PostgreSQL 谁堵塞了谁（锁等待检测）- pg_blocking_pids, pg_safe_snapshot_blocking_pids

2022-04-25 11:53:55

背景

当一个进程处于等待（被堵塞）状态时，是谁干的？可以使用如下函数，快速得到捣蛋（堵塞别人）的PID。

1、请求锁时被堵，是哪些PID堵的?

pg_blocking_pids(int)    int[]    Process ID(s) that are blocking specified server process ID from acquiring a lock

2、请求safe快照时被堵(SSI隔离级别，请求安全快照冲突)，是哪些PID堵的?

pg_safe_snapshot_blocking_pids(int)    int[]    Process ID(s) that are blocking specified server process ID from acquiring a safe snapshot

例子

1、会话1

postgres=# begin;  
BEGIN  
postgres=# select * from tbl limit 1;  
   id   | c1 | c2   
--------+----+----  
 918943 |  1 |  0  
(1 row)  
  
postgres=# select pg_backend_pid();  
 pg_backend_pid   
----------------  
          30862  
(1 row)

2、会话2

postgres=# begin;  
BEGIN  
postgres=# select pg_backend_pid();  
 pg_backend_pid   
----------------  
          30928  
(1 row)  
  
postgres=# truncate tbl;  
  
等待中

3、会话3

postgres=# begin;  
BEGIN  
postgres=# select pg_backend_pid();  
 pg_backend_pid   
----------------  
          30936  
(1 row)  
  
postgres=# select * from tbl limit 1;  
  
等待中

4、会话4

postgres=# select pg_backend_pid();  
 pg_backend_pid   
----------------  
          30999  
(1 row)  
  
postgres=# select * from tbl limit 1;  
  
等待中

5、查看捣蛋PID

postgres=# select pid,pg_blocking_pids(pid),wait_event_type,wait_event,query from pg_stat_activity ;  
  pid  | pg_blocking_pids | wait_event_type |     wait_event      |                                           query                                             
-------+------------------+-----------------+---------------------+-------------------------------------------------------------------------------------------  
 30862 | {}               | Client          | ClientRead          | select pg_backend_pid();  
 30928 | {30862}          | Lock            | relation            | truncate tbl;  
 30936 | {30928}          | Lock            | relation            | select * from tbl limit 1;  
 30999 | {30928}          | Lock            | relation            | select * from tbl limit 1;

代码

src/backend/utils/adt/lockfuncs.c

/*  
 * pg_blocking_pids - produce an array of the PIDs blocking given PID  
 *  
 * The reported PIDs are those that hold a lock conflicting with blocked_pid's  
 * current request (hard block), or are requesting such a lock and are ahead  
 * of blocked_pid in the lock's wait queue (soft block).  
 *  
 * In parallel-query cases, we report all PIDs blocking any member of the  
 * given PID's lock group, and the reported PIDs are those of the blocking  
 * PIDs' lock group leaders.  This allows callers to compare the result to  
 * lists of clients' pg_backend_pid() results even during a parallel query.  
 *  
 * Parallel query makes it possible for there to be duplicate PIDs in the  
 * result (either because multiple waiters are blocked by same PID, or  
 * because multiple blockers have same group leader PID).  We do not bother  
 * to eliminate such duplicates from the result.  
 *  
 * We need not consider predicate locks here, since those don't block anything.  
 */  
Datum  
pg_blocking_pids(PG_FUNCTION_ARGS)  
{  
  
  
...............  
  
/*  
 * pg_safe_snapshot_blocking_pids - produce an array of the PIDs blocking  
 * given PID from getting a safe snapshot  
 *  
 * XXX this does not consider parallel-query cases; not clear how big a  
 * problem that is in practice  
 */  
Datum  
pg_safe_snapshot_blocking_pids(PG_FUNCTION_ARGS)  
{  
...........

src/backend/storage/lmgr/predicate.c

/*  
 * GetSafeSnapshotBlockingPids  
 *              If the specified process is currently blocked in GetSafeSnapshot,  
 *              write the process IDs of all processes that it is blocked by  
 *              into the caller-supplied buffer output[].  The list is truncated at  
 *              output_size, and the number of PIDs written into the buffer is  
 *              returned.  Returns zero if the given PID is not currently blocked  
 *              in GetSafeSnapshot.  
 */  
int  
GetSafeSnapshotBlockingPids(int blocked_pid, int *output, int output_size)  
{  
        int                     num_written = 0;  
        SERIALIZABLEXACT *sxact;  
  
        LWLockAcquire(SerializableXactHashLock, LW_SHARED);  
  
        /* Find blocked_pid's SERIALIZABLEXACT by linear search. */  
        for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))  
        {  
                if (sxact->pid == blocked_pid)  
                        break;  
        }  
  
        /* Did we find it, and is it currently waiting in GetSafeSnapshot? */  
        if (sxact != NULL && SxactIsDeferrableWaiting(sxact))  
        {  
                RWConflict      possibleUnsafeConflict;  
  
                /* Traverse the list of possible unsafe conflicts collecting PIDs. */  
                possibleUnsafeConflict = (RWConflict)  
                        SHMQueueNext(&sxact->possibleUnsafeConflicts,  
                                                 &sxact->possibleUnsafeConflicts,  
                                                 offsetof(RWConflictData, inLink));  
  
                while (possibleUnsafeConflict != NULL && num_written < output_size)  
                {  
                        output[num_written++] = possibleUnsafeConflict->sxactOut->pid;  
                        possibleUnsafeConflict = (RWConflict)  
                                SHMQueueNext(&sxact->possibleUnsafeConflicts,  
                                                         &possibleUnsafeConflict->inLink,  
                                                         offsetof(RWConflictData, inLink));  
                }  
        }  
  
        LWLockRelease(SerializableXactHashLock);  
  
        return num_written;  
}