mysql的thread

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本篇内容主要讲解“mysql 的 thread_running 数量分析”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让丸趣 TV 小编来带大家学习“mysql 的 thread_running 数量分析”吧!

thread pool 的原理：

已在 server 层完成解析；

层创建多组常驻线程，用于接收客户端连接发送的 query 并代为执行，而不是为每个连接单独创建一个线程。

层进行 running thread 数量判断，如果达到阈值则直接报错或 sleep。

状态变量记录了当前并发执行 stmt/command 的数量，执行前加 1 执行后减 1；

突然飙高的诱因：

客户端连接暴增；

系统性能瓶颈，如 CPU,IO 或者 mem swap；

异常 sql；

会表现出 hang 住的假象。

执行，为此引入两个阈值 low_watermark 和 high_watermark，以及变量 threads_running_ctl_mode(selects 或者 all)；

前，检查 thread_running，

若其已达 high_watermark 阈值则直接拒绝执行并返回错误：mysql
server is too busy

若其位于 low 和 high 之间，则 sleep 5ms，然后继续尝试，累计等待 100ms 后则执行

对于已经开启事务和 super 用户，不做限制

控制 query 类型：SELECTS/ALL，默认为 SELECTS，表示只影响 SELECT 语句

源码见注 1

优化为基于 FIFO 的 cond-wait/signal(实现 8 个 FIFO)；

高水位限流(这点保持不变)；

低水位优化；其他解决方案：mariadb 开发 thread pool，percona 在其上实现了优先队列；

优势：思路与 thread pool 一致，但代码更简洁(不到 1000 行)；而且增加了特定 query 的过滤；

代码见注 2

新增 thread_active 记录并发线程数，位于 mysql_execute_command(sql 解析之后)，高水位则在 query 解析之前判断；

只统计 select/DML，而 commit/rollback 则放过。

采用 FIFO，当 thread_active =
thread_running_low_watermark 时进程进入 FIFO 等待，其他线程执行完 sql 后唤醒 FIFO；

内，同时引入 threads_running_wait_timeout 控制线程在 FIFO 最大等待时间，超时则直接报错返回。

引入 8 个 FIFO，降低了进出 FIFO 的锁竞争，线程采用 RR 分配到不同 fifo，每个队列限制并发运行线程为 threads_running_low_watermark/8。

，开始执行 query，[解析后进行低水位判断，若通过则执行]，执行当前 sql 完毕后，thread 可能发起新 query，则重复 [] 过程。

：进入 FIFO 排队最长时间，等待超时后 sql 被拒，默认 100，单位为毫秒 ms。

当前并发 SELECT/INSERT/UPDATE/DELETE 执行的线程数目；

：当前进入到 FIFO 中等待的线程数目；

未打补丁版本，设置 innodb_thread_concurrency=0

未打补丁版本，innodb_thread_concurrency=32

低水位限流补丁版本（活跃线程数不超过 64）

注 1

http://www.gpfeng.com/wp-content/uploads/2013/09/threads_running_control.txt

+static my_bool thread_running_control(THD *thd, ulong tr)
+{
+ int slept_cnt= 0;
+ ulong tr_low, tr_high;
+ DBUG_ENTER(thread_running_control
+
+ /*
+ Super user/slave thread will not be affected at any time,
+ transactions that have already started will continue.
+ */
+ if (thd- security_ctx- master_access SUPER_ACL|| – 对于 super 权限的用户和已经开启的事务不做限制
+ thd- in_active_multi_stmt_transaction() ||
+ thd- slave_thread)
+ DBUG_RETURN(FALSE);
+
+ /*
+ To promise that tr_low will never be greater than tr_high,
+ as values may be changed between these two statements.
+ eg.
+ (low, high) = (200, 500)
+ 1. read low = 200
+ 2. other sessions: set low = 20; set high = 80
+ 3. read high = 80
+ Don t take a lock here to avoid lock contention.
+ */
+ do
+ {
+ tr_low= thread_running_low_watermark;
+ tr_high= thread_running_high_watermark;
+
+ } while (tr_low tr_high);
+
+check_buzy:
+
+ /* tr_high is promised to be non-zero.*/
+ if ((tr_low == 0 tr tr_high) || (tr_low != 0 tr tr_low))
+ DBUG_RETURN(FALSE);
+
+ if (tr = tr_high)
+ {
+ int can_reject= 1;
+
+ /* thread_running_ctl_mode: 0 – SELECTS, 1 – ALL. */
+ if (thread_running_ctl_mode == 0)
+ {
+ int query_is_select= 0;
+ if (thd- query_length() = 8)
+ {
+ char *p= thd- query(); – 读取 query text 的前 6 个字符，以判断是否为 select
+ if (my_toupper(system_charset_info, p[0]) == S
+ my_toupper(system_charset_info, p[1]) == E
+ my_toupper(system_charset_info, p[2]) == L
+ my_toupper(system_charset_info, p[3]) == E
+ my_toupper(system_charset_info, p[4]) == C
+ my_toupper(system_charset_info, p[5]) == T )
+
+ query_is_select= 1;
+ }
+
+ if (!query_is_select)
+ can_reject= 0;
+ }
+
+ if (can_reject)
+ {
+ inc_thread_rejected();
+ DBUG_RETURN(TRUE);
+ }
+ else
+ DBUG_RETURN(FALSE);
+ }
+
+ if (tr_low != 0 tr = tr_low)
+ {
+ /*
+ If total slept time exceed 100ms and thread running does not
+ reach high watermark, let it in.
+ */
+ if (slept_cnt = 20)
+ DBUG_RETURN(FALSE);
+
+ dec_thread_running()
+
+ /* wait for 5ms. */
+ my_sleep(5000UL);
+
+ slept_cnt++;
+ tr= inc_thread_running() – 1;
+
+ goto check_buzy;
+ }
+
+ DBUG_RETURN(FALSE);
+}
+
+/**
Perform one connection-level (COM_XXXX) command.
@param command type of command to perform
@@ -1016,7 +1126,8 @@
thd- set_query_id(get_query_id());
if (!(server_command_flags[command] CF_SKIP_QUERY_ID))
next_query_id();
– inc_thread_running();
+ /* remember old value of thread_running for *thread_running_control*. */
+ int32 tr= inc_thread_running() – 1;
if (!(server_command_flags[command] CF_SKIP_QUESTIONS))
statistic_increment(thd- status_var.questions, LOCK_status);

@@ -1129,6 +1240,13 @@
{
if (alloc_query(thd, packet, packet_length))
break; // fatal error is set
+
+ if (thread_running_control(thd, (ulong)tr))
+ {
+ my_error(ER_SERVER_THREAD_RUNNING_TOO_HIGH, MYF(0));
+ break;
+ }
+
MYSQL_QUERY_START(thd- query(), thd- thread_id, (char *) (thd- db ? thd- db :), thd- security_ctx- priv_user[0])

注 2
http://www.gpfeng.com/wp-content/uploads/2014/01/tr-control.diff_.txt
+/**
Perform one connection-level (COM_XXXX) command.

@param command type of command to perform
@@ -1177,7 +1401,7 @@
command= COM_SHUTDOWN;
}
thd- set_query_id(next_query_id());
– inc_thread_running();
+ int32 tr= inc_thread_running();

if (!(server_command_flags[command] CF_SKIP_QUESTIONS))
statistic_increment(thd- status_var.questions, LOCK_status);
@@ -1209,6 +1433,15 @@
goto done;
}

+ if (command == COM_QUERY alloc_query(thd, packet, packet_length))
+ goto endof_case; // fatal error is set
+
+ if (thread_running_control_high(thd, tr))
+ {
+ my_error(ER_SERVER_THREAD_RUNNING_TOO_HIGH, MYF(0));
+ goto endof_case;
+ }
+
switch (command) {
case COM_INIT_DB:
{
@@ -1311,8 +1544,6 @@
}
case COM_QUERY:
{
– if (alloc_query(thd, packet, packet_length))
– break; // fatal error is set
MYSQL_QUERY_START(thd- query(), thd- thread_id,
(char *) (thd- db ? thd- db :),
thd- security_ctx- priv_user[0],
@@ -1751,6 +1982,7 @@
my_message(ER_UNKNOWN_COM_ERROR, ER(ER_UNKNOWN_COM_ERROR), MYF(0));
break;
}
+endof_case:

done:
DBUG_ASSERT(thd- derived_tables == NULL
@@ -2502,12 +2734,37 @@
Opt_trace_array trace_command_steps(thd- opt_trace, steps

DBUG_ASSERT(thd- transaction.stmt.cannot_safely_rollback() == FALSE);
+ bool count_active= false;

if (need_traffic_control(thd, lex- sql_command))
{
thd- killed = THD::KILL_QUERY;
goto error;
}
+
+ switch (lex- sql_command) {
+
+ case SQLCOM_SELECT:
+ case SQLCOM_UPDATE:
+ case SQLCOM_UPDATE_MULTI:
+ case SQLCOM_DELETE:
+ case SQLCOM_DELETE_MULTI:
+ case SQLCOM_INSERT:
+ case SQLCOM_INSERT_SELECT:
+ case SQLCOM_REPLACE:
+ case SQLCOM_REPLACE_SELECT:
+ count_active= true;
+ break;
+ default:
+ break;
+ }
+
+ if (count_active thread_running_control_low_enter(thd))
+ {
+ my_error(ER_SERVER_THREAD_RUNNING_TOO_HIGH, myf(0));
+ goto error;
+ }
+
status_var_increment(thd- status_var.com_stat[lex- sql_command]);

switch (gtid_pre_statement_checks(thd))
@@ -4990,6 +5247,9 @@

finish:

+ if (count_active)
+ thread_running_control_low_exit(thd);
+
DBUG_ASSERT(!thd- in_active_multi_stmt_transaction() ||
thd- in_multi_stmt_transaction_mode());

+static my_bool thread_running_control_high(THD *thd, int32 tr)
+{
+ int32 tr_high;
+ DBUG_ENTER(thread_running_control_high
+
+ tr_high= (int32)thread_running_high_watermark;
+
+ /* thread_running_ctl_mode: 0 – SELECTS, 1 – ALL. */
+ if ((!tr_high || tr = tr_high) ||
+ thd- transaction.is_active() ||
+ thd- get_command() != COM_QUERY ||
+ thd- security_ctx- master_access SUPER_ACL ||
+ thd- slave_thread)
+ DBUG_RETURN(FALSE);
+
+ const char *query= thd- query();
+ uint32 len= thd- query_length();
+
+ if ((!has_prefix(query, len, SELECT , 6) thread_running_ctl_mode == 0) || – 不再是逐个字符判断
+ has_prefix(query, len, COMMIT , 6) ||
+ has_prefix(query, len, ROLLBACK , 8))
+ DBUG_RETURN(FALSE);
+
+ /* confirm again*/
+ if (tr tr_high get_thread_running() tr_high)
+ {
+ __sync_add_and_fetch(thread_rejected, 1);
+ DBUG_RETURN(TRUE);
+ }
+
+ DBUG_RETURN(FALSE);
+}
+

+static my_bool thread_running_control_low_enter(THD *thd)
+{
+ int res= 0;
+ int32 tr_low;
+ my_bool ret= FALSE;
+ my_bool slept= FALSE;
+ struct timespec timeout;
+ Thread_conc_queue *queue;
+ DBUG_ENTER(thread_running_control_low_enter
+
+ /* update global status */
+ __sync_add_and_fetch(thread_active, 1);
+
+ tr_low= (int32)queue_tr_low_watermark;
+ queue= thread_conc_queues + thd- query_id % N_THREAD_CONC_QUEUE;
+
+ queue- lock();– 问 1：在进行低水位判断前，先锁定 FIFO，避免低水位验证失败时无法获取 FIFO 锁进而不能放入 FIFO；
+
+retry:
+
+ if ((!tr_low || queue- thread_active tr_low) ||
+ (thd- lex- sql_command != SQLCOM_SELECT thread_running_ctl_mode == 0) ||
+ (!slept (thd- transaction.is_active() ||
+ thd- security_ctx- master_access SUPER_ACL || thd- slave_thread)))
+ {
+ queue- thread_active++; – 判断是否满足进入 FIFO 条件，如不满足则立即更新 thread_active++，解锁 queue 并退出；
+ queue- unlock();
+ DBUG_RETURN(ret);
+ }
+
+ if (!slept)
+ {
+ queue- unlock();
+
+ /* sleep for 500 us */
+ my_sleep(500);
+ slept= TRUE;
+ queue- lock();
+
+ goto retry;
+ }
+
+ /* get a free wait-slot */
+ Thread_wait_slot *slot= queue- pop_free();
+
+ /* can t find a free wait slot, must let the query enter */
+ if (!slot)– 当 FIFO 都满了，即无法把当前线程放入，则必须放行让该 sql 正常执行
+ {
+ queue- thread_active++;
+ queue- unlock();
+ DBUG_RETURN(ret);
+ }
+
+ slot- signaled= false;
+ slot- wait_ended= false;
+
+ /* put slot into waiting queue. */
+ queue- push_back_wait(slot);
+ queue- thread_wait++;
+
+ queue- unlock();
+
+ /* update global status */
+ thd_proc_info(thd, waiting in server fifo
+ __sync_sub_and_fetch(thread_active, 1);
+ __sync_add_and_fetch(thread_wait, 1);
+
+ /* cond-wait for at most thread_running_wait_timeout(ms). */
+ set_timespec_nsec(timeout, thread_running_wait_timeout_ns);
+
+ mysql_mutex_lock(slot- mutex);
+ while (!slot- signaled)
+ {
+ res= mysql_cond_timedwait(slot- cond, slot- mutex, timeout);
+ /* no need to signal if cond-wait timedout */
+ slot- signaled= true;
+ }
+ mysql_mutex_unlock(slot- mutex);
+
+ queue- lock();
+ queue- thread_wait–;
+ queue- thread_active++;
+
+ /* remove slot from waiting queue. */
+ queue- remove_wait(slot);
+ /* put slot into the free queue for reuse. */
+ queue- push_back_free(slot);
+
+ queue- unlock();
+
+ /* update global status */
+ __sync_sub_and_fetch(thread_wait, 1);
+ __sync_add_and_fetch(thread_active, 1);
+ thd_proc_info(thd, 0);
+
+ if (res == ETIMEDOUT || res == ETIME)
+ {
+ ret= TRUE; // indicate that query is rejected.
+ __sync_add_and_fetch(thread_rejected, 1);
+ }
+
+ DBUG_RETURN(ret);
+}