Humboldt-Universitȧ. t zu Berlin Institut fu.. r Informatik

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1 WWW: HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik A ThreadAware Debugger with an Open Interface Daniel Schulz Frank Mueller Qcentic Gmbh Institut für Informatik MaxPlanckStr. 39A HumboldtUniversität zu Berlin Cologne (Germany) Berlin (Germany) A ThreadAware Debugger with an Open Interface ISSTA'00 1

2 motivation design active debugging debugger components implementation evaluation HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Overview A ThreadAware Debugger with an Open Interface ISSTA'00 2

3 SMP programming! Threads + POSIX Threads: many implementations + debugging support: no standardization debugging difficult: interleaving of control flows sync. + async. suspension/resumption HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Motivation partial ordering of executions (synchronization) traditionally desirable for all flows threadspecific breakpoints state invisible inqueries synchronization implicit optional control scheduling explicit at context switches breakpoints A ThreadAware Debugger with an Open Interface ISSTA'00 3

4 portability: open interface debugging threads for extensibility: query API $ debugger application flexibility: varying functionality activation: optional shared library TDI: thread debug interface (generic) ) TED: Thread extensions for debugging ) HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Design Debugger TDI TED (implementationdependent) Application A ThreadAware Debugger with an Open Interface ISSTA'00 4

5 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Design Options for Encapsulation Debugger Debugger Coding Decoding TDI I 1 I 2... I n TDI TED Access (generic) Pthreads Impl. I 1 POSIX Threads API Pthreads Impl. I 2... Pthreads Impl. I n TED Impl. 1 Pthreads Impl. TED TED Impl. 2 Pthreads Impl.... TED Impl. n Pthreads Impl. I 1 I 2 I n (a) Non Generic Design (b) Generic Design A ThreadAware Debugger with an Open Interface ISSTA'00 5

6 Active Debugging enhance application code collect information / manipulate execution Passive Debugging (traditional) enhance debugger probe application of thread implementation details to debugger? known no redundancies lower, HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Active Debugging Issue Active Debugging Passive Debugging No (transparent) Yes new thread impl. no changes enhance debugger change/add info from application declarative procedural extract query overhead higher, redundant requests postmortem thread debugging not possible always A ThreadAware Debugger with an Open Interface ISSTA'00 6

7 uniform access to internal thread ADTs set manipulation primitives S r : T DO! T DA (read) S w : T DA T DO! T DA (write) an existing thread API or a debug extension API outs thread API! TED register objects update object relations interface for HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik TED: Thread Extensions for types T of address domain D A / object domain D O map objects (thin layer) onto set iteration! D A attribute access! N [ fnullg A ThreadAware Debugger with an Open Interface ISSTA'00 7

8 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Booch Class Diagram of Object Classes Thread Identifier Address of self Priority State User Function... 1 OwnedBy n n n WaitFor BlockedOn 1 Mutex Identifier Address of self Condtion Variable Identifier Address of self... 1 SignaledBy A ThreadAware Debugger with an Open Interface ISSTA'00 8

9 abstracts from debugger and thread impl. keeps database of application's state TED registers with TDI (thin interface): object updates / queries iteration / attribute access all of these or subset ) supports persistent identifiers (e.g., thread IDs) communicates consistent state to debugger uniform, extensible database query language ) selections of relations with values projections of relations with assignments queries clustered! remove redundancies ) HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik TDI: Thread Debug Interface responses reduced! no copies A ThreadAware Debugger with an Open Interface ISSTA'00 9

10 active debugging ) exchange more data OS support for debugging limits probe bandwidth application stopped at breakpoint ) cannot serve query debugger s handler function in application ) repeated s for large responses ) many context switches fork child in application ) HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Communication Structure ) use IPC for query/response child fills IPC buffers ) avoids consecutive s A ThreadAware Debugger with an Open Interface ISSTA'00 10

11 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Communication between Debugger and Application arget Process IPC Channel Debugger (buffered) IPC.write(<TED Request>) Target.(<RequestHandler>) IPC.read() IPC.write(<Result #1>) IPC.read() (Result #1) subsequent handler s Target.(<RequestHandler>) IPC.write(<Result #n>) IPC.read() (Result #n) Response Phase Request Phase Replicated Target Process exit Target Process IPC Channel (buffered) fork Request Handling IPC.write(<Result #2>) IPC.write(<Result #n>) IPC.write(<TED Request>) Target.(<RequestHandler>) IPC.read() IPC.write(<Result #1>) IPC.read() IPC.read() IPC.read() Debugger (Result #1) (Result #2) (Result #n) ) mutually exclusive execution (b) parallel execution A ThreadAware Debugger with an Open Interface ISSTA'00 11

12 IPC interface to TDI client new user commands query / response handshake: issue query then TDI server handler TDI server parses query updates state using TED formats and sends repsonse HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Debugger Extensions A ThreadAware Debugger with an Open Interface ISSTA'00 12

13 GDB 4.18 LinuxThreads, Solaris Threads, FSU Pthreads, MIT Pthreads application bound to support dynamic linking TDI server as DLL, only activated if debugged (flag in TED) Problem: assure consistent state skew of TDI execution ) deal with probe effect event notification ) postpone signals blocking s ) replacement s HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Implementation ) must prevent preemption / suspension during TDI activity A ThreadAware Debugger with an Open Interface ISSTA'00 13

14 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik POSIX Threads Debugger Signal Handling during Active Debugging Scheduling pthread_debug_tdi_sig_ignore off on Record 1 ptrace(poke...,...) Feedback Timer 3 2 kill(pid, SIGALRM) ptrace(peek...,...)... pthread_debug_tdi_ignored_signals A ThreadAware Debugger with an Open Interface ISSTA'00 14

15 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Response Times: IPC vs. Ptrace response time [sec] ptrace() TDI instantiated threads A ThreadAware Debugger with an Open Interface ISSTA'00 15

16 queries generated by TDI due to user commands parsed, transformed into tuples handled by query evaluator HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Query Language explicitly by experienced user thread:id,entry,state:state == 1 jj mbo == 0 (1) thread:id,prio=10,state: (prio+10<20) && cvwf! =0x10 (2) responses translated into symbolic representation 8 804b238 2 #7 804b238 2 #10 804b238 1 A ThreadAware Debugger with an Open Interface ISSTA'00 16

17 hit breakpoint check running thread ID (depends on thread impl.) upon mismatch, clean up and reset breakpoint resumption (2) may accept signals (and contex switch) breakpoint at context switch trap at nextpc of all suspended threads ) forced suspension signal application ) invoke scheduler HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik ThreadSpecific Breakpoints ) disable signals during (2) A ThreadAware Debugger with an Open Interface ISSTA'00 17

18 HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Resetting a Breakpoint Breakpoint a[n] hit pc trap Inst. B Inst. C a[n] a[n+1] a[n+2] Reset 1 pc Inst. A trap Inst. C a[n] a[n+1] a[n+2] Code Segment Resume 2 Code Segment Ready to Resume Execution pc Inst. A trap Inst. C a[n] a[n+1] a[n+2] Reset 3 pc trap Inst. B Inst. C a[n] a[n+1] a[n+2] Code Segment Code Segment A ThreadAware Debugger with an Open Interface ISSTA'00 18

19 extensive thread info state of synchronization threadspecific breakpoints explicit suspension/resumption threadspecific stack trace breakpoint on next context switch HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik ThreadAware Debugging threadspecific step/next performance overhead No Debugging GDBTDI Overhead Program 14 sec 16 sec 12.5% fft barnes 33 sec 40 sec 17.5% A ThreadAware Debugger with an Open Interface ISSTA'00 19

20 Mach debugger SmartGDB GDB 4.18 Solaris Partop path expressions/actions HPDF: debug command interface MPI message display / TotalView Panorama KDB Fast breakpoints Relational query debugging Our work: active debugging + relational queries for debugging ) functionality, portability threads, HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Related Work A ThreadAware Debugger with an Open Interface ISSTA'00 20

21 open interface for debugging thin layer (extension to thread impl.) threadaware debugging facilities! new features implemented in GDB paradigm of active debugging languageindependent protocols for communication relational query model supports partial or complete TEDs sample impl. for variety of thread impl. types improved efficiency and portability download: HumboldtUniversitȧ. t zu Berlin Institut fu.. r Informatik Conclusion A ThreadAware Debugger with an Open Interface ISSTA'00 21