根據openGauss/MogDB的lwtid檢視執行緒堆疊

在openGauss/MogDB資料庫THREAD_WAIT_STATUS、PG_THREAD_WAIT_STATUS、GLOBAL_THREAD_WAIT_STATUS、LOCAL_ACTIVE_SESSION的幾個檢視裡都有lwtid這一列，他是當前執行緒的輕量級執行緒號。可以根據這一列透過pstack命令去抓取堆疊，供問題的分析。

如下所示，可以用pg_stat_activity 和dbe_perf.thread_wait_status結合，查詢當前正在執行的一些SQL，透過

MogDB=# select  pid,lwtid,state,wait_event,query from pg_stat_activity a,dbe_perf.thread_wait_status s where a.pid=s.tid;
      pid       |  lwtid  | state  | wait_event |                                                       query                                                 
      
----------------+---------+--------+------------+-------------------------------------------------------------------------------------------------------------
------
 23043933468416 | 1724740 | active | none       | select  pid,lwtid,state,wait_event,query from pg_stat_activity a,dbe_perf.thread_wait_status s where a.pid=s
.tid;
 23044103272192 | 1724300 | active | none       | select pg_sleep(30);
 23044453037824 | 1526848 |        | none       | 
 23044479776512 | 1526847 |        | none       | 
 23044508612352 | 1526846 | active | none       | WLM fetch collect info from data nodes
 23044674287360 | 1526842 | idle   | none       | 
 23044771804928 | 1526839 | idle   | none       | 
 23044739823360 | 1526840 | active | none       | 
 23044698928896 | 1526841 | active | none       | 
 23044832098048 | 1526837 |        | none       | 
 23044928567040 | 1526834 | active | none       | 
(11 rows)

我們透過pstack去檢視上邊lwtid=1724300 對應的select pg_sleep(30);的堆疊。

[ptk1@mogdb-kernel-0005 ~]$ pstack 1724300
Thread 1 (Thread 0x14f55f96f700 (LWP 1724300)):
#0  0x000014f98e8d829f in select () from /lib64/libc.so.6
#1  0x0000555ba97ae25d in pg_usleep(long) ()
#2  0x0000555ba87cef4a in pg_sleep(FunctionCallInfoData*) ()
#3  0x0000555ba8fad525 in ?? ()
#4  0x0000555ba8fadb59 in ExecProject(ProjectionInfo*, ExprDoneCond*) ()
#5  0x0000555ba8ffb948 in ExecResult(ResultState*) ()
#6  0x0000555ba8f9c6ee in ExecProcNode(PlanState*) ()
#7  0x0000555ba8f95318 in standard_ExecutorRun(QueryDesc*, ScanDirection, long) ()
#8  0x0000555ba8e25ed5 in ?? ()
#9  0x0000555ba8f956ad in ExecutorRun(QueryDesc*, ScanDirection, long) ()
#10 0x0000555ba8f148b3 in ?? ()
#11 0x0000555ba8f14fa0 in PortalRun(PortalData*, long, bool, _DestReceiver*, _DestReceiver*, char*) ()
#12 0x0000555ba8f095a2 in ?? ()
#13 0x0000555ba8f0ce97 in PostgresMain(int, char**, char const*, char const*) ()
#14 0x0000555ba8e6b501 in ?? ()
#15 0x0000555ba8e89fb8 in int GaussDbThreadMain<(knl_thread_role)1>(knl_thread_arg*) ()
#16 0x0000555ba8e6b585 in ?? ()
#17 0x000014f98ebb117a in start_thread () from /lib64/libpthread.so.0
#18 0x000014f98e8e0dc3 in clone () from /lib64/libc.so.6

可以清晰地看到，堆疊從PostgresMain到後來呼叫了pg_sleep的各個部分，以及後來的pg_usleep休眠部分。

也可以根據lwtid找到想分析的一些關鍵執行緒，例如一些特殊功能的執行緒，如WDRSnapshot、ASP、CheckPointer等、或者是在資料庫中有長時間在跑的SQL的執行緒以及需要分析的等待事件對應的執行緒。

 node_name | db_name  |      thread_name       |     query_id     |      tid       |   sessionid    |  lwtid  | psessionid | tlevel | smpid | wait_status | wa
it_event | locktag | lockmode | block_sessionid | global_sessionid 
-----------+----------+------------------------+------------------+----------------+----------------+---------+------------+--------+-------+-------------+---
---------+---------+----------+-----------------+------------------
 dn_6001   | postgres | gsql                   | 1407374883636205 | 23044103272192 | 23044103272192 | 1724935 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | WLMArbiter             |                0 | 23044453037824 | 23044453037824 | 1526848 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | WorkloadMonitor        |                0 | 23044479776512 | 23044479776512 | 1526847 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | workload               |                0 | 23044508612352 | 23044508612352 | 1526846 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | statement flush thread |                0 | 23044674287360 | 23044674287360 | 1526842 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | WDRSnapshot            | 1407374883553280 | 23044771804928 | 23044771804928 | 1526839 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | PercentileJob          |                0 | 23044739823360 | 23044739823360 | 1526840 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | Asp                    |                0 | 23044698928896 | 23044698928896 | 1526841 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | ApplyLauncher          |                0 | 23044832098048 | 23044832098048 | 1526837 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
 dn_6001   | postgres | JobScheduler           |                0 | 23044928567040 | 23044928567040 | 1526834 |            |      0 |     0 | none        | no
ne       |         |          |                 | 0:0#0
(10 rows)

如下所示，可以看到lwtid=1526839對應的是WDRSnapshot執行緒它當前的一個堆疊情況。

[ptk1@mogdb-kernel-0005 ~]$ pstack 1526839
Thread 1 (Thread 0x14f5876ff700 (LWP 1526839)):
#0  0x000014f98e8d829f in select () from /lib64/libc.so.6
#1  0x0000555ba97ae25d in pg_usleep(long) ()
#2  0x0000555ba8a83e83 in SnapshotNameSpace::SubSnapshotMain() ()
#3  0x0000555ba8a843c5 in SnapshotMain() ()
#4  0x0000555ba8e87e41 in int GaussDbThreadMain<(knl_thread_role)35>(knl_thread_arg*) ()
#5  0x0000555ba8e6b585 in ?? ()
#6  0x000014f98ebb117a in start_thread () from /lib64/libpthread.so.0
#7  0x000014f98e8e0dc3 in clone () from /lib64/libc.so.6