geometric memory growth
Ulf Wiger (AL/EAB)
ulf.wiger@REDACTED
Wed Nov 23 14:54:05 CET 2005
Here's a subtle bug that caused me about one day's worth
of headache:
I have process that holds a gen_tcp session, and
multiplexes "virtual sessions" on top of it.
When running a benchmark, I noticed exponential
memory growth, and it took me a while to notice
that the growth was actually taking place in the
innocent benchmark loop (about 7 lines of code,
basically this:
vcc_rpc2(Channel) ->
vccTransport:msg(Channel, 0, vcc_rex, {self(), ping},
[{temp_channel,true}]),
receive
{_, _, pong} ->
ok;
end.
which amounts to an ets:lookup, a gen_server:call()
and waiting for a message where the third element is
'pong')
17 iterations made the VM allocate 1 GB of RAM.
The problem? (finally diagnosed once Björn G tipped
me off about erts_debug:size(Term) - thanks Björn!)
The second element of the message was a fun:
SendF = fun(To, Msg) ->
msg(State#state.channel,
ChNo, To, Msg)
end,
Horrid, isn't it?
Writing it this way, causes the _whole_ State
to be bound inside the fun. And State of course
contained, among other things, the metadata for
the virtual sessions (e.g. other SendF instances.)
This wasn't much of a problem as long as everything
was local references on the same heap (it probably
would have shown after a while, though), but when
the fun was passed in a message to another process,
all relative references were flattened out, and
the size of the fun doubled in each iteration.
The obvious fix:
Channel = State#state.channel,
SendF = fun(To, Msg) ->
msg(Channel,
ChNo, To, Msg)
end,
Now, wouldn't it be great if the compiler
could figure out how to do this at compile-time?
/Uffe
More information about the erlang-questions
mailing list