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首页 / PostgreSQL大量子事务导致SubtransControlLock等待事件简单分析

PostgreSQL大量子事务导致SubtransControlLock等待事件简单分析

原创 杨明翰 2022-01-06
1700

前端时间某客户生产环境PG数据库出现性能波动问题，多次出现数据库响应超时的异常。异常时间段查询pg_stat_activity视图发现大量进程在等待SubtransControlLock事件。参考官网解释Waiting to read or update sub-transaction information.是指在等待读取或更新子事务的信息。

image.png

什么是子事务？

子事务也被称为嵌套事务（nested transaction），是在一个已经启动的事务中启动的事务。使用子事务可以做部分回滚，这在PG中尤其有用。我们知道在PG中任何一个错误会回滚整个事务，而不是像Oracle或PG那样只回滚报错的statement。在某些场景下为了达到和Oracle、MySQL同样的效果，就需要使用子事务。

SQL 标准定义savepoint创建子事务，ROLLBACK TO SAVEPOINT回滚子事务，RELEASE SAVEPOINT清除子事务。

<savepoint statement> ::=
  SAVEPOINT <savepoint specifier>
                
<savepoint specifier> ::=
  <savepoint name>

<rollback statement> ::=
  ROLLBACK [ WORK ] [ AND [ NO ] CHAIN ] [ <savepoint clause> ]
<savepoint clause> ::=
  TO SAVEPOINT <savepoint specifier>
  
<release savepoint statement> ::=
  RELEASE SAVEPOINT <savepoint specifier>

在PostgreSQL JDBC中还可以设置autosave参数，当配置成always时JDBC driver会在每一个statement前设置一个savepoint，如果该语句报错则回滚到这个savepoint。该参数默认为never，即不会设置savepoint。如果设置成conservative 保守模式，则会在每一个statement前设置一个savepoint，但是只会在极少数情况下回滚，例如 ‘cached statement cannot change return type’ or ‘statement XXX is not valid’ 时JDBC driver会回滚并重试。

SubtransControlLock是在等什么？

SubtransControlLock是在等待读取或更新子事务的信息。在PG中元组（tuple）的可见性是由它的系统列xmin 和 xmax决定的。因为子事务也会分配XID，如果元组存储的XID属于一个子事务，那就需要找到其对应的顶级（父）事务的XID，来判断数据是否可见，因为只有顶级事务才能真正提交。子事务和它的父事务的对应关系是存放在磁盘上的pg_subtrans 目录中。

PG在每个语句开始的时候会对数据库做一个快照，用来确定哪些元组是可见的。快照包括：

  1. 当前最大事务ID，此XID以后的数据不可见
  2. 当前活跃的事务和子事务列表
  3. 当前事务/子事务的最早可见命令的编号

快照会记录所有的活跃顶级事务XID，以及尝试在有限的空间中记录子事务XID。如果有的session有超过64个活动的子事务，则这个快照的子事务数据会被标记为overflowed。如果suboverflowed = true就需要通过映射关系去找子事务和对应的顶级事务。

/*
 * Each backend advertises up to PGPROC_MAX_CACHED_SUBXIDS TransactionIds
 * for non-aborted subtransactions of its current top transaction.  These
 * have to be treated as running XIDs by other backends.
 *
 * We also keep track of whether the cache overflowed (ie, the transaction has
 * generated at least one subtransaction that didn't fit in the cache).
 * If none of the caches have overflowed, we can assume that an XID that's not
 * listed anywhere in the PGPROC array is not a running transaction.  Else we
 * have to look at pg_subtrans.
 */
#define PGPROC_MAX_CACHED_SUBXIDS 64	/* XXX guessed-at value */


	if (nsubxacts > PGPROC_MAX_CACHED_SUBXIDS)
	{
		pgxact->overflowed = true;
		nsubxacts = PGPROC_MAX_CACHED_SUBXIDS;
	}
	
if (pgxact->overflowed)
		suboverflowed = true;

在suboverflowed的情况下，要获取完整的可见性数据只能从pg_subtrans获取。这种情况下通过 perf top命令可以发现SimpleLruReadPage_ReadOnly命令的开销增加。SimpleLruReadPage_ReadOnly可能会触发读取pg_subtrans，而其他事务注册子事务时会去更新pg_subtrans，这可能会导致锁竞争。

/*
 * Interrogate the parent of a transaction in the subtrans log.
 */
TransactionId
SubTransGetParent(TransactionId xid)
{
	int			pageno = TransactionIdToPage(xid);
	int			entryno = TransactionIdToEntry(xid);
	int			slotno;
	TransactionId *ptr;
	TransactionId parent;

	/* Can't ask about stuff that might not be around anymore */
	Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));

	/* Bootstrap and frozen XIDs have no parent */
	if (!TransactionIdIsNormal(xid))
		return InvalidTransactionId;

	/* lock is acquired by SimpleLruReadPage_ReadOnly */

	slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid);
	ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
	ptr += entryno;

	parent = *ptr;

	LWLockRelease(SubtransControlLock);

	return parent;
}





#define NUM_SUBTRANS_BUFFERS    32

/*
 * Initialization of shared memory for SUBTRANS
 */
Size
SUBTRANSShmemSize(void)
{
	return SimpleLruShmemSize(NUM_SUBTRANS_BUFFERS, 0);
}

void
SUBTRANSShmemInit(void)
{
	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
	SimpleLruInit(SubTransCtl, "subtrans", NUM_SUBTRANS_BUFFERS, 0,
				  SubtransControlLock, "pg_subtrans",
				  LWTRANCHE_SUBTRANS_BUFFERS);
	/* Override default assumption that writes should be fsync'd */
	SubTransCtl->do_fsync = false;
}


void
SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
			  LWLock *ctllock, const char *subdir, int tranche_id)
{

		shared->num_slots = nslots;



/*
 * Find a page in a shared buffer, reading it in if necessary.
 * The page number must correspond to an already-initialized page.
 * The caller must intend only read-only access to the page.
 *
 * The passed-in xid is used only for error reporting, and may be
 * InvalidTransactionId if no specific xid is associated with the action.
 *
 * Return value is the shared-buffer slot number now holding the page.
 * The buffer's LRU access info is updated.
 *
 * Control lock must NOT be held at entry, but will be held at exit.
 * It is unspecified whether the lock will be shared or exclusive.
 */
int
SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
{
	SlruShared	shared = ctl->shared;
	int			slotno;

	/* Try to find the page while holding only shared lock */
	LWLockAcquire(shared->ControlLock, LW_SHARED);

	/* See if page is already in a buffer */
	for (slotno = 0; slotno < shared->num_slots; slotno++)
	{
		if (shared->page_number[slotno] == pageno &&
			shared->page_status[slotno] != SLRU_PAGE_EMPTY &&
			shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
		{
			/* See comments for SlruRecentlyUsed macro */
			SlruRecentlyUsed(shared, slotno);
			return slotno;
		}
	}

	/* No luck, so switch to normal exclusive lock and do regular read */
	LWLockRelease(shared->ControlLock);
	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);

	return SimpleLruReadPage(ctl, pageno, true, xid);
}

可以看到PG为了避免直接读取磁盘上pg_subtrans的数据，会先线性查找SLRU缓存。

PG查询子事务ID时，会计算该 ID 在哪个内存页中，然后进行线性搜索以在内存页中查找。如果页面不在缓存中，它会逐出一页并将所需的页面加载到内存中。

SLRU 默认32个page，每个page 默认8k，每个事务ID占4个字节，理论上SLRU最多能缓存65K 事务id。 但大多数时候每页不会存满，极端条件下分配给子事务的单个 XID 可能单独占用整个 SLRU 页面。

在TPS较高且大量使用子事务的系统中，SLRU很快会被写满，导致大量磁盘IO。

这就是SubtransControlLock大量出现的原因。

#define NUM_SUBTRANS_BUFFERS    32
#define TransactionIdToPage(xid) ((xid) / (TransactionId) SUBTRANS_XACTS_PER_PAGE)
void
ExtendSUBTRANS(TransactionId newestXact)
{
	int			pageno;

	/*
	 * No work except at first XID of a page.  But beware: just after
	 * wraparound, the first XID of page zero is FirstNormalTransactionId.
	 */
	if (TransactionIdToEntry(newestXact) != 0 &&
		!TransactionIdEquals(newestXact, FirstNormalTransactionId))
		return;

	pageno = TransactionIdToPage(newestXact);

	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

	/* Zero the page */
	ZeroSUBTRANSPage(pageno);

	LWLockRelease(SubtransControlLock);
}

增加PGPROC_MAX_CACHED_SUBXIDS的值理论上可以减少suboverflowed的出现，但会增加内存开销。

增加NUM_SUBTRANS_BUFFERS的值理论上可以缓解该问题的出现，考虑SLRU 缓存对所需页面进行线性搜索如果盲目增加缓存大小，额外增加的搜索成本可能会导致性能的下降。

如何判断是子事务溢出导致的性能问题

  1. perf top 发现SimpleLruReadPage_ReadOnly函数开销增加
  2. pg_stat_activity视图中大量出现等待SubtransControlLock事件
  3. pg_export_snapshot()导出的快照信息，pg_snapshots 子目录中的文件包含sof:1
  4. 并发增加后 系统负载明显增加。

PG测试

pgbench构造测试数据

-bash-4.2$  /usr/pgsql-12/bin/pgbench -i test
dropping old tables...
NOTICE:  table "pgbench_accounts" does not exist, skipping
NOTICE:  table "pgbench_branches" does not exist, skipping
NOTICE:  table "pgbench_history" does not exist, skipping
NOTICE:  table "pgbench_tellers" does not exist, skipping
creating tables...
generating data...
100000 of 100000 tuples (100%) done (elapsed 0.02 s, remaining 0.00 s)
vacuuming...
creating primary keys...
done.

两个SQL文件，查询和创建大量子事务

echo "\\set id random(1, 10000)
begin;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s1;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s2;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s3;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s4;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s5;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s6;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s7;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s8;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s9;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s10;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s11;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s12;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s13;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s14;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s15;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s16;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s17;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s18;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s19;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s20;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s21;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s22;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s23;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s24;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s25;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s26;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s27;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s28;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s29;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s30;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s31;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s32;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s33;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s34;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s35;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s36;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s37;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s38;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s39;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s40;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s41;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s42;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s43;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s44;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s45;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s46;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s47;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s48;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s49;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s50;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s51;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s52;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s53;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s54;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s55;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s56;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s57;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s58;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s59;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s60;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s61;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s62;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s63;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s64;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s65;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s66;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s67;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s68;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s69;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s70;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s71;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s72;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s73;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s74;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s75;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s76;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s77;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s78;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s79;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s80;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s81;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s82;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s83;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s84;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s85;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s86;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s87;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s88;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s89;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s90;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s91;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s92;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s93;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s94;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s95;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s96;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s97;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s98;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s99;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s100;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s101;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s102;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s103;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s104;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s105;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s106;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s107;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s108;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s109;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s110;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s111;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s112;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s113;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s114;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s115;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s116;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s117;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s118;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s119;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s120;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s121;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s122;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s123;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s124;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s125;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s126;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s127;
update pgbench_accounts set aid = aid where aid = :id;
 savepoint s128;
update pgbench_accounts set aid = aid where aid = :id;
commit;" > updates_with_savepoints.sql




echo "\\set id random(1, 10000)
select * from pgbench_accounts where aid = :id;" > selects.sql

启动pgbench测试，同时开启一个长事务

-bash-4.2$ /usr/pgsql-12/bin/pgbench -rn -P2 -T180 -c25 -j25 -f updates_with_savepoints.sql  -M prepared  test

-bash-4.2$ /usr/pgsql-12/bin/pgbench -rn -P2 -T180 -c25 -j25 -f selects.sql  -M prepared  test

postgres=# select txid_current(), pg_sleep_for('5 minutes');

视图出现大量SubtransControlLock等待事件

postgres=# select wait_event_type,wait_event,state,query from pg_stat_activity  where state='active';   
 wait_event_type |     wait_event      | state  |                                           query                                            
-----------------+---------------------+--------+--------------------------------------------------------------------------------------------
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | subtrans            | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | subtrans            | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
                 |                     | active | select wait_event_type,wait_event,state,query from pg_stat_activity  where state='active';
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | subtrans            | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
                 |                     | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | subtrans            | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 LWLock          | SubtransControlLock | active | update pgbench_accounts set aid = aid where aid = $1;
 Timeout         | PgSleep             | active | select txid_current(), pg_sleep_for('5 minutes');
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | subtrans            | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;
 LWLock          | SubtransControlLock | active | select * from pgbench_accounts where aid = $1;

Perf top 发现SimpleLruReadPage_ReadOnly函数开销增加

[root@mogdb ~]# perf top --no-children --call-graph=fp --dsos=/usr/pgsql-12/bin/postgres
+    8.57%  [.] LWLockRelease
     6.57%  [.] 0x0000000000353fb6
     5.91%  [.] SimpleLruReadPage_ReadOnly
     2.83%  [.] LWLockAcquire
+    1.72%  [.] TransactionIdIsCurrentTransactionId
+    1.39%  [.] hash_search_with_hash_value
     1.33%  [.] 0x0000000000353f93
     1.16%  [.] SubTransGetParent
+    0.66%  [.] heap_hot_search_buffer
     0.57%  [.] SubTransGetTopmostTransaction
+    0.52%  [.] PostgresMain

postgresqlsubtransactionsubtranscontrollocksuboverflowed
最后修改时间：2022-01-06 11:27:38
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