Programming-Language Approaches to Improving Shared-Memory Multithreading: Work-In-Progress Dan Grossman University of Washington Microsoft Research, RiSE July 28, 2009

Dan Grossman: Multithreading Work-In-Progress2 Today A little history / organization: how I got here Informal, broad-not-deep overview of 4 ongoing projects –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Hopefully time for discussion

July 28, 2009Dan Grossman: Multithreading Work-In-Progress3 Biography / group names Me: PLDI, ICFP, POPL “feel like home”, PhD for Cyclone  UW faculty, –Type system, compiler for memory-safe C dialect 30%  80% focus on multithreading, Co-advising 3-4 students with computer architect Luis Ceze, Two groups for “marketing purposes” WASP, wasp.cs.washington.edu SAMPA, sampa.cs.washington.edu

July 28, 2009Dan Grossman: Multithreading Work-In-Progress4 People / other projects Ask me later about: Progress estimation for PigLatin Hadoop queries [Kristi] Composable browser extensions [Ben L.]

July 28, 2009Dan Grossman: Multithreading Work-In-Progress5 Today A little history / organization: how I got here Informal, broad-not-deep overview of 4 ongoing projects –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Hopefully time for discussion

July 28, 2009Dan Grossman: Multithreading Work-In-Progress6 Atomic blocks An easier-to-use and harder-to-implement synchronization primitive void transferFrom(int amt, Acct other){ atomic{ other.withdraw(amt); this.deposit(amt); } “Transactions are to shared-memory concurrency as garbage collection is to memory management” [OOPSLA 07] GC also has key semantic questions most programmers can ignore –Resurrection, serialization, dead assignments, etc.

July 28, 2009Dan Grossman: Multithreading Work-In-Progress7 “Weak” isolation Widespread misconception: “Weak” isolation violates the “all-at-once” property only if corresponding lock code has a race (May still be a bad thing, but smart people disagree.) atomic { y = 1; x = 3; y = x; } x = 2; print(y); //1? 2? 666? initially y==0

July 28, 2009Dan Grossman: Multithreading Work-In-Progress8 It’s worse Privatization: One of several examples where lock code works and weak-isolation transactions do not (Example adapted from [Rajwar/Larus] and [Hudson et al]) ptr fg atomic { r = ptr; ptr = new C(); } assert(r.f==r.g); atomic { ++ptr.f; ++ptr.g; } initially ptr.f == ptr.g

July 28, 2009Dan Grossman: Multithreading Work-In-Progress9 atomic { ++ptr.f; ++ptr.g; } It’s worse Most weak-isolation systems let the assertion fail! Eager-update or lazy-update ptr fg atomic { r = ptr; ptr = new C(); } assert(r.f==r.g); initially ptr.f == ptr.g

July 28, 2009Dan Grossman: Multithreading Work-In-Progress10 The need for semantics Which is wrong: the privatization code or the language implementation? What other “gotchas” exist? Can programmers correctly use transactions without understanding their implementation? Only rigorous programming-language semantics can answer

July 28, 2009Dan Grossman: Multithreading Work-In-Progress11 Separation Static separation: Each thread-shared, mutable object is accessed-inside- transactions xor accessed-outside-transactions throughout its lifetime –Natural in STM Haskell (but not other settings) –Proved sound for eager update [POPL 08 x 2] Dynamic separation: Each thread-shared, mutable object has dynamic metastate explicitly set by programmers to determine “side of the partition” –Designed, proven, implemented by Abadi et al for Bartok

July 28, 2009Dan Grossman: Multithreading Work-In-Progress12 atomic { r = ptr; ptr = new C(); } unprotect(r); assert(r.f==r.g); atomic { ++ptr.f; ++ptr.g; } initially ptr.f == ptr.g Example redux ptr fg

July 28, 2009Dan Grossman: Multithreading Work-In-Progress13 Laura’s work Design, semantics, implementation, and benchmarks for dynamic separation in Haskell Primary contributions: 1.Regions: Change protection state of entire data structures in O(1) time –Cool idioms/benchmarks where this gives 2-6x speedup 2.Lazy-update implementation –Allows protection-state changes from within transactions 3.Interface allowing composable libraries that can be used inside or outside transactions, without breaking Haskell’s types 4.Formal semantics in the style of STM Haskell

July 28, 2009Dan Grossman: Multithreading Work-In-Progress14 Today A little history / organization: how I got here Informal, broad-not-deep overview of 4 ongoing projects –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Hopefully time for discussion

July 28, 2009Dan Grossman: Multithreading Work-In-Progress15 Escape actions Escape actions: –Do not count for memory conflicts –Are not undone if transaction aborts –Possible “strange results” if race with transactional accesses Essentially an unchecked back-door Note: Open nesting is just escape { atomic { s } } –So escaping is the essential primitive atomic { s1; escape { s2; } // perhaps in a callee s3; }

July 28, 2009Dan Grossman: Multithreading Work-In-Progress16 Canonical example If escape actions are hidden behind strong abstractions, we can improve parallelism without affecting program behavior –Clients cannot observe the escaping Unique-id generation type id; id new_id(); bool compare_ids(id, id); Transactions generating ids need not conflict with each other If transaction aborts, no need to undo the id-generation

July 28, 2009Dan Grossman: Multithreading Work-In-Progress17 Matt’s work Formal semantics for escape actions Use to prove the unique-id example is correct –Two implementations, one using escape –Show no client affected by choice of implementation –Fundamentally similar to proving ADTs actually work Gotcha: The theorem is false if a client abuses other escape actions to “leak ids”: –Discovered by attempting the proof! atomic { id x = new_id(); if(compare_ids(x,glbl)) … escape { glbl=x; } … }

July 28, 2009Dan Grossman: Multithreading Work-In-Progress18 Today A little history / organization: how I got here Informal, broad-not-deep overview of 4 ongoing projects –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Hopefully time for discussion

July 28, 2009Dan Grossman: Multithreading Work-In-Progress19 Deterministic C Take arbitrary C + POSIX Threads and make behavior dependent only on inputs (not nondeterministic scheduling) –Helps testing, debugging, reproducibility, replication It’s easy! –Run one thread at a time with deterministic context-switch Example: run for N instructions or until blocking It’s hard! –Need to recover scalability with reasonable overhead Amdahl’s Law is one tough cookie!

July 28, 2009Dan Grossman: Multithreading Work-In-Progress20 How to do it It’s a long and interesting compiler, run-time, and correctness story –Invite Luis over for an hour Key techniques: –Dynamic ownership of memory (run in parallel while threads access what they own) –Buffering (publish buffers deterministically while not violating language’s memory-consistency model) –No promise which deterministic execution programmer will get (tiny change to source code can affect behavior) Performance: –Depends on application –Buffering has better scalability but worse per-thread overhead, so hybrid approaches are sometimes needed

July 28, 2009Dan Grossman: Multithreading Work-In-Progress21 Today A little history / organization: how I got here Informal, broad-not-deep overview of 4 ongoing projects –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Hopefully time for discussion

July 28, 2009Dan Grossman: Multithreading Work-In-Progress22 Code-centric In a shared-memory C/C#/Java program, any heap access might be inter-thread communication –But very few actually are Most prior work to detect/exploit this sparseness is data-centric –What objects are thread-local? –What locks protect what memory? Answers can find bugs, optimize programs, define code metrics, etc. We provide a complementary code-centric view…

July 28, 2009Dan Grossman: Multithreading Work-In-Progress23 Graph Nodes: Code units (e.g., functions) Directed edges: –Source did a write in thread T1 –Target read that write in thread T2 –T1 != T2 Current tool: –Automatically build graph of a (slower) dynamic execution –Manual easy clean-up by programmer –Rely heavily on state-of-the-art dynamic instrumentation (PIN) and graph visualization (Prefuse)

July 28, 2009Dan Grossman: Multithreading Work-In-Progress24 A toy example queue q; // global, mutable void enqueue(T* obj) { … } T* dequeue() { … } void consumer(){ … T t = dequeue(); … } void producer(){ … T* t = …; t->f=…; enqueue(t) … } Program: multiple threads call producer and consumer enqueuedequeueproducer consumer Tool supports “conceptual inlining” to allow multiple abstraction levels

July 28, 2009Dan Grossman: Multithreading Work-In-Progress25 Not just for toys Small and large applications –Example: MySQL (940KLOC); graph clean-up by one grad student in < 1 day without prior source-code knowledge I truly believe: –Great “first day of internship” tool interactive graph essential and not our contribution –Useful way to measure multithreaded behavior Example: Graphs are very sparse thankfully MySQL: >11,000 functions, 423 nodes, 802 edges Example: Graph diff across runs with same input measures the nondeterminism of the program But this is hard-to-evaluate tool work – your thoughts? Future work: Specification of graphs checked during execution

July 28, 2009Dan Grossman: Multithreading Work-In-Progress26 Summary –Better semantics / languages for transactional memory x 2 Dynamic separation for Haskell Semantics / abstraction for “escape actions” –Deterministic Multiprocessing –Code-centric communication graphs Very little published yet, but all Real Soon Now Microsoft has been essential –Transactions (Harris, Abadi, Peyton Jones, many more) –Funding (Scalable Multicore RFP, New Faculty Fellows) Hopefully opportunities to collaborate –Particularly on the (unproven) SE applications of this work