Percona Server 5.7: Key Performance Algorithms
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In this session we will discuss selected areas of InnoDB and XtraDB 5.7 internals that are mostly related to buffer pool management and flushing, from a performance and scalability point of view. We will describe the motivation behind the features in Percona Server XtraDB 5.7 (buffer pool mutex split; multi-threaded LRU flusher; parallel doublewrite), given that MySQL InnoDB 5.7 has reimplemented many similar features that we have in XtraDB 5.6 -- especially for disk-bound RW workloads. The talk will be technical, and assumes you have familiarity with InnoDB internals.
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Percona Server 5.7: Key Performance Algorithms
- 1. XtraDB 5.7: Key Performance Algorithms Laurynas Biveinis Alexey Stroganov Percona firstname.lastname@percona.com
- 2. XtraDB 5.7 Key Performance Algorithms • Focus on the buffer pool, flushing, the doublewrite buffer • Talk assumes familiarity, but feel free to interrupt • What we learned • What we did • How we did it
- 3. InnoDB buffer pool • Memory cache of disk data pages • In-memory data pages accessible through several data structures • 1) Page hash for lookup (space_id; page_id) ? Hash array Data page lists Fold
- 4. InnoDB buffer pool • 2) flush list for dirty page management. Dirtying: Clean page INSERT INTO foo VALUES(bar) Dirty page, LSN = 42 Dirty page, LSN = 25 Dirty page, LSN = 32 Dirty page, LSN = 42 Flush list tail:
- 5. InnoDB buffer pool • 2) flush list for dirty page management. Flushing: Dirty page, LSN = 5 Dirty page, LSN = 7 Dirty page, LSN = 12 Flush list head: Flush up to LSN 10 Clean page Clean page Dirty page, LSN = 12 Flush list head:
- 6. InnoDB buffer pool • 3) LRU list for deciding which pages to evict • Preventing eviction for recently-used pages (making them young): Dirty pageClean page Dirty page Clean page Clean page Page access Dirty pageClean page Dirty page Clean pageClean page
- 7. InnoDB buffer pool • 4) free list for having free space in the buffer pool to read currently non-present pages. Reading: Free page Free page Free page Free page Free page Page read Free page Free page Free page Free page Clean page
- 8. InnoDB buffer pool • 3/4) Evicting/flushing pages from the LRU list and putting them on the free list: Dirty pageClean page Dirty page Clean page Clean page Free page Free page Free page Free page Dirty page Dirty page Clean page Clean page Free page Free page Free page Free page Free page
- 9. The doublewrite buffer Data page Doublewrite buffer in disk Data file Doublewrite buffer in memory Add Flush Write Step 1 Step 2 Step 3
- 10. Buffer pool concurrency flush list LRU list free listpage hash misc. buffer pool mutexflush list mutexpage hash latch
- 11. Buffer pool instances flush list LRU list free list page hash misc. buffer pool mutexflush list mutexpage hash latch buffer pool instance 0 flush list LRU list free list page hash misc. buffer pool mutexflush list mutexpage hash latch buffer pool instance 1
- 12. Buffer pool instances • Problem: some instances are cold and some are hot • “First the accesses to the buffer pools is in no way evenly spread out.” • http://bit.ly/bpsplit • Six year-old quote, still relevant the same today
- 13. Concurrency in XtraDB flush listpage hash flush list mutexpage hash latch LRU list LRU list mutex free list free list mutex misc misc mutex / atomics Patch contributed to MySQL, and merged in 8.0.0 http://bugs.mysql.com/bug.php?id=75534
- 14. Concurrency solutions are compatible flush listpage hash flush list mutexpage hash latch LRU list LRU list mutex free list free list mutex misc misc mutex / atomics buffer pool instance 0 buffer pool instance 1 flush listpage hash flush list mutexpage hash latch LRU list LRU list mutex free list free list mutex misc misc mutex / atomics
- 15. Buffer pool mutexes are so 5.5 Improvement by the buffer pool mutex split Improvement by adaptive flushing
- 16. 5.6+ changed things • In 5.5 and earlier: reduce mutex contention by X%, observe TPS increase by ~X% • Changing flushing heuristics is driven by performance stability, not necessarily by peak performance • Pre-release Percona Server 5.6: reduce mutex contention by X%, observe TPS increase by ~0% • What happened? InnoDB cleaner thread happened
- 17. Buffer pool / flushing concurrency in 5.5 Time Master thread Query thread 1 Query thread 2 flush list flush flush list flush flush list flush make page young make page young LRU list flush make page young LRU list flush make page young
- 18. Buffer pool / flushing concurrency in 5.6+ Time Cleaner thread Query thread 1 Query thread 2 flush list flush flush list flush flush list flush make page young make page young LRU list flush make page young LRU list flush make page young LRU list flush
- 19. Buffer pool / flushing concurrency in 5.6+ • In 5.6+, code-level changes to reduce locking granularity are still important, but • Increasing thread specialization means that… • …flushing - including LRU - heuristics are very important now
- 20. MySQL 5.7 multi-threaded flushing LRU instance #0 flush list instance #0 LRU instance #1 flush list instance #1 LRU instance #2 flush list instance #2 coordinator thread worker thread #0 worker thread #1 time 0 s 1 s LRU… LRU… LRU…
- 22. PFS data is incomplete
- 23. MySQL 5.7.11 OLTP_RW 660 pthread_cond_wait,enter (ib0mutex.h:850), buf_dblwr_write_single_page (ib0mutex.h:850),buf_flush_write_block_low(buf0flu.cc:1096),buf_flush_page (buf0flu.cc1096),buf_flush_single_page_from_LRU (buf0flu.cc:2217), buf_LRU_get_free_block(buf0lru.cc:1401),... 631 pthread_cond_wait,buf_dblwr_write_single_page (buf0dblwr.cc:1213), buf_flush_write_block_low(buf0flu.cc:1096),buf_flush_page (buf0flu.cc:1096), buf_flush_single_page_from_LRU (buf0flu.cc:2217), buf_LRU_get_free_block(buf0lru.cc:1401),... 337 pthread_cond_wait,PolicyMutex<TTASEventMutex<GenericPolicy> (ut0mutex.ic:89),get_next_redo_rseg (trx0trx.cc:1185), trx_assign_rseg_low(trx0trx.cc:1278),trx_set_rw_mode (trx0trx.cc:1278), lock_table(lock0lock.cc:4076),...
- 25. Single-page flushing Is free page available? Single-page flush Take a free page from the free list Query thread needs a free page Yes No Single-page doublewrite Query thread has a free page
- 26. XtraDB innodb_empty_free_list_algorithm=backoff Is free page available? Wait Take a free page from the free list Query thread needs a free page Yes No Single-page doublewrite Query thread has a free page Single-page flush
- 27. MySQL 5.7 multi-threaded flushing LRU instance #0 flush list instance #0 LRU instance #1 flush list instance #1 LRU instance #2 flush list instance #2 coordinator thread worker thread #0 worker thread #1 time 0 s 1 s LRU… LRU… LRU… free pages Single page flushes! free pages
- 28. Percona Server 5.7 multi- threaded flushing LRU flusher #0 LRU flusher #1 LRU instance #0 LRU instance #0 LRU… free pages LRU instance #1 LRU… free pages flush list instance #0 flush list instance #1 coordinator worker #0 time 0 s 1 s flush… flush…
- 30. Percona Server 5.7.10-3 OLTP_RW 2678 nanosleep (libpthread.so.0), … ,buf_LRU_get_free_block (buf0lru.cc:1435), ... 867 pthread_cond_wait,...,log_write_up_to(log0log.cc:1293),... 396 pthread_cond_wait,…, mtr_t::s_lock(sync0rw.ic:433), btr_cur_search_to_nth_level(btr0cur.cc:1022),... 337 libaio::??(libaio.so.1),LinuxAIOHandler::collect (os0file.cc: 2325), ... 240 poll(libc.so. 6),...,Protocol_classic::read_packet(protocol_classic.cc:810),...
- 31. 2678 nanosleep, …, buf_LRU_get_free_block
- 32. Percona Server 5.7.10-3 OLTP_RW flushers only
- 33. Legacy doublewrite buffer: adding pages
- 34. Percona Server 5.7.10-3 OLTP_RW flushers only 139 libaio::??(libaio.so.1),LinuxAIOHandler::collect (os0file.cc:2448), LinuxAIOHandler::poll(os0file.cc:2594),... 56 pthread_cond_wait,…,buf_dblwr_add_to_batch (buf0dblwr.cc:1111),…,buf_flush_LRU_list_batch (buf0flu.cc:1555), ...,buf_lru_manager(buf0flu.cc:2334),... 25 pthread_cond_wait,…,os_event_wait_low (os0event.cc:534),buf_flush_page_cleaner_worker(buf0flu.cc:3482),... 21 pthread_cond_wait, …, PolicyMutex<TTASEventMutex<GenericPolicy> (ut0mutex.ic:89),buf_page_io_complete (buf0buf.cc:5966), fil_aio_wait(fil0fil.cc:5754),io_handler_thread(srv0start.cc:330),... 8 pthread_cond_timedwait,…,buf_flush_page_cleaner_coordinator (buf0flu.cc:2726),...
- 35. 56 pthread_cond_wait, …, buf_dblwr_add_to_batch
- 36. Legacy doublewrite buffer: flushing buffer
- 37. Parallel doublewrite buffer: adding pages
- 38. Parallel doublewrite buffer: flushing buffers
- 39. Percona Server 5.7.11-4 OLTP_RW flushers only
- 40. Percona Server 5.7.11-4 OLTP_RW flushers only 112 libaio::??(libaio.so.1),LinuxAIOHandler::collect (os0file.cc:2455),...,io_handler_thread(srv0start.cc:330),... 54 pthread_cond_wait,…,buf_dblwr_flush_buffered_writes (buf0dblwr.cc:1287),…,buf_flush_LRU_list (buf0flu.cc:2341),buf_lru_manager(buf0flu.cc:2341),... 35 pthread_cond_wait, …, PolicyMutex<TTASEventMutex<GenericPolicy> (ut0mutex.ic:89), buf_page_io_complete(buf0buf.cc:5986), …, io_handler_thread(srv0start.cc:330),... 27 pthread_cond_wait,...,buf_flush_page_cleaner_worker(buf0flu.cc:3489),... 10 pthread_cond_wait,…,enter(ib0mutex.h:845), buf_LRU_block_free_non_file_page(ib0mutex.h:845), buf_LRU_block_free_hashed_page(buf0lru.cc:2567), …,buf_page_io_complete(buf0buf.cc:6070), …,io_handler_thread (srv0start.cc:330),...
- 43. Summary: 5.7 story • I/O-bound workloads: high demand for free pages, provided by LRU batch flushing or single-page flushing • Single-page flushes are bad, w/ and w/o doublewrite • Removed it • Made batch LRU flusher truly parallel • Doublewrite buffer negates parallel flushing gains • Made it parallel too