在之前的文章《latch free:cache buffer handles造成的SQL性能問題》中我介紹了cache buffer handle latch的一些知識，在這里我們復習一下： "當會話需要pin住buffer header時它首先要獲去buffer handle，得到buffer handle的過程中首先要搶占cache buffer handles栓，為了避免對于cache buffer handles栓的過度爭用，每個會話被允許cache一小撮buffer handles,也叫保留集(reserved set)。該保留集的上限由隱式參數_db_handles_cached(默認為5)所控制，在此基礎上會話在執行不是十分復雜的SQL時不必反復申請栓。 同時存在一些十分復雜的語句，需要同時pin住大量的緩存，此時若存在空閑的handle，會話被允許索要手頭5個cached buffer handles以外的handle。也為了限制單個會話的資源占用量，Oracle通過內部算法總是盡量讓每個進程能夠pin住”合理份額“的buffer，由此單個會話同一時間所能持有的總handle數等于db_block_buffers/processes，隱式參數_cursor_db_buffers_pinned指出了該值。另cache buffer handles并沒有子栓，僅有父栓本身，所以如果系統中有大量復雜SQL，會話所能cache的buffer handles遠不足以滿足需求時，該栓就成為整個系統性能的瓶頸，甚至可能出現潛在的死鎖/餓死問題。" 應網友在T.ASKMACLEAN.COM上的提問，我們介紹更多cache Buffer handle相關的知識： ? cache buffer handle 結構: ? ------------------------------ | Buffer state object | ------------------------------ | Place to hang the buffer | ------------------------------ | Consistent Get? | ------------------------------ | Proc Owning SO | ------------------------------ | Flags(RIR) | ------------------------------ ? 來看一個 cache buffer handle SO: 70000046fdfe530, type: 24, owner: 70000041b018630, flag: INIT/-/-/0x00 (buffer) (CR) PR: 70000048e92d148 FLG: 0x500000 lock rls: 0, class bit: 0 kcbbfbp: [BH: 7000001c7f069b0, LINK: 70000046fdfe570] where: kdswh02: kdsgrp, why: 0 BH (7000001c7f069b0) file#: 12 rdba: 0x03061612 (12/398866) class: 1 ba: 7000001c70ee000 set: 75 blksize: 8192 bsi: 0 set-flg: 0 pwbcnt: 0 dbwrid: 2 obj: 66209 objn: 48710 tsn: 6 afn: 12 hash: [700000485f12138,700000485f12138] lru: [70000025af67790,700000132f69ee0] lru-flags: hot_buffer ckptq: [NULL] fileq: [NULL] objq: [700000114f5dd10,70000028bf5d620] use: [70000046fdfe570,70000046fdfe570] wait: [NULL] st: SCURRENT md: SHR tch: 0 flags: affinity_lock LRBA: [0x0.0.0] HSCN: [0xffff.ffffffff] HSUB: [65535] where: kdswh02: kdsgrp, why: 0 # Example: #?? (buffer) (CR) PR: 37290 FLG:??? 0 #?? kcbbfbp??? : [BH: befd8, LINK: 7836c] (WAITING) Buffer handle (X$KCBBF) kernel cache, buffer buffer_handles Query x$kcbbf? - lists all the buffer handles 相關參數 _db_handles ?? ???????? System-wide simultaneous buffer operations ,no of buffer handles _db_handles_cached ??? ?Buffer handles cached each process , no of processes? default 5 _cursor_db_buffers_pinned? additional number of buffers a cursor can pin at once _session_kept_cursor_pins? ??? ?Number of cursors pins to keep in a session When a buffer is pinned it is attached to buffer state object. ? ? 接下來 我們實際體驗一下 cache buffer handles latch 和 buffer pin的影響： ? ? SESSION A :SQL> select * from v$version;BANNER ---------------------------------------------------------------- Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi PL/SQL Release 10.2.0.5.0 - Production CORE??? 10.2.0.5.0????? Production TNS for Linux: Version 10.2.0.5.0 - Production NLSRTL Version 10.2.0.5.0 - ProductionSQL> create table test_cbc_handle(t1 int);Table created.SQL> insert into test_cbc_handle values(1);1 row created.SQL> commit;Commit complete.SQL> select rowid from test_cbc_handle; ROWID ------------------ AAANO6AABAAAQZSAAASQL> select * from test_cbc_handle where rowid='AAANO6AABAAAQZSAAA';T1 ----------1 SQL> select addr,name from v$latch_parent where name='cache buffer handles';ADDR???????????? NAME ---------------- -------------------------------------------------- 00000000600140A8 cache buffer handlesSQL> select to_number('00000000600140A8','xxxxxxxxxxxxxxxxxxxx') from dual;TO_NUMBER('00000000600140A8','XXXXXXXXXXXXXXXXXXXX') ----------------------------------------------------1610694824注意cache buffer handles只有一個parent latch 而沒有 child latch我們讓SESSION A hold 住唯一的一個cache buffer handles parent latch 這會用到 oradebug call kslgetl 命令， kslgetl是oracle內部get latch的函數SQL> oradebug setmypid; Statement processed.SQL> oradebug call kslgetl 1610694824 1; Function returned 1另外開一個SESSION B 來觀察：SQL> select * from v$latchholder;PID??????? SID LADDR??????????? NAME?????????????????????????????????????????????????????????????????? GETS ---------- ---------- ---------------- ---------------------------------------------------------------- ----------15??????? 141 00000000600140A8 cache buffer handles??????????????????????????????????????????????????? 119cache buffer handles latch 確實被session A hold住了,這將阻塞其他進程acquire cache buffer handle latchSQL> select * from test_cbc_handle where rowid='AAANO6AABAAAQZSAAA';T1 ----------1但此時其他Server Process還是可以正常 read buffer， 這是因為隱藏參數"_db_handles_cached", 默認process會cache 5個 cache buffer handle 修改"_db_handles_cached"=0，讓process不再擁有5個緩存的cache buffer handle ， 這將導致 process 若需要pin buffer，則必須hold cache buffer handle latch以便分配一個cache buffer handleSQL> alter system set "_db_handles_cached"=0 scope=spfile;System altered.并重啟實例 shutdown immediate; startup;session A:SQL> oradebug setmypid; Statement processed.SQL> oradebug call kslgetl 1610694824 1; Function returned 1session B:select * from test_cbc_handle where rowid='AAANO6AABAAAQZSAAA';session B hang!!WHY?SQL> oradebug setmypid; Statement processed.SQL> oradebug dump systemstate 266; Statement processed.SO: 0x11b30b7b0, type: 2, owner: (nil), flag: INIT/-/-/0x00(process) Oracle pid=22, calls cur/top: (nil)/0x11b453c38, flag: (0) -int error: 0, call error: 0, sess error: 0, txn error 0(post info) last post received: 0 0 0last post received-location: No postlast process to post me: nonelast post sent: 0 0 0last post sent-location: No postlast process posted by me: none(latch info) wait_event=0 bits=8holding??? (efd=4) 600140a8 cache buffer handles level=3SO: 0x11b305810, type: 2, owner: (nil), flag: INIT/-/-/0x00(process) Oracle pid=10, calls cur/top: 0x11b455ac0/0x11b450a58, flag: (0) -int error: 0, call error: 0, sess error: 0, txn error 0(post info) last post received: 0 0 0last post received-location: No postlast process to post me: nonelast post sent: 0 0 0last post sent-location: No postlast process posted by me: none(latch info) wait_event=0 bits=2Location from where call was made: kcbzgs:waiting for 600140a8 cache buffer handles level=3FBD93353:000019F0??? 10?? 162 10005?? 1 KSL WAIT BEG [latch: cache buffer handles] 1610694824/0x600140a8 125/0x7d 0/0x0 FF936584:00002761??? 10?? 144 10005?? 1 KSL WAIT BEG [latch: cache buffer handles] 1610694824/0x600140a8 125/0x7d 0/0x0PID=22 holding 持有cache buffer handles latchPID=10 等待 cache buffer handles latch， 這是因為"_db_handles_cached"=0 導致 process自身沒有緩存cache buffer handles這個systemstate中找不到 kcbbfbp cache buffer handle對象， 因為 "_db_handles_cached"=0 且 cache buffer handles latch被hold 住了我們釋放cache buffer handles latch ， 來觀察 buffer被pin住而不是釋放時的情境session A exit session B:SQL> select * from v$latchholder;no rows selectedSQL> insert into test_cbc_handle values(2);1 row created.SQL> commit;Commit complete.SQL> SQL> select t1,rowid from test_cbc_handle;T1 ROWID ---------- ------------------1 AAANPAAABAAAQZSAAA2 AAANPAAABAAAQZSAABSQL> select spid,pid from v$process where addr = ( select paddr from v$session where sid=(select distinct sid from v$mystat));SPID??????????????? PID ------------ ---------- 19251??????????????? 10用 GDB 對 SPID=19215 做debug ， 使用 kcbrls 函數作為breakpoint 這會讓 進程無法release buffer[oracle@vrh8 ~]$ gdb $ORACLE_HOME/bin/oracle 19251 GNU gdb (GDB) Red Hat Enterprise Linux (7.0.1-37.el5) Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.? Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-redhat-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... Reading symbols from /s01/oracle/product/10.2.0.5/db_1/bin/oracle...(no debugging symbols found)...done. Attaching to program: /s01/oracle/product/10.2.0.5/db_1/bin/oracle, process 19251 Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libskgxp10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libskgxp10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libhasgen10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libhasgen10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libskgxn2.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libskgxn2.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libocr10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libocr10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libocrb10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libocrb10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libocrutl10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libocrutl10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libjox10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libjox10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libclsra10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libclsra10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libdbcfg10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libdbcfg10.so Reading symbols from /s01/oracle/product/10.2.0.5/db_1/lib/libnnz10.so...(no debugging symbols found)...done. Loaded symbols for /s01/oracle/product/10.2.0.5/db_1/lib/libnnz10.so Reading symbols from /usr/lib64/libaio.so.1...(no debugging symbols found)...done. Loaded symbols for /usr/lib64/libaio.so.1 Reading symbols from /lib64/libdl.so.2...(no debugging symbols found)...done. Loaded symbols for /lib64/libdl.so.2 Reading symbols from /lib64/libm.so.6...(no debugging symbols found)...done. Loaded symbols for /lib64/libm.so.6 Reading symbols from /lib64/libpthread.so.0...(no debugging symbols found)...done. [Thread debugging using libthread_db enabled] Loaded symbols for /lib64/libpthread.so.0 Reading symbols from /lib64/libnsl.so.1...(no debugging symbols found)...done. Loaded symbols for /lib64/libnsl.so.1 Reading symbols from /lib64/libc.so.6...(no debugging symbols found)...done. Loaded symbols for /lib64/libc.so.6 Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done. Loaded symbols for /lib64/ld-linux-x86-64.so.2 Reading symbols from /lib64/libnss_files.so.2...(no debugging symbols found)...done. Loaded symbols for /lib64/libnss_files.so.2 0x00000035c000d940 in __read_nocancel () from /lib64/libpthread.so.0(gdb) break kcbrlsBreakpoint 1 at 0x10e5d24session B:select * from test_cbc_handle where rowid='AAANPAAABAAAQZSAAA'; select hang !!GDB(gdb) c Continuing.Breakpoint 1, 0x00000000010e5d24 in kcbrls () (gdb) bt #0? 0x00000000010e5d24 in kcbrls () #1? 0x0000000002e87d25 in qertbFetchByUserRowID () #2? 0x00000000030c62b8 in opifch2 () #3? 0x00000000032327f0 in kpoal8 () #4? 0x00000000013b7c10 in opiodr () #5? 0x0000000003c3c9da in ttcpip () #6? 0x00000000013b3144 in opitsk () #7? 0x00000000013b60ec in opiino () #8? 0x00000000013b7c10 in opiodr () #9? 0x00000000013a92f8 in opidrv () #10 0x0000000001fa3936 in sou2o () #11 0x000000000072d40b in opimai_real () #12 0x000000000072d35c in main ()SQL> oradebug setmypid; Statement processed.SQL> oradebug dump systemstate 266; Statement processed.此時可以看到 kcbbfbp buffer cache handle 將? SO state object 和 BH BUFFER HEADER? link在一起----------------------------------------SO: 0x11b452348, type: 3, owner: 0x11b305810, flag: INIT/-/-/0x00(call) sess: cur 11b41bd18, rec 0, usr 11b41bd18; depth: 0----------------------------------------SO: 0x1182dc750, type: 24, owner: 0x11b452348, flag: INIT/-/-/0x00(buffer) (CR) PR: 0x11b305810 FLG: 0x108000class bit: (nil)kcbbfbp: [BH: 0xf2fc69f8, LINK: 0x1182dc790]where: kdswh05: kdsgrp, why: 0BH (0xf2fc69f8) file#: 1 rdba: 0x00410652 (1/67154) class: 1 ba: 0xf297c000set: 3 blksize: 8192 bsi: 0 set-flg: 2 pwbcnt: 272dbwrid: 0 obj: 54208 objn: 54202 tsn: 0 afn: 1hash: [f2fc47f8,1181f3038] lru: [f2fc6b88,f2fc6968]obj-flags: object_ckpt_listckptq: [1182ecf38,1182ecf38] fileq: [1182ecf58,1182ecf58] objq: [108712a28,108712a28]use: [1182dc790,1182dc790] wait: [NULL]st: XCURRENT md: SHR tch: 12flags: buffer_dirty gotten_in_current_mode block_written_onceredo_since_readLRBA: [0xc7.73b.0] HSCN: [0x0.1cbe52] HSUB: [1]Using State Objects----------------------------------------SO: 0x1182dc750, type: 24, owner: 0x11b452348, flag: INIT/-/-/0x00(buffer) (CR) PR: 0x11b305810 FLG: 0x108000class bit: (nil)kcbbfbp: [BH: 0xf2fc69f8, LINK: 0x1182dc790]where: kdswh05: kdsgrp, why: 0buffer tsn: 0 rdba: 0x00410652 (1/67154)scn: 0x0000.001cbe52 seq: 0x01 flg: 0x02 tail: 0xbe520601frmt: 0x02 chkval: 0x0000 type: 0x06=trans datatab 0, row 0, @0x1f9a tl: 6 fb: --H-FL-- lb: 0x0? cc: 1 col? 0: [ 2]? c1 02 tab 0, row 1, @0x1f94 tl: 6 fb: --H-FL-- lb: 0x2? cc: 1 col? 0: [ 2]? c1 15 end_of_block_dump?? ??? ?(buffer) (CR) PR: 0x11b305810 FLG: 0x108000st: XCURRENT md: SHR tch: 12 該 buffer header的status= XCURRENT mode=KCBMSHARE KCBMSHR???? current share也可以通過? x$kcbbf 找到相關的 cache buffer handleSQL> select distinct KCBBPBH from? x$kcbbf ;KCBBPBH ---------------- 00 00000000F2FC69F8??????????? ==>0xf2fc69f8SQL> select * from x$kcbbf where kcbbpbh='00000000F2FC69F8';ADDR?????????????????? INDX??? INST_ID KCBBFSO_TYP KCBBFSO_FLG KCBBFSO_OWN ---------------- ---------- ---------- ----------- ----------- ----------------KCBBFFLG??? KCBBFCR??? KCBBFCM KCBBFMBR???????? KCBBPBH ---------- ---------- ---------- ---------------- ---------------- KCBBPBF????????? X0KCBBPBH??????? X0KCBBPBF??????? X1KCBBPBH ---------------- ---------------- ---------------- ---------------- X1KCBBPBF??????? KCBBFBH??????????? KCBBFWHR?? KCBBFWHY ---------------- ---------------- ---------- ---------- 00000001182DC750??????? 748????????? 1????????? 24?????????? 1 000000011B4523481081344????????? 1????????? 0 00?????????????? 00000000F2FC69F8 00000001182DC750 00?????????????? 00000001182DC750 00 00000001182DC7F8 00????????????????????? 583????????? 0SQL> desc x$kcbbf;Name????????????????????????????????????? Null???? Type----------------------------------------- -------- ----------------------------ADDR?????????????????????????????????????????????? RAW(8)INDX?????????????????????????????????????????????? NUMBERINST_ID??????????????????????????????????????????? NUMBERKCBBFSO_TYP??????????????????????????????????????? NUMBERKCBBFSO_FLG??????????????????????????????????????? NUMBERKCBBFSO_OWN??????????????????????????????????????? RAW(8)KCBBFFLG?????????????????????????????????????????? NUMBERKCBBFCR??????????????????????????????????????????? NUMBERKCBBFCM??????????????????????????????????????????? NUMBERKCBBFMBR?????????????????????????????????????????? RAW(8)KCBBPBH??????????????????????????????????????????? RAW(8)KCBBPBF??????????????????????????????????????????? RAW(8)X0KCBBPBH????????????????????????????????????????? RAW(8)X0KCBBPBF????????????????????????????????????????? RAW(8)X1KCBBPBH????????????????????????????????????????? RAW(8)X1KCBBPBF????????????????????????????????????????? RAW(8)KCBBFBH??????????????????????????????????????????? RAW(8)KCBBFWHR?????????????????????????????????????????? NUMBERKCBBFWHY?????????????????????????????????????????? NUMBERgdb 退出 讓process能夠順利完成kcbrls release buffer后 相關的cache buffer handle被清理SQL> select distinct KCBBPBH from? x$kcbbf ;KCBBPBH ---------------- 00

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