{"id":3303,"date":"2020-05-10T17:21:40","date_gmt":"2020-05-10T17:21:40","guid":{"rendered":"https:\/\/nenadnoveljic.com\/blog\/?p=3303"},"modified":"2021-02-11T16:59:19","modified_gmt":"2021-02-11T16:59:19","slug":"tracing-library-cache-locks","status":"publish","type":"post","link":"https:\/\/nenadnoveljic.com\/blog\/tracing-library-cache-locks\/","title":{"rendered":"Tracing Library Cache Locks"},"content":{"rendered":"<p>While working on a <a href=\"https:\/\/nenadnoveljic.com\/blog\/deadlock-row-cache-lock-library-cache-lock\/\">recent problem<\/a>, I missed a possibility of tracing library cache locks. In contrast, row cache locks can be traced with the event 10222:<\/p>\n<pre><code>...\nkqrpre: optimistic lookup: success\nkqrpre: done po=7fffbd990200 cid=8 flg=0 eq=0 pso=99184ec8 dur=CALL\nkqrpre: keyIndex=0 hash=162240d8 d53980ff 0\nkqrpre: obobn=690 obname=OPTSTAT_HIST_CONTROL$ obtyp=2 obsta=1 obflg=4194304\nkqrpre: returnVal: TRUE\n...<\/code><\/pre>\n<p>As far as I know, there isn&#8217;t anything similar for library cache locks. If you were thinking of querying <i>x$kgllk<\/i> or dumping the library cache, they provide just a momentary snapshot of the current state. But sometimes, it&#8217;s useful to observe the library cache locks over time. SQL trace shows that information, but only for blocked requests.<\/p>\n<p>In order to close this gap I, first, examined the following two Oracle C functions on the release 19.6.0.0.200114: <i>kgllkal<\/i> and <i>kglGetSO<\/i>.<\/p>\n<p><i>kgllkal<\/i> allocates a library cache lock. It receives the following arguments:<\/p>\n<ul>\n<li><i>rdx<\/i>: handle address<\/li>\n<li><i>rcx<\/i>: lock mode<\/li>\n<\/ul>\n<p>Further, it calls <i>kglGetSO<\/i> to allocate the library cache state object. <i>kglGetSO<\/i> returns the lock address.<\/p>\n<p>That being said, it&#8217;s easy to come up with the DTrace script which assembles the useful information:<\/p>\n<pre><code>#pragma D option quiet\n\npid$target:oracle:kgllkal:entry\n{\n  printf(\"%Y\\n\", walltimestamp);\n  ustack();\n  self-&gt;kgllkhdl = arg2;\n  self-&gt;kgllkmod = arg3;\n}\n\npid$target:oracle:kglGetSO:entry\n\/ self-&gt;kgllkhdl \/\n{\n  self-&gt;kglGetSO = 1;\n}\n\npid$target:oracle:kglGetSO:return\n\/ self-&gt;kglGetSO \/\n{\n  printf(\"\\n\");\n  printf(\"KGLLKHDL KGLLKMOD KGLLKADR\\n\");\n  printf(\"-------- -------- --------\\n\");\n  printf(\"%X %8d %X\\n\" , self-&gt;kgllkhdl, self-&gt;kgllkmod, arg1 );\n  printf(\"\\n\");\n  printf(\"==========================\\n\");\n  printf(\"\\n\");\n  self-&gt;kglGetSO = 0;\n}\n\npid$target:oracle:kgllkal:return\n{\n  self-&gt;kgllkhdl = 0;\n  self-&gt;kgllkmod = 0;\n}<\/code><\/pre>\n<p>Here&#8217;s a sample output:<\/p>\n<pre><code>\n...\n==========================\n\n2020 May  8 22:02:18\n\n              oracle`kgllkal\n              oracle`kglLockCursor+0x83c\n              oracle`kkspbd0+0x161\n              oracle`<span style=\"color:blue\">kksParseCursory<\/span>+0x1a5\n              oracle`opiosq0+0xb72\n              oracle`opipls+0x1a52\n              oracle`opiodr+0x439\n              oracle`rpidrus+0xdc\n              oracle`skgmstack+0x59\n              oracle`rpidru+0x9f\n              oracle`rpiswu2+0x22e\n              oracle`rpidrv+0x551\n              oracle`psddr0+0x22f\n              oracle`psdnal+0x31a\n              oracle`pevm_EXECC+0x14a\n              oracle`pfrinstr_EXECC+0x41\n              oracle`pfrrun_no_tool+0x3c\n              oracle`pfrrun+0x54a\n              oracle`plsql_run+0x2c0\n              oracle`peicnt+0x16c\n\nKGLLKHDL KGLLKMOD KGLLKADR\n-------- -------- --------\n<span style=\"color:blue\">9E4C2908<\/span>        1 <span style=\"color:green\">99B2BFA8<\/span>\n\n==========================\n\n2020 May  8 22:02:18\n\n              oracle`kgllkal\n              oracle`kglLockCursor+0x83c\n              oracle`kxsGetLookupLock+0x67\n              oracle`kkscsCheckCursor+0x1b2\n              oracle`<span style=\"color:red\">kkscsSearchChildList<\/span>+0x398\n              oracle`kksfbc+0xabd\n              oracle`kkspbd0+0x39f\n              oracle`kksParseCursor+0x1a5\n              oracle`opiosq0+0xb72\n              oracle`opipls+0x1a52\n              oracle`opiodr+0x439\n              oracle`rpidrus+0xdc\n              oracle`skgmstack+0x59\n              oracle`rpidru+0x9f\n              oracle`rpiswu2+0x22e\n              oracle`rpidrv+0x551\n              oracle`psddr0+0x22f\n              oracle`psdnal+0x31a\n              oracle`pevm_EXECC+0x14a\n              oracle`pfrinstr_EXECC+0x41\n\nKGLLKHDL KGLLKMOD KGLLKADR\n-------- -------- --------\n<span style=\"color:red\">9C7093A8<\/span>        1 <span style=\"color:green\">99B2BDD8<\/span>\n\n==========================<\/code><\/pre>\n<p>The script shows the call stacks, thus, providing the context of each lock allocation. In the example above, the first entry is for parsing a <span style=\"color:blue\">parent<\/span> cursor, the second for a <span style=\"color:red\">child<\/span> cursor.<\/p>\n<p>The script also shows the columns from <i>x$kgllk<\/i>, which is, by the way, the SQL interface to the shared memory structure underpinning library cache locks:<\/p>\n<ul>\n<li><i>KGLLKHDL<\/i>: handle address (parameter 1 of library cache lock wait event)<\/li>\n<li><i>KGLLKADR<\/i>: lock address (parameter 2 of library cache lock wait event)<\/li>\n<li><i>KGLLKMOD<\/i>: lock mode (contained in the parameter 3 of library cache lock wait event)<\/li>\n<\/ul>\n<p>You can use these values for querying the database:<\/p>\n<pre><code>select kgllkhdl,kglhdpar,kgllkmod,kglnaobj,kgllkuse from x$kgllk \n  where kgllkadr in (<span style=\"color:green\">'0000000099B2BFA8','0000000099B2BDD8'<\/span>) ;\n\nKGLLKHDL         KGLHDPAR           KGLLKMOD KGLNAOBJ                                                     KGLLKUSE\n---------------- ---------------- ---------- ------------------------------------------------------------ ----------------\n<span style=\"color:red\">000000009C7093A8<\/span> <span style=\"color:blue\">000000009E4C2908<\/span>          1 UPDATE \/*+ OPT_PARAM('_parallel_syspls_obey_force' 'false')  00000000BA662B50\n<span style=\"color:blue\">000000009E4C2908<\/span> <span style=\"color:blue\">000000009E4C2908<\/span>          1 UPDATE \/*+ OPT_PARAM('_parallel_syspls_obey_force' 'false')  00000000BA662B50<\/code><\/pre>\n<p>Note that the output above confirms the relationship between the <span style=\"color:blue\">parent<\/span> and the <span style=\"color:red\">child<\/span> cursor that we deduced by observing the call stacks.<\/p>\n<p>Once more, DTrace proved invaluable for extending the observability in the Oracle database.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Tracing Library Cache Locks with DTrace <a href=\"https:\/\/nenadnoveljic.com\/blog\/tracing-library-cache-locks\/\" class=\"more-link\">Continue Reading <span class=\"meta-nav\">&rarr;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[24,39,5],"tags":[],"class_list":["post-3303","post","type-post","status-publish","format-standard","hentry","category-dtrace","category-library-cache","category-oracle"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - 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