[erlang-bugs] Simple floating-point lists operation hungs Erlang system

Denis Nesterov <>
Fri Nov 25 23:03:15 CET 2011


Hello!

I wrote simple benchmark code to measure floating-point Erlang performance.
And seems that code gets Erlang system into a deadlock :-p

The thing is that my code doing VERY simple thing - it first generates big
list of pairs of Euclidian vectors, then in loop performs sequential
mult-add operation on each element.
When I start one such process per CPU core and list is big enough, the
Erlang systems quickly stops responding.

 The code is:

-module ( bug ).
-export ( [start/0] ).

-record ( vec3, { x=0, y=0, z=0 } ).


vec3ma( V0, V1, K ) when is_record( V0, vec3 ) and is_record( V1, vec3 )
and is_number( K ) ->
#vec3{ x = V0#vec3.x + V1#vec3.x * K, y = V0#vec3.y + V1#vec3.y * K, z =
V0#vec3.z + V1#vec3.z * K }.

%create test data
makeVecTestList( L, 0 ) -> L;
makeVecTestList( L, N ) ->
E = { #vec3{ x = N * 10.0, y = 20.0, z = 30.0 }, #vec3{ x=10.0, y=20.0,
z=-40.0 } },
makeVecTestList( [ E|L ], N - 1 ).

%Test list-processing code
proc( L ) -> proc( L, [] ).
proc( [], Acc ) -> Acc;
proc( [ { P, V } | T ], Acc ) -> proc( T, [ { vec3ma( P, V, 0.11 ), V } |
Acc ] ).

%process loop
vecTestLoop( L, T, I ) ->
Now = now(),
DT = timer:now_diff( Now, T ) * 1.0e-3,
io:format( "Step~p: ~p ms\n", [I, DT] ),
vecTestLoop( proc( L ), Now, I ).

start() ->
L = makeVecTestList( [], 1000000 ),
io:format( "Running test with: ~p\n", [length(L)] ),
spawn( fun() -> vecTestLoop( L, now(), 1 ) end ),
spawn( fun() -> vecTestLoop( L, now(), 2 ) end ),
spawn( fun() -> vecTestLoop( L, now(), 3 ) end ),
spawn( fun() -> vecTestLoop( L, now(), 4 ) end ).


I used next test-cases:

AMD 64 2 core, 3.1 GHZ, Windows XP, Erlang R14B04:
- Works well with one process and any list that fits into memory
- Works well with two processes and list of 500000 elements
- Hangs in about 30-60 seconds with list of 750000 elements
- Hangs almost immediately with list of 1000000 elements

Intel Quad 2.67 GHz, Windows 7 x64, Erlang R14B04:
- Works well with one process and any list that fits into memory
- Hangs in about 30-60 seconds with list of 1000000 elements
- Hangs almost immediately with list of 1500000 elements

 This looks very similar to deadlock - when normal iterations freeezes, the
erl.exe cannot be stopped with Ctrl+C/Break, typing "q()." etc, and starts
to load ONLY ONE CORE. Though while it still alive, it consumes all cores.

Memory consumption by erl.exe was about or even under 1 GB in all cases.
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