opening tables mysql_MySQL 大量 Opening tables 案例分析-爱可生
因為一時間沒法排查出具體原因，所以針對 MySQL 進程打了個快照，方便事后詳細排查。
二、分析棧幀
用 pt-pmp 工具處理下快照后，內容比較多，這里只顯示一些比較重要的堆棧：
1402
pthread_cond_wait,wait(os0event.cc:165),os_event::wait_low(os0event.cc:165),sync_array_wait_event(sync0arr.cc:475),TTASEventMutex::wait(ut0mutex.ic:89),spin_and_try_lock(ib0mutex.h:850),enter(ib0mutex.h:850),PolicyMutex(ib0mutex.h:850),dict_table_open_on_name(dict0dict.cc:1238),ha_innobase::open_dict_table(ha_innodb.cc:6250),ha_innobase::open(ha_innodb.cc:5888),handler::ha_open(handler.cc:2759),open_table_from_share(table.cc:3353),open_table(sql_base.cc:3559),open_and_process_table(sql_base.cc:5145),open_tables(sql_base.cc:5145),open_tables_for_query(sql_base.cc:6531),execute_sqlcom_select(sql_parse.cc:5127),mysql_execute_command(sql_parse.cc:2792),mysql_parse(sql_parse.cc:5582),dispatch_command(sql_parse.cc:1458),do_command(sql_parse.cc:999),handle_connection(connection_handler_per_thread.cc:300),pfs_spawn_thread(pfs.cc:2190),start_thread(libpthread.so.0),clone(libc.so.6)
36
nanosleep(libpthread.so.0),os_thread_sleep(os0thread.cc:279),buf_flush_wait_flushed(buf0flu.cc:2074),log_preflush_pool_modified_pages(log0log.cc:1531),log_checkpoint_margin(log0log.cc:1973),log_check_margins(log0log.cc:1973),log_free_check(log0log.ic:491),row_ins_sec_index_entry(log0log.ic:491),row_ins_index_entry(row0ins.cc:3460),row_ins_index_entry_step(row0ins.cc:3460),row_ins(row0ins.cc:3460),row_ins_step(row0ins.cc:3460),row_insert_for_mysql_using_ins_graph(row0mysql.cc:1738),ha_innobase::write_row(ha_innodb.cc:7566),handler::ha_write_row(handler.cc:7991),write_record(sql_insert.cc:1873),Sql_cmd_insert::mysql_insert(sql_insert.cc:769),Sql_cmd_insert::execute(sql_insert.cc:3105),mysql_execute_command(sql_parse.cc:3566),mysql_parse(sql_parse.cc:5582),dispatch_command(sql_parse.cc:1458),do_command(sql_parse.cc:999),handle_connection(connection_handler_per_thread.cc:300),pfs_spawn_thread(pfs.cc:2190),start_thread(libpthread.so.0),clone(libc.so.6)
1
pthread_cond_wait,wait(os0event.cc:165),os_event::wait_low(os0event.cc:165),buf_dblwr_flush_buffered_writes(buf0dblwr.cc:979),buf_dblwr_add_to_batch(buf0dblwr.cc:1154),buf_flush_write_block_low(buf0flu.cc:1099),buf_flush_page(buf0flu.cc:1099),buf_flush_try_neighbors(buf0flu.cc:1453),buf_flush_page_and_try_neighbors(buf0flu.cc:1453),buf_do_flush_list_batch(buf0flu.cc:1780),buf_flush_batch(buf0flu.cc:1780),buf_flush_do_batch(buf0flu.cc:1780),pc_flush_slot(buf0flu.cc:2874),buf_flush_page_cleaner_worker(buf0flu.cc:3504),start_thread(libpthread.so.0),clone(libc.so.6)
1
nanosleep(libpthread.so.0),os_thread_sleep(os0thread.cc:279),buf_flush_wait_flushed(buf0flu.cc:2074),log_preflush_pool_modified_pages(log0log.cc:1531),log_checkpoint_margin(log0log.cc:1973),log_check_margins(log0log.cc:1973),log_free_check(log0log.ic:491),que_run_threads_low(log0log.ic:491),que_run_threads(log0log.ic:491),trx_purge(trx0purge.cc:1883),srv_do_purge(srv0srv.cc:2631),srv_purge_coordinator_thread(srv0srv.cc:2631),start_thread(libpthread.so.0),clone(libc.so.6)
1
nanosleep(libpthread.so.0),os_thread_sleep(os0thread.cc:279),buf_flush_wait_flushed(buf0flu.cc:2074),log_preflush_pool_modified_pages(log0log.cc:1531),log_checkpoint_margin(log0log.cc:1973),log_check_margins(log0log.cc:1973),log_free_check(log0log.ic:491),que_run_threads_low(log0log.ic:491),que_run_threads(log0log.ic:491),que_eval_sql(que0que.cc:1232),dict_stats_exec_sql(dict0stats.cc:311),dict_stats_save(dict0stats.cc:2415),dict_stats_update(dict0stats.cc:3112),dict_stats_process_entry_from_recalc_pool(dict0stats_bg.cc:356),dict_stats_thread(dict0stats_bg.cc:356),start_thread(libpthread.so.0),clone(libc.so.6)
其中 1402 個線程是處于自旋狀態，因為無法拿到對應的 Mutex 鎖而進行忙等，應該就是對應我們看到的大量 Opening tables 的線程，但是具體什么鎖需要跟蹤下 MySQL 源碼才能知道。
36 個線程都是 insert 操作，最后都再等待 buf_flush_wait_flushed，貌似是等待刷盤操作。
最后三個線程分別是：
1. page_cleaner：后臺刷臟線程，最后處于 buf_dblwr_flush_buffered_writes。
2. purge_thread：purge 線程(清理 undo)，最后處于 buf_flush_wait_flushed。
3. dict_stats_thread：統計信息更新線程，最后處于 buf_flush_wait_flushed。
簡單分析下這些線程，發現 insert、pugre_thread、dict_stats_thread 的最后一部分堆棧調用都是如下：
log_free_check
-> log_check_margins
-> log_checkpoint_margin
-> log_preflush_pool_modified_pages
-> buf_flush_wait_flushed
-> os_thread_sleep
三、源碼跟蹤
Server version: 5.7.22-log MySQL Community Server (GPL)
這里我把源碼分析主要拆成 2 塊，第一塊是分析 1402 個線程的堆棧，看下是在等待什么鎖資源，第二塊是分析這個鎖資源被哪個線程占用了，為啥不釋放？
先來看第一部分，過程就是從 MySQL 快照中找到一個如下類似的線程(這里去掉了一些參數顯示)，然后從 MySQL 源碼中一步一步分析函數調用。
#0 0x00007efc61303965 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1 0x0000000000ffb03b in wait at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/os/os0event.cc:165
#2 os_event::wait_low at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/os/os0event.cc:335
#3 0x00000000010a0ce9 in sync_array_wait_event at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/sync/sync0arr.cc:475
#4 0x0000000000f8c1c4 in TTASEventMutex::wait at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/include/ut0mutex.ic:89
#5 0x0000000000f8c33b in spin_and_try_lock at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/include/ib0mutex.h:850
#6 enter at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/include/ib0mutex.h:707
#7 PolicyMutex >::enter at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/include/ib0mutex.h:987
#8 0x0000000001152f41 in dict_table_open_on_name at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0dict.cc:1238
#9 0x0000000000f72a73 in ha_innobase::open_dict_table at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/handler/ha_innodb.cc:6250
#10 0x0000000000f8273b in ha_innobase::open at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/handler/ha_innodb.cc:5888
#11 0x000000000081b33e in handler::ha_open at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/handler.cc:2759
#12 0x0000000000dc239a in open_table_from_share at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/table.cc:3353
#13 0x0000000000cc18b9 in open_table at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_base.cc:3559
#14 0x0000000000cc52b6 in open_and_process_table at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_base.cc:5145
#15 open_tables at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_base.cc:5756
#16 0x0000000000cc5e62 in open_tables_for_query at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_base.cc:6531
#17 0x0000000000d14ff6 in execute_sqlcom_select at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_parse.cc:5127
#18 0x0000000000d18bce in mysql_execute_command at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_parse.cc:2792
#19 0x0000000000d1aaad in mysql_parse at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_parse.cc:5582
#20 0x0000000000d1bcca in dispatch_command at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_parse.cc:1458
#21 0x0000000000d1cb74 in do_command at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/sql_parse.cc:999
#22 0x0000000000dedaec in handle_connection at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/sql/conn_handler/connection_handler_per_thread.cc:300
#23 0x0000000001256a94 in pfs_spawn_thread at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/perfschema/pfs.cc:2190
#24 0x00007efc612ffdd5 in start_thread () from /lib64/libpthread.so.0
#25 0x00007efc5fdb8ead in clone () from /lib64/libc.so.6
分析過后，發現這種線程，最終是在 dict_table_open_on_name 函數中(注意函數行數為 1238，與堆棧一致)，因為無法獲取 dict_sys->mutex，這把數據字典鎖而產生自旋等待。
再來看第二部分，dict_sys->mutex被哪個線程占了，為啥不釋放？這里有個小技巧，就是只需要分析總數為 1 的函數調用，因為這把數據字典鎖同一時間只能被一個線程占用，那這里主要是就是最后 3 個線程。先從最可疑的線程開始，即 dict_stats_thread，因為它也有一個 dict~
#0 0x00007efc61306e3d in nanosleep () from /lib64/libpthread.so.0
#1 0x0000000000ffb7a0 in os_thread_sleep at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/os/os0thread.cc:279
#2 0x0000000001135779 in buf_flush_wait_flushed at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/buf/buf0flu.cc:2074
#3 0x0000000000fd4589 in log_preflush_pool_modified_pages at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/log/log0log.cc:1531
#4 0x0000000000fd7a69 in log_checkpoint_margin at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/log/log0log.cc:1973
#5 log_check_margins at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/log/log0log.cc:2065
#6 0x000000000101af4d in log_free_check at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/include/log0log.ic:491
#7 que_run_threads_low at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/que/que0que.cc:1109
#8 que_run_threads at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/que/que0que.cc:1155
#9 0x000000000101b86e in que_eval_sql at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/que/que0que.cc:1232
#10 0x0000000001176b9c in dict_stats_exec_sql at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0stats.cc:311
#11 0x0000000001179346 in dict_stats_save at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0stats.cc:2415
#12 0x000000000117a9ca in dict_stats_update at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0stats.cc:3112
#13 0x000000000117cc3a in dict_stats_process_entry_from_recalc_pool at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0stats_bg.cc:356
#14 dict_stats_thread at /export/home/pb2/build/sb_0-27500212-1520171728.22/mysql-5.7.22/storage/innobase/dict/dict0stats_bg.cc:445
#15 0x00007efc612ffdd5 in start_thread () from /lib64/libpthread.so.0
#16 0x00007efc5fdb8ead in clone () from /lib64/libc.so.6
同樣根據該線程的函數調用，去 MySQL 源碼里一步一步跟蹤，看下是否占用了數據字典鎖，比較幸運，在 dict_stats_save 函數中發現其拿走了 數據字典鎖，然后進入 dict_stats_exec_sql函數執行更新統計信息的 SQL，而且期間并未釋放數據字典鎖(這塊需要去函數分支中檢查，因為函數較多，這里省略說明)，如下：
找到數據字典鎖被 dict_stats_thread占用后就得分析為啥它不釋放呢，我們繼續往下看堆棧，這塊就得去著重分析下 log_free_check 開始往下的函數，之前也提到過 insert、pugre_thread、dict_stats_thread 等線程最后都是跑得這段邏輯。
dict_stats_save
-> dict_stats_exec_sql
-> que_eval_sql
-> que_run_threads
-> que_run_threads_low
-> log_free_check /* Check that there is enough space in the log to accommodate possible log entries by this query step. */
-> log_check_margins /* Checks that there is enough free space in the log to start a new query step.Flushes the log buffer or makes a new checkpoint if necessary. */
-> log_checkpoint_margin /* Tries to establish a big enough margin of free space in the log groups, such that a new log entry can be catenated without an immediate need for a checkpoint. */
-> log_preflush_pool_modified_pages /* Advances the smallest lsn for which there are unflushed dirty blocks in the buffer pool. */
-> buf_flush_wait_flushed /* Waits until a flush batch of the given lsn ends */
-> os_thread_sleep
這段邏輯主要是先判斷 log buffer 可用空間是否達到臨界值(log->buf_free > log->max_buf_free)，達到的話需要將 log buffer 中的日志先刷到 redo log 中。
然后判斷 innodb buffer pool 中臟頁占比是否達到臨界值((log->lsn - oldest_lsn) > log->max_modified_age_sync)，達到的話需要進行同步刷臟頁，這個過程可能會比較久，且阻塞當前其他寫入進程。
最后判斷 checkpoint 是否達到臨界值(對比 log->lsn - log->last_checkpoint_lsn、max_checkpoint_age、max_checkpoint_age_async3 個值的大小)，分別選擇進行同步 checkpoint、異步 checkpoint、或是不做 checkpoint。
這里最后進入的是 log_preflush_pool_modified_pages，就是`同步刷臟頁`的過程，刷臟頁的過程就是喚醒 page clean thread 進行寫入，而自身則處于buf_flush_wait_flushed函數中，一直等待刷臟頁完成，就如我們之前堆棧中看到的一樣，insert、pugre_thread、dict_stats_thread 等線程均在等待 page clean thread 刷新臟頁。
這塊可能我理解的不是很透徹~，大家有興趣可以去看下 MySQL · 引擎特性 · InnoDB redo log漫游，里面介紹的非常詳細。
MySQL · 引擎特性 · InnoDB redo log漫游
四、總結
根據上述堆棧分析，總結下來，還是由于磁盤寫入性能太差，導致了 innodb buffer pool 中臟頁LSN 落后當前 LSN 太多，達到臨界值后，觸發了 MySQL 的同步刷臟機制，期間會卡住所有正在進行寫入的線程，即諸多 insert 語句、purge 線程、還有 dict_stats_thread(統計信息更新線程)均被阻塞。
我們一開始看到的許多會話處于 Opening tables 的現象，可以說是一個比較巧合的事。當某張表的數據量變化達到一定量級(默認 10%)，就觸發了后臺的 dict_stats_thread，它會拿著 dict_sys->mutex(數據字典鎖)去更新對應表上面的統計信息，但是更新過程中卡在同步刷臟頁步驟，無法及時釋放 dict_sys->mutex，因此很多 SQL 會因為無法拿到 dict_sys->mutex 而處于 Opening tables 狀態。
總結
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