Using perf to analyze any(almost) internal gpdb functions

On my previous post I have shown how to investigate particular function execution, actually I’ve just borrowed a ready to use piece of code here: Profiling with perf

In this post you will see how to go much deeper

Let’s consider this fuction ExecInitSeqScan as an example

You can see this function exported in ../gpdb/src/include/executor/nodeSeqscan.h and having a signature as

extern SeqScanState *ExecInitSeqScan(SeqScan *node, EState *estate, int eflags);

parameters of the function are having the following definitions in turn

typedef Scan SeqScan; --plannodes.h

--=======================================================

typedef struct Scan   --plannodes.h
{
	Plan		plan;
	Index		scanrelid;		/* relid is index into the range table */
} Scan;

--===================================================

typedef struct Plan  --plannodes.h 
{
	NodeTag		type;
	/*
	 * estimated execution costs for plan (see costsize.c for more info)
	 */
	Cost		startup_cost;	/* cost expended before fetching any tuples */
	Cost		total_cost;		/* total cost (assuming all tuples fetched) */

	/*
	 * planner's estimate of result size of this plan step
	 */
	double		plan_rows;		/* number of rows plan is expected to emit */
	int			plan_width;		/* average row width in bytes */

	/*
	 * information needed for parallel query
	 */
	bool		parallel_aware; /* engage parallel-aware logic? */
	bool		parallel_safe;	/* OK to use as part of parallel plan? */

	/*
	 * Common structural data for all Plan types.
	 */
	int			plan_node_id;	/* unique across entire final plan tree */
	List	   *targetlist;		/* target list to be computed at this node */
	List	   *qual;			/* implicitly-ANDed qual conditions */
	struct Plan *lefttree;		/* input plan tree(s) */
	struct Plan *righttree;
	List	   *initPlan;		/* Init Plan nodes (un-correlated expr
								 * subselects) */

	/*
	 * Information for management of parameter-change-driven rescanning
	 *
	 * extParam includes the paramIDs of all external PARAM_EXEC params
	 * affecting this plan node or its children.  setParam params from the
	 * node's initPlans are not included, but their extParams are.
	 *
	 * allParam includes all the extParam paramIDs, plus the IDs of local
	 * params that affect the node (i.e., the setParams of its initplans).
	 * These are _all_ the PARAM_EXEC params that affect this node.
	 */
	Bitmapset  *extParam;
	Bitmapset  *allParam;

	/*
	 * MPP needs to keep track of the characteristics of flow of output
	 * tuple of Plan nodes.
	 */
	Flow		*flow;			/* Flow description.  Initially NULL.
	 * Set during parallelization.
	 */

	/**
	 * How much memory (in KB) should be used to execute this plan node?
	 */
	uint64 operatorMemKB;
} Plan;

When you have you code compiled with debug information – you can actually “perf” not only entry point of the fuction but rather many other lines of code, you can check which ones by

 sudo perf probe --line ExecInitSeqScan  -x /usr/local/gpdb/bin/postgres

Let’s proceed with our function of choice though

-- At first we delete potentially already set probe for ExecInitSeqScan
sudo perf probe -d ExecInitSeqScan
-- we tranverse those SeqScan node parameter via SeqScan->Plan-> Plan.plan_rows
sudo perf probe -x /usr/local/gpdb/bin/postgres  ExecInitSeqScan 'node->plan.plan_rows:s64'

after that we can start probing (on running gpdb instance)

sudo perf record -e  probe_postgres:ExecInitSeqScan -u gpadmin
-- -u gbadmin means all processes under gpadmin user

we are starting some workload in a separate session/terminal, then stop recording via Ctrl-C
several probe results are saved in perf.data(default) file.
we can see “formatted “results as

sudo perf script -i perf.data

You will see many rows like

        postgres 25226 [003]  9568.804588: probe_postgres:ExecInitSeqScan: (9a7960) plan_rows=0
        postgres 25313 [003]  9585.033424: probe_postgres:ExecInitSeqScan: (9a7960) plan_rows=0
        postgres 25330 [000]  9585.193666: probe_postgres:ExecInitSeqScan: (9a7960) plan_rows=0
        postgres 25335 [002]  9585.329603: probe_postgres:ExecInitSeqScan: (9a7960) plan_rows=0
        postgres 25335 [002]  9585.362406: probe_postgres:ExecInitSeqScan: (9a7960) plan_rows=0