PostgreSQL 9.6 等待事件出炉

2016 Postgres大象会官方报名通道： 点此报名

作者:digoal

标签: PostgreSQL , 9.6 , 等待事件 , wait_event

背景

PostgreSQL 9.6动态视图pg_stat_activity新增了wait_event_type, wait_event的等待事件展示。

当会话处于等待状态时，wait_event与wait_event_type非空，表示会话正在等待的类型。

根据等待信息，可以了解当前会话的状态。

将来也可以通过插件的形式，掌握数据库在某个时间段内的等待事件统计，更好的诊断数据库的健康状态。

已有的插件如下: https://github.com/postgrespro/pg_wait_sampling 用于对等待事件进行采样。

例子

pg_wait_sampling_history view – history of wait events obtained by sampling into in-memory ring buffer.

pg_wait_sampling_profile view – profile of wait events obtained by sampling into in-memory hash table.

pg_wait_sampling_reset_profile() function resets the profile.

The work of wait event statistics collector worker is controlled by following GUCs.

PostgreSQL 9.6 等待事件

详见: https://www.postgresql.org/docs/9.6/static/monitoring-stats.html

pg_stat_activity 视图新增等待事件列，可以观察到会话当前的等待。

1. wait_event_type

表示等待时间的类别，如果backend处于等待状态则有内容，否则为空。

类别如下

1.1 LWLockNamed:

命名的轻量锁，这种锁的目的是用于保护内存中的数据结构，防止并发的问题。

The backend is waiting for a specific named lightweight lock.

Each such lock protects a particular data structure in shared memory.

1.2 LWLockTranche:

分组轻量锁，没有细分名字，只是笼统的分类。

The backend is waiting for one of a group of related lightweight locks.

All locks in the group perform a similar function;

1.3 Lock:

重量级锁，用于保护SQL可见对象，例如表。 也可以用于保护存储，例如扩展表时。

见 src/include/storage/lock.h

The backend is waiting for a heavyweight lock.

Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables.

However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension.

1.4 BufferPin:

bufferpin用于保护数据库data buffer中的数据，例如保护游标访问的数据。

The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer.

Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question.

2. wait_event

表示wait_event_type中对应的详细的等待事件。

如果当前backend处于等待状态，则有值，否则为空

Wait event name if backend is currently waiting, otherwise NULL.

2.1 LWLockNamed

ShmemIndexLock

　　Waiting to find or allocate space in shared memory.

OidGenLock

　　Waiting to allocate or assign an OID.

XidGenLock

　　通常出现在高并发的请求事务号时

　　Waiting to allocate or assign a transaction id.

ProcArrayLock

　　通常出现在高并发的请求事务号，并且开启了old_snapshot_threshold时

　　Waiting to get a snapshot or clearing a transaction id at transaction end.

SInvalReadLock

　　Waiting to retrieve or remove messages from shared invalidation queue.

SInvalWriteLock

　　Waiting to add a message in shared invalidation queue.

WALBufMappingLock

　　Waiting to replace a page in WAL buffers.

WALWriteLock

　　wal刷盘较慢时，可以提高wal writer频率，或者加大BUFFER，或者提高目标盘的IOPS，降低目标盘的RT。

　　Waiting for WAL buffers to be written to disk.

ControlFileLock

　　如果产生XLOG确实很频繁，并且没有办法降低，可以使用较大的XLOG文件，最大64MB。

　　通常这个很少见。

　　Waiting to read or update the control file or creation of a new WAL file.

CheckpointLock

　　Waiting to perform checkpoint.

CLogControlLock

　　Waiting to read or update transaction status.

SubtransControlLock

　　Waiting to read or update sub-transaction information.

MultiXactGenLock

　　Waiting to read or update shared multixact state.

MultiXactOffsetControlLock

　　Waiting to read or update multixact offset mappings.

MultiXactMemberControlLock

　　Waiting to read or update multixact member mappings.

RelCacheInitLock

　　Waiting to read or write relation cache initialization file.

CheckpointerCommLock

　　检查点分三步（write, sync_file_range, fsync），表示fsync请求出现等待，需要提高IO，或者减少fsync时的dirty page。

　　Waiting to manage fsync requests.

TwoPhaseStateLock

　　Waiting to read or update the state of prepared transactions.

TablespaceCreateLock

　　Waiting to create or drop the tablespace.

BtreeVacuumLock

　　频繁出现，说明索引字段被频繁更新。

　　Waiting to read or update vacuum-related information for a B-tree index.

AddinShmemInitLock

　　Waiting to manage space allocation in shared memory.

AutovacuumLock

　　Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers.

AutovacuumScheduleLock

　　说明autovacuum单表比较慢，看看是否可以关闭autovacuum的SLEEP调度。

　　Waiting to ensure that the table it has selected for a vacuum still needs vacuuming.

SyncScanLock

　　Waiting to get the start location of a scan on a table for synchronized scans.

RelationMappingLock

　　Waiting to update the relation map file used to store catalog to filenode mapping.

AsyncCtlLock

　　Waiting to read or update shared notification state.

AsyncQueueLock

　　Waiting to read or update notification messages.

SerializableXactHashLock

　　Waiting to retrieve or store information about serializable transactions.

SerializableFinishedListLock

　　Waiting to access the list of finished serializable transactions.

SerializablePredicateLockListLock

　　Waiting to perform an operation on a list of locks held by serializable transactions.

OldSerXidLock

　　Waiting to read or record conflicting serializable transactions.

SyncRepLock

　　Waiting to read or update information about synchronous replicas.

BackgroundWorkerLock

　　Waiting to read or update background worker state.

DynamicSharedMemoryControlLock

　　Waiting to read or update dynamic shared memory state.

AutoFileLock

　　Waiting to update the postgresql.auto.conf file.

ReplicationSlotAllocationLock

　　Waiting to allocate or free a replication slot.

ReplicationSlotControlLock

　　Waiting to read or update replication slot state.

CommitTsControlLock

　　Waiting to read or update transaction commit timestamps.

CommitTsLock

　　Waiting to read or update the last value set for the transaction timestamp.

ReplicationOriginLock

　　Waiting to setup, drop or use replication origin.

MultiXactTruncationLock

　　Waiting to read or truncate multixact information.

OldSnapshotTimeMapLock

　　Waiting to read or update old snapshot control information.

2.2 LWLockTranche

clog

　　通常很少见，可能出现在在非常高并发的极小写事务时，文件IO出现等待，使用cgroup可以很容易复现。

　　Waiting for I/O on a clog (transaction status) buffer.

commit_timestamp

　　Waiting for I/O on commit timestamp buffer.

subtrans

　　Waiting for I/O a subtransaction buffer.

multixact_offset

　　Waiting for I/O on a multixact offset buffer.

multixact_member

　　Waiting for I/O on a multixact_member buffer.

async

　　Waiting for I/O on an async (notify) buffer.

oldserxid

　　Waiting to I/O on an oldserxid buffer.

wal_insert

　　Waiting to insert WAL into a memory buffer.

buffer_content

　　指 数据库 shared buffer

　　Waiting to read or write a data page in memory.

buffer_io

　　指 数据库 shared buffer

　　Waiting for I/O on a data page.

replication_origin

　　Waiting to read or update the replication progress.

replication_slot_io

　　Waiting for I/O on a replication slot.

proc

　　Waiting to read or update the fast-path lock information.

buffer_mapping

　　Waiting to associate a data block with a buffer in the buffer pool.

lock_manager

　　Waiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query).

predicate_lock_manager

　　Waiting to add or examine predicate lock information.

2.3 Lock

relation

　　Waiting to acquire a lock on a relation.

extend

　　Waiting to extend a relation.

page

　　Waiting to acquire a lock on page of a relation.

tuple

　　Waiting to acquire a lock on a tuple.

transactionid

　　Waiting for a transaction to finish.

virtualxid

　　Waiting to acquire a virtual xid lock.

speculative token

　　Waiting to acquire a speculative insertion lock.

object

　　Waiting to acquire a lock on a non-relation database object.

userlock

　　Waiting to acquire a userlock.

advisory

　　Waiting to acquire an advisory user lock.

2.4 BufferPin

BufferPin

　　Waiting to acquire a pin on a buffer.

3. 获取当指定PID当前的等待信息。

pg_stat_get_backend_wait_event_type(integer)

　　Wait event type name if backend is currently waiting, otherwise NULL. See Table 28-4 for details.

pg_stat_get_backend_wait_event(integer)

　　Wait event name if backend is currently waiting, otherwise NULL. See Table 28-4 for details.

2016 Postgres大象会官方报名通道：http://www.huodongxing.com/event/8352217821400

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