www.cs.rochester.edu/u/scott/ Dagstuhl, January 2015 MLS ! 1 ! Whe - - PowerPoint PPT Presentation

MLS! 1!

Dete term rministic inistic P Paralle llel Sc l Scripting: ripting: Ma Making the ing the Ea Easy C sy Case se Ea Easy sy Michael L. Scott

www.cs.rochester.edu/u/scott/ Dagstuhl, January 2015

MLS! 2!

Whe Where re in the in the C Curric urriculum ulum?

languages, SW engg., sci. comp. graphics, HCI, web computing computer literacy data structures networks,

• dist. comp.

OS, arch., par. comp., DBMS

MLS! 3!

Wha What La t Langua nguage ge D Do We

• We U

Use se?

• Lamport: This is the wrong question.

» Imagine an art historian answering how would you describe impressionist painting? by saying in French.

• MLS: This is the

wrong analogy.

» Imagine an art teacher answering how would you introduce pointillism? by saying in oils.

• Notation matters!

MLS! 4!

Whe Where re D Doe

• es Ea

s Easine siness Ma ss Matte tter? r?

• Dynamic (scripting) languages!

» Increasing share of modern programs in Python, Ruby, PHP, Matlab, R, JavaScript, . . . » Increasing share of introductory curricula » Ease of programming trumps performance » But concurrency can still make a difference on multicore machine

• Deterministic Parallel Ruby (DPR)

» Thesis work of Li Lu (now at Hortonworks)\ » Emphasis on independence & proper nesting

MLS! 5!

DPR PR C Construc

• nstructs (1

ts (1)

• Independent blocks (co operator):

def qsort(a) return a if a.length <= 1 m = a.length/2 pivot = a[m] left = qsort(a.select{|v| v < pivot}) mid = a.select{|v| v == pivot} right = qsort(a.select{|v| v > pivot}) return left + mid + right end

MLS! 6!

DPR PR C Construc

• nstructs (1

ts (1)

• Independent blocks (co operator):

def qsort(a) return a if a.length <= 1 m = a.length/2 pivot = a[m] left = qsort(a.select{|v| v < pivot}) mid = a.select{|v| v == pivot} right = qsort(a.select{|v| v > pivot}) return left + mid + right end

MLS! 7!

DPR PR C Construc

• nstructs (1

ts (1)

• Independent blocks (co operator):

def qsort(a) return a if a.length <= 1 m = a.length/2 pivot = a[m] co ->{ left = qsort(a.select{|v| v < pivot}) }, },

• >{ mid = a.select{|v| v == pivot} },
• >{ right = qsort(a.select{|v| v > pivot}) }

return left + mid + right end

MLS! 8!

DPR PR C Construc

• nstructs (2

ts (2)

• Independent unordered iteration:

myset.all {|e| op }

• Reductions:

sum = Reduction.new(IntegerAdd) myset.all {|e| sum.push(1) if P(e) } count = sum.get

MLS! 9!

DPR PR C Construc

• nstructs (3

ts (3)

• Atomic commutative operations — consider:

sum = Reduction.new(IntegerAdd) myset.all {|e| sum.push(f(e)) }

where f is a very expensive function. Instead, do:

mtab = new MemoTable(f) sum = Reduction.new(IntegerAdd) myset.all {|e| sum.push(mtab.eval(e)) }

where MemoTable is defined (in advance, by an expert) as ...

MLS! 10!

DPR PR C Construc

• nstructs (3

ts (3)

class MemoTable def initialize(f) @f = f @table = Hash.new end self.setACOps :eval end def eval(key) atomic val = @table[key] end if val == nil val = @f.call(key) atomic @table[key] = val end end return val end

MLS! 11!

DPR PR C Construc

• nstructs (4

ts (4)

• Pure futures:

f = Future.new(->{|x1, x2, ...| op }, arg1, arg2, ...) ... v = f.get()

• Pipelines

p = ->{|x| s1} >> ->{|x| s2} >> ->{|x| s3} p.setInStream(MyInStream.new) p.setOutStream(MyOutStream.new) p.run

» Stage i on object n must be independent of stage j on object m

if (and only if) i+n = j+m

MLS! 12!

DPR PR Im Imple plementa ntation tion

• Tiny sequential library (few dozen lines);

(larger) parallel library

» Both run on any Ruby VM

• Built-in version on JRuby

» Thread pool with Cilk-like work stealing scheduler » Modest scalability: 2-6x on 12 cores

• Independence treated as an assertion

» Checked at run-time: TARDIS determinism checker [PLDI’14] » Cf. subscript or type checking

• Not solid enough to use for teaching yet !

» But I really think something along these lines is what we need

www.cs.rochester.edu/u/scott/