openGauss/MOGDB与PG等待事件

原创阎书利 2022-02-10

1235

数据库版本：PG12.1 openGauss/MOGDB 2.1.0

最近看到了许多关于PG等待事件的文章，对等待事件这部分也有了很大的兴趣。

等待事件是一个累计的统计信息，表明一个server process要继续完成作业，必须等待一个时间的结束；因为系统资源有限，那么完成某些工作，所需资源就要轮流使用，那么在这个过程当中，就会产生等待资源的情况。数据库会用不同类型的定义，来描述这个事情，称之为等待事件。

openGauss/MOGDB数据库是基于PG研发的，PG是从9.6版本加入了等待事件特性，可以通过查询pg_stat_activity中的wait_event_type和wait_event了解到每个sql进程在当前更详细的执行状态。openGauss/MOGDB在PG的基础上有许多优化及改动，把一些等待事件重新定义，等待事件在保留了部分原等待事件的基础上也增加了一部分。

在分析问题的时候，等待事件对于我们还是较为重要的，我们可以根据等待事件，初步定位问题，并结合相关测试进行验证，看到了熟悉的等待事件，我们甚至能大概猜出问题所在。相对于ORACLE来说，PG以及openGauss/MOGDB的等待事件种类和数量较少。在等待事件这方面可能还有极大优化的空间，如果能把等待事件的细粒程度增加，应该会帮助我们更好的了解数据库状态，解决数据库问题。

一般来说产生等待事件的几种情况：
1.请求的资源忙，需要资源释放
2.会话处于空闲状态，等待任务
3.会话被阻塞，需要等待阻塞解除

以下内容对比可能根据PG版本有变化，如果有误，欢迎帮我指正交流。

一、ORACLE与PG与openGauss/MOGDB等待事件种类

在ORACLE 11G里,共有13类等待事件，包含了1367个等待事件，如下所示：

SYS@orcl11g> select distinct wait_class from v$event_name order by 1;
 
 	WAIT_CLASS
 	--------------------------------------
 	Administrative		--管理类
 	Application			--应用类
 	Cluster				    --集群类
 	Commit				--提交类
 	Concurrency		--并发
 	Configuration		--配置
 	Idle				        --空闲
 	Network				--网络
 	Other				    --其他
 	Queueing				--队列
 	Scheduler				--任务调度
 	System I/O			--系统I/O
 	User I/O				--用户I/O
 
 	13 rows selected.
 	
 SYS@orcl11g>select count(name) from v$event_name; 
 
 COUNT(NAME)
 -------------------
 	              1367

而PG数据库里，有着9类等待事件

 /* ----------
  * Wait Classes
  * ----------
  */
 #define PG_WAIT_LWLOCK				0x01000000U   /* 等待LWLock */
 #define PG_WAIT_LOCK				0x03000000U   /* 等待Lock */
 #define PG_WAIT_BUFFER_PIN			0x04000000U   /* 等待访问数据缓冲区 */
 #define PG_WAIT_ACTIVITY			0x05000000U   /* 服务器进程处于空闲状态 */
 #define PG_WAIT_CLIENT				0x06000000U   /* 等待应用客户端程序在套接字中进行操作 */
 #define PG_WAIT_EXTENSION			0x07000000U   /* 等待扩展模块中的操作 */
 #define PG_WAIT_IPC					0x08000000U   /* 等待进程间通信 */
 #define PG_WAIT_TIMEOUT				0x09000000U   /* 等待达到超时时间 */
 #define PG_WAIT_IO					0x0A000000U   /* 等待IO操作完成 */

openGauss/MOGDB里，有着5类等待事件

 /* ----------
  * Wait Event Classes
  * ----------
  */
 #define WAIT_EVENT_END 0x00000000U   /* 等待事件结束*/
 #define PG_WAIT_LWLOCK 0x01000000U  /* 等待LWLock */
 #define PG_WAIT_LOCK 0x03000000U     /* 等待Lock */
 #define PG_WAIT_IO 0x0A000000U    /* 等待IO操作完成 */
 #define PG_WAIT_SQL 0x0B000000U  /* 等待SQL的类型 */

二、PG和openGauss/MOGDB等待事件对比

1.WAIT_EVENT_END

在类型定义里，WAIT_EVENT_END更像是一种声明等待事件结束的状态，可以看到代码使用部分，在调用pgstat_report_waitevent 函数报告某个等待事件之后，进行相关处理，最后再调用一次pgstat_report_waitevent 函数报告WAIT_EVENT_END，类似于声明操作结束，等待结束。

pgstat_report_waitevent 函数部分代码如下，这个函数会从服务器进程需要等待的地方调用，会报告等待事件信息，等待信息被存储作为4字节。

2.PG_WAIT_LOCK

Lock类的等待事件表示backend后台进程等待重量级的锁，通常是指 relation、tuple、page、transactionid 等子类型锁。

在PG里共有10种，

 /*
  * LOCKTAG is the key information needed to look up a LOCK item in the
  * lock hashtable.  A LOCKTAG value uniquely identifies a lockable object.
  *
  * The LockTagType enum defines the different kinds of objects we can lock.
  * We can handle up to 256 different LockTagTypes.
  */
 typedef enum LockTagType
 {
 	LOCKTAG_RELATION,			/* whole relation */
 	LOCKTAG_RELATION_EXTEND,	/* the right to extend a relation */
 	LOCKTAG_PAGE,				/* one page of a relation */
 	LOCKTAG_TUPLE,				/* one physical tuple */
 	LOCKTAG_TRANSACTION,		/* transaction (for waiting for xact done) */
 	LOCKTAG_VIRTUALTRANSACTION, /* virtual transaction (ditto) */
 	LOCKTAG_SPECULATIVE_TOKEN,	/* speculative insertion Xid and token */
 	LOCKTAG_OBJECT,				/* non-relation database object */
 	LOCKTAG_USERLOCK,			/* reserved for old contrib/userlock code */
 	LOCKTAG_ADVISORY			/* advisory user locks */
 } LockTagType;

而openGauss/MOGDB的LOCK类等待事件增加了LOCKTAG_PARTITION、LOCKTAG_PARTITION_SEQUENCE、LOCKTAG_CSTORE_FREESPACE、LOCKTAG_RELFILENODE、LOCKTAG_SUBTRANSACTION，分别是分区、分区序列、cstore的空闲空间、relfilenode以及子事务的等待。

 /*
  * LOCKTAG is the key information needed to look up a LOCK item in the
  * lock hashtable.	A LOCKTAG value uniquely identifies a lockable object.
  *
  * The LockTagType enum defines the different kinds of objects we can lock.
  * We can handle up to 256 different LockTagTypes.
  */
 typedef enum LockTagType {
     LOCKTAG_RELATION, /* whole relation */
     /* ID info for a relation is DB OID + REL OID; DB OID = 0 if shared */
     LOCKTAG_RELATION_EXTEND, /* the right to extend a relation */
     /* same ID info as RELATION */
     LOCKTAG_PARTITION,          /*partition*/
     LOCKTAG_PARTITION_SEQUENCE, /*partition sequence*/
     LOCKTAG_PAGE,               /* one page of a relation */
     /* ID info for a page is RELATION info + BlockNumber */
     LOCKTAG_TUPLE, /* one physical tuple */
     /* ID info for a tuple is PAGE info + OffsetNumber */
     LOCKTAG_TRANSACTION, /* transaction (for waiting for xact done) */
     /* ID info for a transaction is its TransactionId */
     LOCKTAG_VIRTUALTRANSACTION, /* virtual transaction (ditto) */
     /* ID info for a virtual transaction is its VirtualTransactionId */
     LOCKTAG_OBJECT, /* non-relation database object */
     /* ID info for an object is DB OID + CLASS OID + OBJECT OID + SUBID */
     LOCKTAG_CSTORE_FREESPACE, /* cstore free space */
 
     /*
      * Note: object ID has same representation as in pg_depend and
      * pg_description, but notice that we are constraining SUBID to 16 bits.
      * Also, we use DB OID = 0 for shared objects such as tablespaces.
      */
     LOCKTAG_USERLOCK, /* reserved for old contrib/userlock code */
     LOCKTAG_ADVISORY, /* advisory user locks */
     /* same ID info as spcoid, dboid, reloid */
     LOCKTAG_RELFILENODE, /* relfilenode */
     LOCKTAG_SUBTRANSACTION, /* subtransaction (for waiting for subxact done) */
     /* ID info for a transaction is its TransactionId + SubTransactionId */
     LOCK_EVENT_NUM
 } LockTagType;

3.PG_WAIT_IO

如下的IO类部分为openGauss/MOGDB和PG12.1对比所不具有的，可以看到PG12.1比openGauss/MOGDB多了逻辑复制查询重写，Reorder Buffer的读写等待、时间线历史文件的同步、WAL同步、WAL BOOTSTRAP的同步、写等待等等。

	WAIT_EVENT_DSM_FILL_ZERO_WRITE，
    	WAIT_EVENT_LOCK_FILE_RECHECKDATADIR_READ,
    	WAIT_EVENT_LOGICAL_REWRITE_CHECKPOINT_SYNC,
    	WAIT_EVENT_LOGICAL_REWRITE_MAPPING_SYNC,
    	WAIT_EVENT_LOGICAL_REWRITE_MAPPING_WRITE,
    	WAIT_EVENT_LOGICAL_REWRITE_SYNC,
    	WAIT_EVENT_LOGICAL_REWRITE_TRUNCATE,
    	WAIT_EVENT_LOGICAL_REWRITE_WRITE,
        WAIT_EVENT_REORDER_BUFFER_READ,
    	WAIT_EVENT_REORDER_BUFFER_WRITE,
    	WAIT_EVENT_REORDER_LOGICAL_MAPPING_READ,
    	WAIT_EVENT_TIMELINE_HISTORY_FILE_SYNC,
    	WAIT_EVENT_TIMELINE_HISTORY_FILE_WRITE,
    	WAIT_EVENT_TIMELINE_HISTORY_READ,
    	WAIT_EVENT_TIMELINE_HISTORY_SYNC,
    	WAIT_EVENT_TIMELINE_HISTORY_WRITE,
    	WAIT_EVENT_WALSENDER_TIMELINE_HISTORY_READ,
    	WAIT_EVENT_WAL_BOOTSTRAP_SYNC,
    	WAIT_EVENT_WAL_BOOTSTRAP_WRITE,
        WAIT_EVENT_WAL_SYNC,

而如下部分为PG12.1不具有而openGauss/MOGDB独有的，多了undo文件相关，doublerwite文件读写等等、

     WAIT_EVENT_BUF_HASH_SEARCH,
     WAIT_EVENT_BUF_STRATEGY_GET,
     WAIT_EVENT_UNDO_FILE_EXTEND,
     WAIT_EVENT_UNDO_FILE_PREFETCH,
     WAIT_EVENT_UNDO_FILE_READ,
     WAIT_EVENT_UNDO_FILE_WRITE,
     WAIT_EVENT_UNDO_FILE_FLUSH,
     WAIT_EVENT_UNDO_FILE_SYNC,
     WAIT_EVENT_WAL_BUFFER_ACCESS,
     WAIT_EVENT_WAL_BUFFER_FULL,
     WAIT_EVENT_DW_READ,
     WAIT_EVENT_DW_WRITE,
     WAIT_EVENT_DW_SINGLE_POS,
     WAIT_EVENT_DW_SINGLE_WRITE,
     WAIT_EVENT_PREDO_PROCESS_PENDING,
     WAIT_EVENT_PREDO_APPLY,
     WAIT_EVENT_DISABLE_CONNECT_FILE_READ,
     WAIT_EVENT_DISABLE_CONNECT_FILE_SYNC,
     WAIT_EVENT_DISABLE_CONNECT_FILE_WRITE,
     WAIT_EVENT_MPFL_INIT,
     WAIT_EVENT_MPFL_READ,
     WAIT_EVENT_MPFL_WRITE,
     WAIT_EVENT_OBS_LIST,
     WAIT_EVENT_OBS_READ,
     WAIT_EVENT_OBS_WRITE,
     WAIT_EVENT_LOGCTRL_SLEEP,
     WAIT_EVENT_COMPRESS_ADDRESS_FILE_FLUSH,
     WAIT_EVENT_COMPRESS_ADDRESS_FILE_SYNC,

4.PG_WAIT_LWLOCK

LWLock的等待事件主要包含两种：LWLockNamed 和LWLockTranche ，前者表示backend后台进程等待某种特定的轻量级锁，后者表示表示backend后台进程等待一组相关轻量级锁。

这一部分的等待事件较多，就不一一列举了，但是可以看到，openGauss/MOGDB和PG12.1的LWLockNamed 类等待事件只有少部分一致，这个可能与openGauss/MOGDB基于PG 9.2.4版本研发有关，等待事件重新定义了。而LWLockTranche 这部分还是有一部分是一致的，但明显openGauss/MOGDB补充的等待事件数量也更加多。

5.PG_WAIT_SQL

这一类的等待事件是openGauss/MOGDB分的一类关于SQL的，可以看到等待的SQL类型。

    /* ----------
     * Wait Events - SQL
     *
     * Using this to indicate the type of  SQL DML event.
     * ----------
     */
    typedef enum WaitEventSQL {
        WAIT_EVENT_SQL_SELECT = PG_WAIT_SQL,
        WAIT_EVENT_SQL_UPDATE,
        WAIT_EVENT_SQL_INSERT,
        WAIT_EVENT_SQL_DELETE,
        WAIT_EVENT_SQL_MERGEINTO,
        WAIT_EVENT_SQL_DDL,
        WAIT_EVENT_SQL_DML,
        WAIT_EVENT_SQL_DCL,
        WAIT_EVENT_SQL_TCL
    } WaitEventSQL;

如下是相应的pgstat_report_wait_count 函数使用的关于这个等待事件的部分，可以看到它主要是使用pg atomic函数根据wait_event_info为用户添加sql计数。

        /* Using pg atomic function to add count for corresponsible WaitEventSQL */
        if (classId == PG_WAIT_SQL) {
            WaitEventSQL w = (WaitEventSQL)wait_event_info;
            switch (w) {
                case WAIT_EVENT_SQL_SELECT: {
                    UPDATE_SQL_COUNT(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_select,
                        WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.selectElapse);
                } break;
                case WAIT_EVENT_SQL_UPDATE: {
                    UPDATE_SQL_COUNT(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_update,
                        WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.updateElapse);
                } break;
                case WAIT_EVENT_SQL_INSERT: {
                    UPDATE_SQL_COUNT(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_insert,
                        WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.insertElapse);
                } break;
                case WAIT_EVENT_SQL_DELETE: {
                    UPDATE_SQL_COUNT(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_delete,
                        WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.deleteElapse);
                } break;
                case WAIT_EVENT_SQL_MERGEINTO:
                    pg_atomic_fetch_add_u64(&(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_mergeinto), 1);
                    break;
                case WAIT_EVENT_SQL_DDL:
                    pg_atomic_fetch_add_u64(&(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_ddl), 1);
                    break;
                case WAIT_EVENT_SQL_DML:
                    pg_atomic_fetch_add_u64(&(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_dml), 1);
                    break;
                case WAIT_EVENT_SQL_DCL:
                    pg_atomic_fetch_add_u64(&(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_dcl), 1);
                    break;
                case WAIT_EVENT_SQL_TCL:
                    pg_atomic_fetch_add_u64(&(WaitCountStatusCell->WaitCountArray[dataid].wc_cnt.wc_sql_tcl), 1);
                    break;
                default:
                    break;
            }
        }
        LWLockRelease(WaitCountHashLock);
    }