.
> Well, yes, avoiding central points of contention (such as blessed
> processes or, you guessed it, ETS tables) is certainly good engineering
> practice, but see first part of this email for other considerations.
> BR,
> 	-- Jachym
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