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“Locking without blocking” LFthreads: A lock-free thread library

“Locking without blocking” LFthreads: A lock-free thread library. Anders Gidenstam Joint work with: Marina Papatriantafilou Distributed Computing and Systems group Department of Computer Science and Engineering, Chalmers University of Technology, Göteborg, Sweden. Outline. Introduction

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“Locking without blocking” LFthreads: A lock-free thread library

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  1. “Locking without blocking”LFthreads: A lock-free thread library Anders Gidenstam Joint work with: Marina Papatriantafilou Distributed Computing and Systems group Department of Computer Science and Engineering, Chalmers University of Technology, Göteborg, Sweden

  2. Outline • Introduction • Lock-free synchronization • The Problem & Background • LFthreads • Overview • Lock-free thread-blocking synchronization • Experiments • Conclusions Anders Gidenstam, Max-Planck Institute for Computer Science

  3. Synchronization on a shared object • Lock-free synchronization • Concurrent operations without enforcing mutual exclusion • Avoids: • Blocking (or busy waiting), convoy effects and priority inversion • Progress Guarantee • At least one operation always makes progress • Synchronization primitives • Built into CPU and memory system • Atomic read-modify-write (i.e. a critical section of one instruction) • Examples: Compare-and-Swap, Load-Linked / Store-Conditional Anders Gidenstam, Max-Planck Institute for Computer Science

  4. O1 O2 O3 Correctness of a concurrent object • Desired semantics of a shared data object • Linearizability [Herlihy & Wing, 1990] • For each operation invocation there must be one single time instant during its duration where the operation appears to takeeffect. • The observed effectsshould be consistentwith a sequentialexecution of the operationsin that order. Anders Gidenstam, Max-Planck Institute for Computer Science

  5. O1 O2 O3 O1 O3 O2 Correctness of a concurrent object • Desired semantics of a shared data object • Linearizability [Herlihy & Wing, 1990] • For each operation invocation there must be one single time instant during its duration where the operation appears to takeeffect. • The observed effectsshould be consistentwith a sequentialexecution of the operationsin that order. Anders Gidenstam, Max-Planck Institute for Computer Science

  6. The Problem • Multithreading on a multiprocessor • Thread multiplexing and scheduling • Thread synchronization objects • Aim: The POSIX pthread API implemented in a lock-free way • Motivation: Why lock-free? • Processors should always be able to do useful work • In spite of others being slow: page faults, interrupts, h/w problems • Improved performance? Running threads Ready Queue Processors T2 T1 P1 T3 Waiting runnablethreads P2 T5 P3 T4 Anders Gidenstam, Max-Planck Institute for Computer Science

  7. The Problem(s) • Contention on the ready queue • Non-blocking work-stealing; Hood,[Blumhofe et al, 1994],[Blumhofe et al, 1998] • Lesser Bear,[Oguma et al, 2001] • In LFthreads: a lock-free queue Ready Queue Processors T2 T1 P1 Dequeue Dequeue P2 Enqueue P3 T4 Anders Gidenstam, Max-Planck Institute for Computer Science

  8. The Problem(s) • Blocking synchronization • Often undesirable • Hinders progress • Expensive context switches • Sometimes required • Waiting for some event • Legacy applications • Goal in LFthreads: • Lock-free for processors • Resilience to slow operations/processors Processors P1 T3 T5 P2 Block T5 P3 T4 Anders Gidenstam, Max-Planck Institute for Computer Science

  9. The Problem(s) • Implementing blocking synchronization • Keep common case fast • Often no contention • User / kernel level split implementation • E.g. Linux, Solaris • Critical sections are short • Spinning v.s. blocking[Zahorjan et al, 1991] Processors P1 T3 P2 T5 Acquire P3 T4 Anders Gidenstam, Max-Planck Institute for Computer Science

  10. The Problem(s) • Implementing blocking synchronization • Keep common case fast • Often no contention • User / kernel level split implementation • E.g. Linux, Solaris • Critical sections are short • Spinning v.s. blocking[Zahorjan et al, 1991] • Enlisting the scheduler • [Devi et al, 2006] • [Kontothanassis et al, 1997] Processors P1 T3 T2 P2 T5 Owner P3 T4 Block Anders Gidenstam, Max-Planck Institute for Computer Science

  11. The Problem(s) • Implementing blocking synchronization • There can be concurrent operations – synchronization needed • Mutual exclusion? • A slow processor could force others to spin • Slow: e.g. page faults, interrupts, h/w problems Kernel partial solution: • spin lock + disable IRQ Processors P1 T3 T5 P2 Blocking Blocking P3 T4 Anders Gidenstam, Max-Planck Institute for Computer Science

  12. More Related Work • Lock-free threading • Hood. R. Blumofe, D. Papadopoulos, 1998. • Lesser Bear. H. Oguma, Y. Nakayama, 2001. • Lock-free operating systems kernels • Synthesis. H. Massalin, 1992. • Cache Kernel. M. Greenwald, D. Cheriton, 1999. • A. Gavare, P. Tsigas, 2005. Anders Gidenstam, Max-Planck Institute for Computer Science

  13. LFthreads – system overview Processors Ready Queue Running threads T2 T1 P1 T3 Waiting runnable threads P2 T5 T6 T4 P3 Synchronization object w. blocked threads Anders Gidenstam, Max-Planck Institute for Computer Science

  14. Thread synchronization objects • Mutual exclusion object – mutex • Two states: unlocked / locked • Three operations for threads • lock(m) - Locks m. If m is already locked the thread is blocked. • trylock(m) - Tries to lock m. Returns false if m is already locked. • unlock(m) - Unlocks m. Anders Gidenstam, Max-Planck Institute for Computer Science

  15. Mutex implementation • “Typical” mutex implementation • Note: The spin-lock protects the mutex_t record from concurrent updates by different processors • Operations cannot overlap • Processors might be forced to spin waiting for others type mutex_t isrecord state : enum (UNLOCKED, LOCKED); waiting : Queue of thread_t; slock : spin_lock_t; Anders Gidenstam, Max-Planck Institute for Computer Science

  16. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-freeQueue of thread_t;hand-off : integer; lock(M) TA count 0 TX 0 Hand-off TY Anders Gidenstam, Max-Planck Institute for Computer Science

  17. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; Increase Increase lock(M) TA count 1 TX 0 Hand-off TY Anders Gidenstam, Max-Planck Institute for Computer Science

  18. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; Increase Increase lock(M) TA count 2 lock(M) TX 0 Hand-off TY Anders Gidenstam, Max-Planck Institute for Computer Science

  19. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; Increase Enqueue lock(M) TA count 2 TX lock(M) TX 0 Hand-off TY Anders Gidenstam, Max-Planck Institute for Computer Science

  20. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 2 TX lock(M) TX 0 Hand-off TY Anders Gidenstam, Max-Planck Institute for Computer Science

  21. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 TX lock(M) TX 0 Hand-off TY Decrease Decrease Anders Gidenstam, Max-Planck Institute for Computer Science

  22. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TX TY Dequeue Enqueue on ready queue Dequeue and Enqueue on ready queue Anders Gidenstam, Max-Planck Institute for Computer Science

  23. Lock-free mutex implementationThe Hand-off method type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TX TY Dequeue and Enqueue on ready queue Dequeue Enqueue on ready queue Done! Anders Gidenstam, Max-Planck Institute for Computer Science

  24. Lock-free mutex implementation:Slow lock (i) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TY Decrease Decrease Anders Gidenstam, Max-Planck Institute for Computer Science

  25. Lock-free mutex implementation:Slow lock (i) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TY Decrease Uhu?! Empty? Anders Gidenstam, Max-Planck Institute for Computer Science

  26. Lock-free mutex implementation:Slow lock (i) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX TA Hand-off TY Set hand-off flag Decrease Anders Gidenstam, Max-Planck Institute for Computer Science

  27. Lock-free mutex implementation:Slow lock (i) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX TA Hand-off TY Set hand-off flag Still empty? Anders Gidenstam, Max-Planck Institute for Computer Science

  28. Lock-free mutex implementation:Slow lock (i) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX TA Hand-off TY Set hand-off flag Still empty? Done! Anders Gidenstam, Max-Planck Institute for Computer Science

  29. Lock-free mutex implementation:Slow lock (ii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 TX lock(M) TX TA Hand-off TY Set hand-off flag Still empty? Anders Gidenstam, Max-Planck Institute for Computer Science

  30. Lock-free mutex implementation:Slow lock (ii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 TX lock(M) TX TA Hand-off TY Set hand-off flag Still empty? Erase if unchanged Anders Gidenstam, Max-Planck Institute for Computer Science

  31. Lock-free mutex implementation:Slow lock (ii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TX TY Set hand-off flag Still empty? Erase if unchanged Dequeue and Enqueue on ready queue Dequeue Enqueue on ready queue Anders Gidenstam, Max-Planck Institute for Computer Science

  32. Lock-free mutex implementation:Slow lock (ii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 lock(M) TX 0 Hand-off TX TY Set hand-off flag Still empty? Erase if unchanged Dequeue and Enqueue on ready queue Dequeue Enqueue on ready queue Done! Anders Gidenstam, Max-Planck Institute for Computer Science

  33. Lock-free mutex implementation:Slow lock (iii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 TX lock(M) TX Z Hand-off TY Set hand-off flag Still empty? Erase if unchanged Anders Gidenstam, Max-Planck Institute for Computer Science

  34. Lock-free mutex implementation:Slow lock (iii) type mutex_t isrecord count : integer; waiting : Lock-free Queue of thread_t; hand-off : integer; lock(M) unlock(M) TA count 1 TX lock(M) TX Z Hand-off TY Set hand-off flag Still empty? Erase if unchanged Done! Anders Gidenstam, Max-Planck Institute for Computer Science

  35. Proof-of-concept implementation • LFthreads user-level thread library • Linux/IA32 platform • “Cloned” processes as virtual CPUs • Share the sameaddress space,file descriptor table etc • User-level thread contextsbased on • SUSv2 setcontext(2) / getcontext(2) • Non-preemptive scheduling (so far) • POSIX compliance a goal Virtual CPUs CPUs Threads VP1 T3 P1 Ready Queue VP2 T5 T2 T1 VP3 P2 VP4 T4 Kernel space User space Anders Gidenstam, Max-Planck Institute for Computer Science

  36. Experimental evaluation • Micro benchmark • Threads competing for a critical section • High contention • Work: 1 -1 • Low contention • Work: 1 - 1000 • Configurations • LFthreads with 1, 2, 4, 8,16 virtual CPUs • lock-free mutex • spin-lock-based mutex • Platforms standard pthreads (2.6.x kernel) • 2x dual-core AMD Opteron processors Anders Gidenstam, Max-Planck Institute for Computer Science

  37. Experimental evaluation (i) Anders Gidenstam, Max-Planck Institute for Computer Science

  38. Experimental evaluation (i) Anders Gidenstam, Max-Planck Institute for Computer Science

  39. Experimental evaluation (i) Anders Gidenstam, Max-Planck Institute for Computer Science

  40. Experimental evaluation (i) Anders Gidenstam, Max-Planck Institute for Computer Science

  41. Experimental evaluation (i) Anders Gidenstam, Max-Planck Institute for Computer Science

  42. Experimental evaluation (i) Why? Anders Gidenstam, Max-Planck Institute for Computer Science

  43. Experimental evaluation (i) Why? Remember spinning v.s. blocking? Anders Gidenstam, Max-Planck Institute for Computer Science

  44. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  45. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  46. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  47. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  48. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  49. Experimental evaluation (ii) Anders Gidenstam, Max-Planck Institute for Computer Science

  50. Open Problems • Condition variables • Seem to need atomic move thread operation. • Other synchronization objects • Semaphores, barriers, readers-writers locks, … • Signals • Support required by POSIX • Can interrupt blocking • Must be able to remove any thread control block in e.g. a waiting queue. • Thread cancellation • Blocking IO Anders Gidenstam, Max-Planck Institute for Computer Science

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