MongoDB Europe 2016 - Building WiredTiger
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Hidden inside MongoDB is the WiredTiger data engine, an Open Source, pluggable storage engine that became the database's default in 3.2. Written in C, WiredTiger uses a variety of techniques to provide unmatched performance, low latency and scalability. This talk will explore data structures and techniques C/C++ programmers can use to support heavily threaded applications on modern hardware, using examples from the WiredTiger code base. Data structures and techniques to be covered include hazard pointers, skiplists, ticket locks, atomic instructions and memory barriers.
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MongoDB Europe 2016 - Building WiredTiger
- 1. And now for something completely different...
- 2. WiredTiger: Fast data structures in C
- 3. Keith Bostic MongoDB WiredTiger team keith.bostic@mongodb.com
- 4. #MDBE16 You are here: database layers Middleware Networking Query APIs Storage Engine
- 5. #MDBE16 Storage engines are performance critical Middleware Networking Query APIs mmapV1 Storage Engine RocksDB Storage Engine WiredTiger Storage Engine ACID transactional guarantees
- 6. #MDBE16 WiredTiger • From (some of) the folks that brought you Berkeley DB • High performance data engine • scalable throughput with low latency • MongoDB’s default storage engine • a general-purpose workhorse
- 7. Next Ø Hardware (is the problem) • Hazard pointers • Skiplists • Ticket locks
- 8. #MDBE16 Modern servers have many CPUs/cores core 3 core 2 core 1 core N
- 9. #MDBE16 Each core has multiple memory caches core 3 core 2 core 1 core N two or more caches two or more caches two or more caches two or more caches
- 10. #MDBE16 Cache coherence: cores “snoop” on writes core 3 core 2 core 1 core N two or more caches two or more caches two or more caches Main Memory two or more caches
- 11. #MDBE16 Traditional data engines struggle with this architecture • Writing “shared” memory is slow • but databases exist to manage shared access to data! • Snoopy cache-coherence scales poorly
- 12. #MDBE16 Programmers solve with locking • Locks are complex objects • get exclusive access to the lock state • review and update the lock state • “publish” (ensure every CPU sees the changes) • release exclusive access
- 13. #MDBE16 Locking is slow • Every operation requires exclusive access • even shared (“read”) locks require a lock/unlock cycle • thread stall is inevitable • Locks require notification of every CPU • Locks require exclusive access to the memory bus
- 14. #MDBE16 Locking is expensive • A lock per object is too much memory • POSIX locks cache-aligned, up to 128B • grouping objects under locks makes contention worse • More complexity to make locks “fair” and avoid starvation • add thread queues • wake-up the next thread waiting for the lock
- 15. #MDBE16 We need to find something else If we can’t use locks, what do we use instead? Today we’re going to talk about ways to get rid of locks.
- 16. #MDBE16 WiredTiger is written in C • Java or C++ are better choices for system programming • automatic memory management vs. malloc/free • exception handling vs. explicit error paths • widespread availability of reusable components • Giving up programmer productivity
- 17. #MDBE16 C is “portable assembler” • Marshall typed values to/from unaligned memory • streaming compression, encryption, checksums • unstructured I/O to/from stable storage • Light-weight access to shared data • use the underlying machine primitives that make up locks • algorithms/structures based on those primitives
- 18. You may have seen this last year:
- 19. Next • Hardware Ø Hazard pointers • Skiplists • Ticket locks
- 20. #MDBE16 Pages in the WiredTiger cache page 2 page 6 page 8 page 9 Lots and lots (and lots) of pages MongoDB worker threads read from disk WiredTiger server threads evict to disk
- 21. #MDBE16 A reasonable page-locking implementation • MongoDB worker threads read, modify pages • WiredTiger server threads evict pages from the cache • Allocate a lock per page • MongoDB worker threads share pages • WiredTiger eviction threads require exclusive access
- 22. #MDBE16 Page locking in the WiredTiger cache page 2 page 6 page 8 page 9 eviction lock lock lock lock writer reader thread stall on read locks! vulnerable to starvation too much memory
- 23. #MDBE16 Introducing memory barriers • Memory barriers • order reads, writes or both across a line of code • compiler won’t cache values or reorder across a barrier • Locks imply memory barriers
- 24. #MDBE16 Something faster • Hazard pointers: a technique for avoiding locks • MongoDB worker threads • “log” intention to access a page • publish: a memory barrier to ensure global CPU visibility • Write to a per-thread memory location • write won’t collide with other worker threads
- 25. #MDBE16 What about eviction starvation? • Add a per-page “blocker” • MongoDB worker won’t proceed if the page is blocked • Cheap: • it’s only a bit of information • a read-only operation for workers
- 26. #MDBE16 Worker threads • Publish intent to access the page • Memory barrier to ensure global CPU visibility • If the page not blocked, it’s accessible • Clear intent to access when done
- 27. #MDBE16 Hazard pointers for workers page 2 page 6 page 8 page 9 flag writer reader flag flag flag page 9 page 2 page 6 page 2 page 9
- 28. #MDBE16 Eviction server • Block future worker thread access • Memory barrier to ensure global CPU visibility • Review worker thread access intentions • can either wait or quit • Unblock worker thread access when done
- 29. #MDBE16 Hazard pointers for workers and eviction page 2 page 6 page 8 page 9 flag flag flag flag writer reader page 9 page 2 page 6 page 2 page 9 eviction
- 30. #MDBE16 Something faster: hazard pointers Replaces two lock/unlock pairs for each page access ... with a single memory barrier instruction. • Transfers work to the eviction server • MongoDB worker latency is what we care about • Memory costs • per-worker-thread list • per-page blocking flag
- 31. Next • Hardware • Hazard pointers Ø Skiplists • Ticket locks
- 32. #MDBE16 Introducing atomic instructions • Atomic increment or decrement • read a value • change it and store it back without the possibility of racing • Based on compare-and-swap (CAS) instruction • read value • update value if the value is unchanged • but fail if the value has changed
- 33. #MDBE16 Atomic prepend to singly-linked list Update head if (and only if), head’s value is unchanged head NEW new.next = head compare_and_swap(head, new.next, new)
- 34. #MDBE16 How WiredTiger uses skiplists • WiredTiger pages start with a disk image • a compact representation we don’t want to modify • Inserts and updates for the disk image stored in skiplists
- 35. #MDBE16 Skiplists start with a linked list Singly-linked list with sorted values: 7, 10, 13, 18, 21, 24 7 10 211813 24
- 36. #MDBE16 Skiplists: add additional linked lists Each higher level “skips” over more of the list 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 37. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 38. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 39. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 40. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 41. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 42. #MDBE16 Search for 18 search starts at the top-level 1:7 3:7 2:7 1:10 1:211:181:13 1:24 2:13 2:21 3:21 2:24
- 43. #MDBE16 Skiplists, the great Replaces a lock/unlock pair over the entire skiplist with one atomic memory instruction per object level • Insert without locking • Search without locking, while inserting • Forward & backward traversal without locking, while inserting
- 44. #MDBE16 Skiplists, the good • Simpler code than a Btree • WiredTiger binary search ~200 lines of code • a typical skiplist search < 20 • Fast search • a Btree guarantees search in logarithmic time • skiplists don’t offer a guarantee, but are usually close
- 45. #MDBE16 Skiplists, the not-so-good • Cache-unfriendly • every indirection a CPU cache miss • Memory-unfriendly • needs more memory for a data set than a Btree • Removal requires locking • WiredTiger is an MVCC engine (multiple values per key) • removal less important to WiredTiger
- 46. Next • Hardware • Hazard pointers • Skiplists Ø Ticket locks
- 47. #MDBE16 Ticket locks • WiredTiger still needs to lock objects • but we can make locks faster • Ticket locks • customers take a unique ticket number • customers served in ticket order
- 48. #MDBE16 Ticket locks Please Take a Number 42 43414039 Now Serving
- 49. #MDBE16 Ticket locks • Two incrementing counters: ticket: the next available ticket number serving: the ticket number now being served • Thread takes a ticket number • Thread increments “next available” • Thread waits for “serving” to match its ticket number • When thread finishes, increments “serving”
- 50. #MDBE16 Ticket locks serialize threads 40 Now Serving 39 Thread A 39 40 39 40 41 Thread B
- 51. #MDBE16 Ticket locks are almost what we need • Ticket locks avoid starvation and are “fair” • Smaller memory footprint • Can be made significantly faster than POSIX locks • remember our compare-and-swap instructions! • But POSIX locks are shared between readers
- 52. #MDBE16 Ticket locks: shared vs. exclusive • Three incrementing counters: ticket: the next available ticket number readers: the next reader to be served writers: the next writer to be served
- 53. #MDBE16 Readers run in parallel 40 Writers Readers 39 Thread A 39 40 41 41 39 40 41 42 39 40 41 42 Thread B Thread C
- 54. #MDBE16 Multiple variable updates without locking • Updating multiple counters would require locking ... but we can write the bus width atomically • Encode the entire lock state in a single 8B value lock { uint16_t readers; uint16_t writers; uint16_t ticket; // 64K simultaneous threads uint16_t unused; }
- 55. #MDBE16 Ticket locks Replaces two higher-level lock/unlock calls ... with two atomic instructions.
- 56. #MDBE16 That’s a (very) fast introduction.... • Hazard pointers • Skiplists • Ticket locks Open Source implementations are available in WiredTiger, including Public Domain ticket locks.
- 57. #MDBE16 WiredTiger distribution • Standalone application database toolkit library • key-value store (NoSQL) • row-store, column-store and LSM engines • schema layer includes data types and indexes • Another MongoDB Open Source contribution • WiredTiger available for other applications • https://github.com/wiredtiger