Multicore Computing Using Erlang Art Gittleman California State University Long Beach

Motivation David Patterson – 2006 article The number of processors per chip is expected to double every two years Educating faculty to train students in concurrency is not a trivial problem

Using Multiple Processors Most languages Shared memory – must protect against simultaneous modification Erlang Asynchronous message passing Create processes Scheduler uses available processors

Erlang Functional programming language Developed at Ericsson Open Source, runs on Windows, Linux, Mac Applications Amazon SimpleDB Facebook chat (70 million+ users)

Teaching Erlang Survey of Programming Languages Five week module on functional programming Used machines with two cores Basic Erlang and some concurrency Advanced Programming Languages Ten-week unit Used Beowulf cluster, 30 processors

Erlang Intro Binding variables Uses pattern matching X = 4. binds X to 4 Y = X + 2. binds Y to 6 X = 3. error – X bound to 4 Z + 1 = Y. binds Z to 5

Data Structures Tuples – fixed size X = {hat, cat, fat}. {A, cat, fat} = X. binds A to hat Lists – variable size L = [3, 4, 5, 6]. [X | Y] = L. binds X to 3, Y to [4,5,6]

Functions Recursive definition -module(append). -export([append/2]). append([ ], L) → L; append([H | T], L) → [H | append(T, L).

Concurrency Primitives Create a process Pid = spawn(Fun) % Fun executes % in new process Send a process a message Pid ! Message Receive a message receive … end

-module(times2). -export([run/1]). times2() -> % executed in a process receive {From, X} -> From ! 2*X, % send back ans times2() end. run(Num) → % run(10) returns 20 Pid = spawn(fun times2/0), % create Pid ! {self(), Num}, % send receive Result -> Result end.

Three Assignments 1. Write three recursive functions Generate a list of squares of random numbers Sum two lists, element by element Count the number of items < 1 in a list 2. Estimate pi. Send number of random trials to a process that computes pi and returns result. 3. Estimate pi using four processes. Speedup shown with two-core machines

Higher-Order Functions Map L = [1,2,3,4]. lists:map(fun(X) → 2*X end, L). % returns [2,4,6,8] List-comprehension [2*X || X ← L].

Ten most frequent words From Adam Turoff, Haskell article Used text of Emma by Jane Austen Develop simple functions to find the ten most frequent words Read text as list, split into sublists of equal words, create sublists of [freq, word] pairs, reverse sort, return top ten. Many functions such as 'sort' are provided.

Strassen Matrix Multiplication Recursive, more efficient than standard method NxN matrix requires 7 N/2xN/2 multiplications Used seven machines on a Beowulf cluster. Compare 14 processors to 1. Time in seconds 256x x x | |

Conclusions Erlang was easy to use for concurrency. The programmer spawns processes and the scheduler uses available processors either on the same machine or distributed. Erlang fits well in a programing languages course.