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diff --git a/doc/src/sgml/html/dynamic-trace.html b/doc/src/sgml/html/dynamic-trace.html new file mode 100644 index 0000000..42690bf --- /dev/null +++ b/doc/src/sgml/html/dynamic-trace.html @@ -0,0 +1,301 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>28.5. Dynamic Tracing</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="progress-reporting.html" title="28.4. Progress Reporting" /><link rel="next" href="diskusage.html" title="Chapter 29. Monitoring Disk Usage" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">28.5. Dynamic Tracing</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="progress-reporting.html" title="28.4. Progress Reporting">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="monitoring.html" title="Chapter 28. Monitoring Database Activity">Up</a></td><th width="60%" align="center">Chapter 28. Monitoring Database Activity</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="diskusage.html" title="Chapter 29. Monitoring Disk Usage">Next</a></td></tr></table><hr /></div><div class="sect1" id="DYNAMIC-TRACE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">28.5. Dynamic Tracing</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="dynamic-trace.html#COMPILING-FOR-TRACE">28.5.1. Compiling for Dynamic Tracing</a></span></dt><dt><span class="sect2"><a href="dynamic-trace.html#TRACE-POINTS">28.5.2. Built-in Probes</a></span></dt><dt><span class="sect2"><a href="dynamic-trace.html#USING-TRACE-POINTS">28.5.3. Using Probes</a></span></dt><dt><span class="sect2"><a href="dynamic-trace.html#DEFINING-TRACE-POINTS">28.5.4. Defining New Probes</a></span></dt></dl></div><a id="id-1.6.15.10.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> provides facilities to support + dynamic tracing of the database server. This allows an external + utility to be called at specific points in the code and thereby trace + execution. + </p><p> + A number of probes or trace points are already inserted into the source + code. These probes are intended to be used by database developers and + administrators. By default the probes are not compiled into + <span class="productname">PostgreSQL</span>; the user needs to explicitly tell + the configure script to make the probes available. + </p><p> + Currently, the + <a class="ulink" href="https://en.wikipedia.org/wiki/DTrace" target="_top">DTrace</a> + utility is supported, which, at the time of this writing, is available + on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux. The + <a class="ulink" href="https://sourceware.org/systemtap/" target="_top">SystemTap</a> project + for Linux provides a DTrace equivalent and can also be used. Supporting other dynamic + tracing utilities is theoretically possible by changing the definitions for + the macros in <code class="filename">src/include/utils/probes.h</code>. + </p><div class="sect2" id="COMPILING-FOR-TRACE"><div class="titlepage"><div><div><h3 class="title">28.5.1. Compiling for Dynamic Tracing</h3></div></div></div><p> + By default, probes are not available, so you will need to + explicitly tell the configure script to make the probes available + in <span class="productname">PostgreSQL</span>. To include DTrace support + specify <code class="option">--enable-dtrace</code> to configure. See <a class="xref" href="install-procedure.html" title="17.4. Installation Procedure">Section 17.4</a> for further information. + </p></div><div class="sect2" id="TRACE-POINTS"><div class="titlepage"><div><div><h3 class="title">28.5.2. Built-in Probes</h3></div></div></div><p> + A number of standard probes are provided in the source code, + as shown in <a class="xref" href="dynamic-trace.html#DTRACE-PROBE-POINT-TABLE" title="Table 28.47. Built-in DTrace Probes">Table 28.47</a>; + <a class="xref" href="dynamic-trace.html#TYPEDEFS-TABLE" title="Table 28.48. Defined Types Used in Probe Parameters">Table 28.48</a> + shows the types used in the probes. More probes can certainly be + added to enhance <span class="productname">PostgreSQL</span>'s observability. + </p><div class="table" id="DTRACE-PROBE-POINT-TABLE"><p class="title"><strong>Table 28.47. Built-in DTrace Probes</strong></p><div class="table-contents"><table class="table" summary="Built-in DTrace Probes" border="1"><colgroup><col class="col1" /><col class="col2" /><col class="col3" /></colgroup><thead><tr><th>Name</th><th>Parameters</th><th>Description</th></tr></thead><tbody><tr><td><code class="literal">transaction-start</code></td><td><code class="literal">(LocalTransactionId)</code></td><td>Probe that fires at the start of a new transaction. + arg0 is the transaction ID.</td></tr><tr><td><code class="literal">transaction-commit</code></td><td><code class="literal">(LocalTransactionId)</code></td><td>Probe that fires when a transaction completes successfully. + arg0 is the transaction ID.</td></tr><tr><td><code class="literal">transaction-abort</code></td><td><code class="literal">(LocalTransactionId)</code></td><td>Probe that fires when a transaction completes unsuccessfully. + arg0 is the transaction ID.</td></tr><tr><td><code class="literal">query-start</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the processing of a query is started. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-done</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the processing of a query is complete. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-parse-start</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the parsing of a query is started. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-parse-done</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the parsing of a query is complete. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-rewrite-start</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the rewriting of a query is started. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-rewrite-done</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires when the rewriting of a query is complete. + arg0 is the query string.</td></tr><tr><td><code class="literal">query-plan-start</code></td><td><code class="literal">()</code></td><td>Probe that fires when the planning of a query is started.</td></tr><tr><td><code class="literal">query-plan-done</code></td><td><code class="literal">()</code></td><td>Probe that fires when the planning of a query is complete.</td></tr><tr><td><code class="literal">query-execute-start</code></td><td><code class="literal">()</code></td><td>Probe that fires when the execution of a query is started.</td></tr><tr><td><code class="literal">query-execute-done</code></td><td><code class="literal">()</code></td><td>Probe that fires when the execution of a query is complete.</td></tr><tr><td><code class="literal">statement-status</code></td><td><code class="literal">(const char *)</code></td><td>Probe that fires anytime the server process updates its + <code class="structname">pg_stat_activity</code>.<code class="structfield">status</code>. + arg0 is the new status string.</td></tr><tr><td><code class="literal">checkpoint-start</code></td><td><code class="literal">(int)</code></td><td>Probe that fires when a checkpoint is started. + arg0 holds the bitwise flags used to distinguish different checkpoint + types, such as shutdown, immediate or force.</td></tr><tr><td><code class="literal">checkpoint-done</code></td><td><code class="literal">(int, int, int, int, int)</code></td><td>Probe that fires when a checkpoint is complete. + (The probes listed next fire in sequence during checkpoint processing.) + arg0 is the number of buffers written. arg1 is the total number of + buffers. arg2, arg3 and arg4 contain the number of WAL files added, + removed and recycled respectively.</td></tr><tr><td><code class="literal">clog-checkpoint-start</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the CLOG portion of a checkpoint is started. + arg0 is true for normal checkpoint, false for shutdown + checkpoint.</td></tr><tr><td><code class="literal">clog-checkpoint-done</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the CLOG portion of a checkpoint is + complete. arg0 has the same meaning as for <code class="literal">clog-checkpoint-start</code>.</td></tr><tr><td><code class="literal">subtrans-checkpoint-start</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the SUBTRANS portion of a checkpoint is + started. + arg0 is true for normal checkpoint, false for shutdown + checkpoint.</td></tr><tr><td><code class="literal">subtrans-checkpoint-done</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the SUBTRANS portion of a checkpoint is + complete. arg0 has the same meaning as for + <code class="literal">subtrans-checkpoint-start</code>.</td></tr><tr><td><code class="literal">multixact-checkpoint-start</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the MultiXact portion of a checkpoint is + started. + arg0 is true for normal checkpoint, false for shutdown + checkpoint.</td></tr><tr><td><code class="literal">multixact-checkpoint-done</code></td><td><code class="literal">(bool)</code></td><td>Probe that fires when the MultiXact portion of a checkpoint is + complete. arg0 has the same meaning as for + <code class="literal">multixact-checkpoint-start</code>.</td></tr><tr><td><code class="literal">buffer-checkpoint-start</code></td><td><code class="literal">(int)</code></td><td>Probe that fires when the buffer-writing portion of a checkpoint + is started. + arg0 holds the bitwise flags used to distinguish different checkpoint + types, such as shutdown, immediate or force.</td></tr><tr><td><code class="literal">buffer-sync-start</code></td><td><code class="literal">(int, int)</code></td><td>Probe that fires when we begin to write dirty buffers during + checkpoint (after identifying which buffers must be written). + arg0 is the total number of buffers. + arg1 is the number that are currently dirty and need to be written.</td></tr><tr><td><code class="literal">buffer-sync-written</code></td><td><code class="literal">(int)</code></td><td>Probe that fires after each buffer is written during checkpoint. + arg0 is the ID number of the buffer.</td></tr><tr><td><code class="literal">buffer-sync-done</code></td><td><code class="literal">(int, int, int)</code></td><td>Probe that fires when all dirty buffers have been written. + arg0 is the total number of buffers. + arg1 is the number of buffers actually written by the checkpoint process. + arg2 is the number that were expected to be written (arg1 of + <code class="literal">buffer-sync-start</code>); any difference reflects other processes flushing + buffers during the checkpoint.</td></tr><tr><td><code class="literal">buffer-checkpoint-sync-start</code></td><td><code class="literal">()</code></td><td>Probe that fires after dirty buffers have been written to the + kernel, and before starting to issue fsync requests.</td></tr><tr><td><code class="literal">buffer-checkpoint-done</code></td><td><code class="literal">()</code></td><td>Probe that fires when syncing of buffers to disk is + complete.</td></tr><tr><td><code class="literal">twophase-checkpoint-start</code></td><td><code class="literal">()</code></td><td>Probe that fires when the two-phase portion of a checkpoint is + started.</td></tr><tr><td><code class="literal">twophase-checkpoint-done</code></td><td><code class="literal">()</code></td><td>Probe that fires when the two-phase portion of a checkpoint is + complete.</td></tr><tr><td><code class="literal">buffer-read-start</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</code></td><td>Probe that fires when a buffer read is started. + arg0 and arg1 contain the fork and block numbers of the page (but + arg1 will be -1 if this is a relation extension request). + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer. + arg6 is true for a relation extension request, false for normal + read.</td></tr><tr><td><code class="literal">buffer-read-done</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool)</code></td><td>Probe that fires when a buffer read is complete. + arg0 and arg1 contain the fork and block numbers of the page (if this + is a relation extension request, arg1 now contains the block number + of the newly added block). + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer. + arg6 is true for a relation extension request, false for normal + read. + arg7 is true if the buffer was found in the pool, false if not.</td></tr><tr><td><code class="literal">buffer-flush-start</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid)</code></td><td>Probe that fires before issuing any write request for a shared + buffer. + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation.</td></tr><tr><td><code class="literal">buffer-flush-done</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid)</code></td><td>Probe that fires when a write request is complete. (Note + that this just reflects the time to pass the data to the kernel; + it's typically not actually been written to disk yet.) + The arguments are the same as for <code class="literal">buffer-flush-start</code>.</td></tr><tr><td><code class="literal">buffer-write-dirty-start</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid)</code></td><td>Probe that fires when a server process begins to write a dirty + buffer. (If this happens often, it implies that + <a class="xref" href="runtime-config-resource.html#GUC-SHARED-BUFFERS">shared_buffers</a> is too + small or the background writer control parameters need adjustment.) + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation.</td></tr><tr><td><code class="literal">buffer-write-dirty-done</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid)</code></td><td>Probe that fires when a dirty-buffer write is complete. + The arguments are the same as for <code class="literal">buffer-write-dirty-start</code>.</td></tr><tr><td><code class="literal">wal-buffer-write-dirty-start</code></td><td><code class="literal">()</code></td><td>Probe that fires when a server process begins to write a + dirty WAL buffer because no more WAL buffer space is available. + (If this happens often, it implies that + <a class="xref" href="runtime-config-wal.html#GUC-WAL-BUFFERS">wal_buffers</a> is too small.)</td></tr><tr><td><code class="literal">wal-buffer-write-dirty-done</code></td><td><code class="literal">()</code></td><td>Probe that fires when a dirty WAL buffer write is complete.</td></tr><tr><td><code class="literal">wal-insert</code></td><td><code class="literal">(unsigned char, unsigned char)</code></td><td>Probe that fires when a WAL record is inserted. + arg0 is the resource manager (rmid) for the record. + arg1 contains the info flags.</td></tr><tr><td><code class="literal">wal-switch</code></td><td><code class="literal">()</code></td><td>Probe that fires when a WAL segment switch is requested.</td></tr><tr><td><code class="literal">smgr-md-read-start</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</code></td><td>Probe that fires when beginning to read a block from a relation. + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer.</td></tr><tr><td><code class="literal">smgr-md-read-done</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</code></td><td>Probe that fires when a block read is complete. + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer. + arg6 is the number of bytes actually read, while arg7 is the number + requested (if these are different it indicates trouble).</td></tr><tr><td><code class="literal">smgr-md-write-start</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</code></td><td>Probe that fires when beginning to write a block to a relation. + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer.</td></tr><tr><td><code class="literal">smgr-md-write-done</code></td><td><code class="literal">(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</code></td><td>Probe that fires when a block write is complete. + arg0 and arg1 contain the fork and block numbers of the page. + arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs + identifying the relation. + arg5 is the ID of the backend which created the temporary relation for a + local buffer, or <code class="symbol">InvalidBackendId</code> (-1) for a shared buffer. + arg6 is the number of bytes actually written, while arg7 is the number + requested (if these are different it indicates trouble).</td></tr><tr><td><code class="literal">sort-start</code></td><td><code class="literal">(int, bool, int, int, bool, int)</code></td><td>Probe that fires when a sort operation is started. + arg0 indicates heap, index or datum sort. + arg1 is true for unique-value enforcement. + arg2 is the number of key columns. + arg3 is the number of kilobytes of work memory allowed. + arg4 is true if random access to the sort result is required. + arg5 indicates serial when <code class="literal">0</code>, parallel worker when + <code class="literal">1</code>, or parallel leader when <code class="literal">2</code>.</td></tr><tr><td><code class="literal">sort-done</code></td><td><code class="literal">(bool, long)</code></td><td>Probe that fires when a sort is complete. + arg0 is true for external sort, false for internal sort. + arg1 is the number of disk blocks used for an external sort, + or kilobytes of memory used for an internal sort.</td></tr><tr><td><code class="literal">lwlock-acquire</code></td><td><code class="literal">(char *, LWLockMode)</code></td><td>Probe that fires when an LWLock has been acquired. + arg0 is the LWLock's tranche. + arg1 is the requested lock mode, either exclusive or shared.</td></tr><tr><td><code class="literal">lwlock-release</code></td><td><code class="literal">(char *)</code></td><td>Probe that fires when an LWLock has been released (but note + that any released waiters have not yet been awakened). + arg0 is the LWLock's tranche.</td></tr><tr><td><code class="literal">lwlock-wait-start</code></td><td><code class="literal">(char *, LWLockMode)</code></td><td>Probe that fires when an LWLock was not immediately available and + a server process has begun to wait for the lock to become available. + arg0 is the LWLock's tranche. + arg1 is the requested lock mode, either exclusive or shared.</td></tr><tr><td><code class="literal">lwlock-wait-done</code></td><td><code class="literal">(char *, LWLockMode)</code></td><td>Probe that fires when a server process has been released from its + wait for an LWLock (it does not actually have the lock yet). + arg0 is the LWLock's tranche. + arg1 is the requested lock mode, either exclusive or shared.</td></tr><tr><td><code class="literal">lwlock-condacquire</code></td><td><code class="literal">(char *, LWLockMode)</code></td><td>Probe that fires when an LWLock was successfully acquired when the + caller specified no waiting. + arg0 is the LWLock's tranche. + arg1 is the requested lock mode, either exclusive or shared.</td></tr><tr><td><code class="literal">lwlock-condacquire-fail</code></td><td><code class="literal">(char *, LWLockMode)</code></td><td>Probe that fires when an LWLock was not successfully acquired when + the caller specified no waiting. + arg0 is the LWLock's tranche. + arg1 is the requested lock mode, either exclusive or shared.</td></tr><tr><td><code class="literal">lock-wait-start</code></td><td><code class="literal">(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</code></td><td>Probe that fires when a request for a heavyweight lock (lmgr lock) + has begun to wait because the lock is not available. + arg0 through arg3 are the tag fields identifying the object being + locked. arg4 indicates the type of object being locked. + arg5 indicates the lock type being requested.</td></tr><tr><td><code class="literal">lock-wait-done</code></td><td><code class="literal">(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</code></td><td>Probe that fires when a request for a heavyweight lock (lmgr lock) + has finished waiting (i.e., has acquired the lock). + The arguments are the same as for <code class="literal">lock-wait-start</code>.</td></tr><tr><td><code class="literal">deadlock-found</code></td><td><code class="literal">()</code></td><td>Probe that fires when a deadlock is found by the deadlock + detector.</td></tr></tbody></table></div></div><br class="table-break" /><div class="table" id="TYPEDEFS-TABLE"><p class="title"><strong>Table 28.48. Defined Types Used in Probe Parameters</strong></p><div class="table-contents"><table class="table" summary="Defined Types Used in Probe Parameters" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Type</th><th>Definition</th></tr></thead><tbody><tr><td><code class="type">LocalTransactionId</code></td><td><code class="type">unsigned int</code></td></tr><tr><td><code class="type">LWLockMode</code></td><td><code class="type">int</code></td></tr><tr><td><code class="type">LOCKMODE</code></td><td><code class="type">int</code></td></tr><tr><td><code class="type">BlockNumber</code></td><td><code class="type">unsigned int</code></td></tr><tr><td><code class="type">Oid</code></td><td><code class="type">unsigned int</code></td></tr><tr><td><code class="type">ForkNumber</code></td><td><code class="type">int</code></td></tr><tr><td><code class="type">bool</code></td><td><code class="type">unsigned char</code></td></tr></tbody></table></div></div><br class="table-break" /></div><div class="sect2" id="USING-TRACE-POINTS"><div class="titlepage"><div><div><h3 class="title">28.5.3. Using Probes</h3></div></div></div><p> + The example below shows a DTrace script for analyzing transaction + counts in the system, as an alternative to snapshotting + <code class="structname">pg_stat_database</code> before and after a performance test: +</p><pre class="programlisting"> +#!/usr/sbin/dtrace -qs + +postgresql$1:::transaction-start +{ + @start["Start"] = count(); + self->ts = timestamp; +} + +postgresql$1:::transaction-abort +{ + @abort["Abort"] = count(); +} + +postgresql$1:::transaction-commit +/self->ts/ +{ + @commit["Commit"] = count(); + @time["Total time (ns)"] = sum(timestamp - self->ts); + self->ts=0; +} +</pre><p> + When executed, the example D script gives output such as: +</p><pre class="screen"> +# ./txn_count.d `pgrep -n postgres` or ./txn_count.d <PID> +^C + +Start 71 +Commit 70 +Total time (ns) 2312105013 +</pre><p> + </p><div class="note"><h3 class="title">Note</h3><p> + SystemTap uses a different notation for trace scripts than DTrace does, + even though the underlying trace points are compatible. One point worth + noting is that at this writing, SystemTap scripts must reference probe + names using double underscores in place of hyphens. This is expected to + be fixed in future SystemTap releases. + </p></div><p> + You should remember that DTrace scripts need to be carefully written and + debugged, otherwise the trace information collected might + be meaningless. In most cases where problems are found it is the + instrumentation that is at fault, not the underlying system. When + discussing information found using dynamic tracing, be sure to enclose + the script used to allow that too to be checked and discussed. + </p></div><div class="sect2" id="DEFINING-TRACE-POINTS"><div class="titlepage"><div><div><h3 class="title">28.5.4. Defining New Probes</h3></div></div></div><p> + New probes can be defined within the code wherever the developer + desires, though this will require a recompilation. Below are the steps + for inserting new probes: + </p><div class="procedure"><ol class="procedure" type="1"><li class="step"><p> + Decide on probe names and data to be made available through the probes + </p></li><li class="step"><p> + Add the probe definitions to <code class="filename">src/backend/utils/probes.d</code> + </p></li><li class="step"><p> + Include <code class="filename">pg_trace.h</code> if it is not already present in the + module(s) containing the probe points, and insert + <code class="literal">TRACE_POSTGRESQL</code> probe macros at the desired locations + in the source code + </p></li><li class="step"><p> + Recompile and verify that the new probes are available + </p></li></ol></div><p><strong>Example: </strong> + Here is an example of how you would add a probe to trace all new + transactions by transaction ID. + </p><div class="procedure"><ol class="procedure" type="1"><li class="step"><p> + Decide that the probe will be named <code class="literal">transaction-start</code> and + requires a parameter of type <code class="type">LocalTransactionId</code> + </p></li><li class="step"><p> + Add the probe definition to <code class="filename">src/backend/utils/probes.d</code>: +</p><pre class="programlisting"> +probe transaction__start(LocalTransactionId); +</pre><p> + Note the use of the double underline in the probe name. In a DTrace + script using the probe, the double underline needs to be replaced with a + hyphen, so <code class="literal">transaction-start</code> is the name to document for + users. + </p></li><li class="step"><p> + At compile time, <code class="literal">transaction__start</code> is converted to a macro + called <code class="literal">TRACE_POSTGRESQL_TRANSACTION_START</code> (notice the + underscores are single here), which is available by including + <code class="filename">pg_trace.h</code>. Add the macro call to the appropriate location + in the source code. In this case, it looks like the following: + +</p><pre class="programlisting"> +TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId); +</pre><p> + </p></li><li class="step"><p> + After recompiling and running the new binary, check that your newly added + probe is available by executing the following DTrace command. You + should see similar output: +</p><pre class="screen"> +# dtrace -ln transaction-start + ID PROVIDER MODULE FUNCTION NAME +18705 postgresql49878 postgres StartTransactionCommand transaction-start +18755 postgresql49877 postgres StartTransactionCommand transaction-start +18805 postgresql49876 postgres StartTransactionCommand transaction-start +18855 postgresql49875 postgres StartTransactionCommand transaction-start +18986 postgresql49873 postgres StartTransactionCommand transaction-start +</pre><p> + </p></li></ol></div><p> + There are a few things to be careful about when adding trace macros + to the C code: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + You should take care that the data types specified for a probe's + parameters match the data types of the variables used in the macro. + Otherwise, you will get compilation errors. + </p></li><li class="listitem"><p> + On most platforms, if <span class="productname">PostgreSQL</span> is + built with <code class="option">--enable-dtrace</code>, the arguments to a trace + macro will be evaluated whenever control passes through the + macro, <span class="emphasis"><em>even if no tracing is being done</em></span>. This is + usually not worth worrying about if you are just reporting the + values of a few local variables. But beware of putting expensive + function calls into the arguments. If you need to do that, + consider protecting the macro with a check to see if the trace + is actually enabled: + +</p><pre class="programlisting"> +if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED()) + TRACE_POSTGRESQL_TRANSACTION_START(some_function(...)); +</pre><p> + + Each trace macro has a corresponding <code class="literal">ENABLED</code> macro. + </p></li></ul></div><p> + + </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="progress-reporting.html" title="28.4. Progress Reporting">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="monitoring.html" title="Chapter 28. 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