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Κόμπος : A Concurrency-Agnostic Debugger An Example for Domain-Specific Online Debugging C. Torres Lopez S. Marr D. Aumayr H. Mössenböck E. Gonzalez Boix IFIP WG 2.11, July 17-20, 2017, Koblenz

What am I doing? A simple VM for all Making dynamic Combining concurrency models languages fast, easily! Concurrency Models (incl. metaobject actors, CSP, STM, protocols) fork/join, data flow, threads+locks, … Ownership-based Meta-Tracing vs. Tooling/Debugging metaobject protocol Par;al Evalua;on 2

ENIAC's main control panel, U. S. Army Photo For a brief bit of history: ENIAC’s recessive gene Marcus Mitch, and Akera Atsushi. Penn Printout (March 1996) h_p://www.upenn.edu/compu`ng/printout/archive/v12/4/pdf/gene.pdf

Decades of Research and Solu`ons for “ Everything” 4

But no Silver Bullet Actors CSP Locks, Monitors, … Fork/Join Transac;onal Memory Data Flow … 5

In Prac`ce: Concurrency Abstrac`ons are Combined • Uses Locks and Atomic* • Mul`ple async. future/ > 4500 “deadlock” bugs task abstrac`ons > 530 “race condi`on” bugs • Mul`ple ‘transac`on’ systems Study on Scala: S. Tasharofi, P. Dinges, and R. E. Johnson. Why Do Scala Developers Mix the Actor Model with other Concurrency Models? In Proc. of ECOOP, volume 7920 of LNCS, pages 302–326. Springer, 2013. 6

I want to reason about actors, fork/join tasks, transac`ons … And not some implementa`on-level shared memory HOW TO DEBUG SUCH SYSTEMS? 7

Demo of Kómpos 8

Custom Debugger Features Stepping Opera`ons Breakpoints Sequen`al 1 5 Generic 4 2 Actors 6 5 CSP 4 2 STM 3 3 Threads&Locks 4 2 22 19 (not intended to be complete)

Debugger Architecture Debugger Interpreter Frontend Debugger Protocol 10

Kómpos Architecture Actors Threads CSP … STM F/J Kómpos Debugger SOM NS Debugger Protocol Interpreter (Web Socket, JSON) Java, Truffle+Graal 11

Kómpos Architecture Actors Threads Breakpoint CSP … 22 Breakpoint Support STM Types F/J Stepping 19 Stepping Kómpos Support Opera`ons Debugger SOM NS Visualiza`ons Debugger Protocol Interpreter Various Support (Web Socket, JSON) Visualiza`ons Java, Truffle+Graal How to Abstract from Concurrency Models? 12

A Concurrency-Agnos`c Debugger Protocol THE KÓMPOS PROTOCOL 13

Abstract from Concurrency Models • Breakpoint Types • Stepping Opera`ons Actors Threads • Ac`vity Types, Dynamic scopes Breakpoint CSP … Support STM F/J Stepping Kómpos Support Debugger SOM NS Debugger Interface Visualiza`ons Interpreter Support (Web Socket, JSON) • Interac`ons modeled via data • Debugger agnos`c of concepts 14

What’s a Breakpoint? message send message receive promise resolver promise resolu`on prom := aResult <-: get. • Name/Type • Source Loca`ons Truffle AST § Iden`fied by Source Tags Var Write Send Debugger Requirements #get MsgSendTag § Annotated AST Var Read 15

What’s a Stepping Opera`on? Step into Step over Return Step to Receiver Step to Promise Resolver • Name/Type Step to Promise Resolu`on prom := • Source Loca`ons aResult <-: get. – Iden`fied by Source Tags • Current Type of Ac`vity – Actor, Process, Fork/Join Task, Thread • Current Dynamic Scope – Held Monitor, Transac`on 16

Kómpos Protocol Metadata EntityType id: typeId name: string ActivityType DynamicScopeType icon: string BreakpointType SteppingType name: string name: string label: string label: string applicableTo: Tag[] applicableTo: Tag[] activities: ActivityType[] scopes: DynamicScopeType[] 17

Kómpos Protocol Messages Source BreakpointUpdate URI: URI location: Coord sourceText: string type: BreakpointType locations: TaggedCoord[] Stopped Step activityId: id activityId: id location: Coord type: SteppingType actType: ActivityType scopes: DynamicScopeType[] 18

Example for Transac`ons before transac`on to int: {BreakpointUpdate, before commit loc: 1:1:6, awer commit tags: Atomic type: beforeCommit} atomic { from int: {Stopped, int b = this.fieldB; actId: 1, this.fieldA = b + 1; loc: 3:3:20, scopes: [tx]} } resume to int: {Step, step next actId: 1, step over type: afterCommit} before commit awer commit scopes: tx 19

How concurrency-agnos`c is the protocol design? EVALUATION 20

Implemented Interac`ons No changes to frontend required! Stepping Opera`ons Breakpoints Sequen`al 1 5 Generic 4 2 Actors 6 5 CSP 4 2 STM 3 3 Threads&Locks 4 2 22 19 (not intended to be complete)

CONCLUSION 22

Too many Concepts, Too many Varia`ons 18/07/17 23

Solu`on: Model Domain with Metadata • Breakpoint Types • Stepping Opera`ons Actors Threads • Ac`vity Types, Dynamic Scopes Breakpoint … CSP Support STM F/J Stepping Kómpos Support Debugger SOM NS Visualiza`ons Debugger Interface Interpreter Support (Web Socket, JSON) 24

Paper Deadline: 14 August 2017 Workshop Proposals: 1 October 2017 25

Kómpos: Plaxorm for Debugging Complex Concurrent Applica`ons • Concurrent Debugger – Concept-agnos`c • Visualiza`on, Breakpoints, Stepping, Tracing • Replay, Asser`ons, … • For Actors, CSP, Fork/Join, STM, Locks&Threads, … 26