Make EXPLAIN sampling smarter, to avoid excessive sampling delay.
Martijn van Oosterhout
This commit is contained in:
Bruce Momjian
parent
53d669e5c6
commit
87bd07d979
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@ -7,7 +7,7 @@
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.201 2006/04/27 00:33:41 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.202 2006/05/30 14:01:57 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -1306,7 +1306,7 @@ ExecCallTriggerFunc(TriggerData *trigdata,
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* one "tuple returned" (really the number of firings).
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*/
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if (instr)
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InstrStopNode(instr + tgindx, true);
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InstrStopNode(instr + tgindx, 1);
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return (HeapTuple) DatumGetPointer(result);
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}
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@ -2154,7 +2154,7 @@ AfterTriggerExecute(AfterTriggerEvent event,
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* one "tuple returned" (really the number of firings).
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*/
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if (instr)
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InstrStopNode(instr + tgindx, true);
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InstrStopNode(instr + tgindx, 1);
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}
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@ -12,7 +12,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/execProcnode.c,v 1.54 2006/03/05 15:58:26 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/executor/execProcnode.c,v 1.55 2006/05/30 14:01:57 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -423,7 +423,7 @@ ExecProcNode(PlanState *node)
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}
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if (node->instrument)
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InstrStopNode(node->instrument, !TupIsNull(result));
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InstrStopNode(node->instrument, TupIsNull(result) ? 0 : 1 );
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return result;
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}
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@ -7,16 +7,78 @@
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* Copyright (c) 2001-2006, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.14 2006/03/05 15:58:26 momjian Exp $
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* $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.15 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include <unistd.h>
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#include <math.h>
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#include "executor/instrument.h"
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/* This is the function that is used to determine the sampling intervals. In
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* general, if the function is f(x), then for N tuples we will take on the
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* order of integral(1/f(x), x=0..N) samples. Some examples follow, with the
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* number of samples that would be collected over 1,000,000 tuples.
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f(x) = x => log2(N) 20
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f(x) = x^(1/2) => 2 * N^(1/2) 2000
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f(x) = x^(1/3) => 1.5 * N^(2/3) 15000
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* I've chosen the last one as it seems to provide a good compromise between
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* low overhead but still getting a meaningful number of samples. However,
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* not all machines have the cbrt() function so on those we substitute
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* sqrt(). The difference is not very significant in the tests I made.
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*/
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#ifdef HAVE_CBRT
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#define SampleFunc cbrt
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#else
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#define SampleFunc sqrt
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#endif
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#define SAMPLE_THRESHOLD 50
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static double SampleOverhead;
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static bool SampleOverheadCalculated;
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static void
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CalculateSampleOverhead()
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{
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Instrumentation instr;
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int i;
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/* We want to determine the sampling overhead, to correct
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* calculations later. This only needs to be done once per backend.
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* Is this the place? A wrong value here (due to a mistimed
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* task-switch) will cause bad calculations later.
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*
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* To minimize the risk we do it a few times and take the lowest.
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*/
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SampleOverhead = 1.0e6;
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for( i = 0; i<5; i++ )
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{
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int j;
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double overhead;
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memset( &instr, 0, sizeof(instr) );
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/* Loop SAMPLE_THRESHOLD times or 100 microseconds, whichever is faster */
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for( j=0; j<SAMPLE_THRESHOLD && INSTR_TIME_GET_DOUBLE(instr.counter) < 100e-6; i++ )
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{
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InstrStartNode( &instr );
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InstrStopNode( &instr, 1 );
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}
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overhead = INSTR_TIME_GET_DOUBLE(instr.counter) / instr.samplecount;
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if( overhead < SampleOverhead )
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SampleOverhead = overhead;
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}
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SampleOverheadCalculated = true;
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}
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/* Allocate new instrumentation structure(s) */
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Instrumentation *
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@ -25,7 +87,10 @@ InstrAlloc(int n)
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Instrumentation *instr = palloc0(n * sizeof(Instrumentation));
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/* we don't need to do any initialization except zero 'em */
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/* Calculate overhead, if not done yet */
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if( !SampleOverheadCalculated )
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CalculateSampleOverhead();
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return instr;
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}
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@ -34,49 +99,69 @@ void
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InstrStartNode(Instrumentation *instr)
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{
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if (INSTR_TIME_IS_ZERO(instr->starttime))
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INSTR_TIME_SET_CURRENT(instr->starttime);
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{
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/* We always sample the first SAMPLE_THRESHOLD tuples, so small nodes are always accurate */
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if (instr->tuplecount < SAMPLE_THRESHOLD)
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instr->sampling = true;
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else
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{
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/* Otherwise we go to sampling, see the comments on SampleFunc at the top of the file */
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if( instr->tuplecount > instr->nextsample )
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{
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instr->sampling = true;
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/* The doubling is so the random will average 1 over time */
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instr->nextsample += 2.0 * SampleFunc(instr->tuplecount) * (double)rand() / (double)RAND_MAX;
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}
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}
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if (instr->sampling)
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INSTR_TIME_SET_CURRENT(instr->starttime);
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}
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else
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elog(DEBUG2, "InstrStartNode called twice in a row");
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}
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/* Exit from a plan node */
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void
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InstrStopNode(Instrumentation *instr, bool returnedTuple)
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InstrStopNode(Instrumentation *instr, double nTuples)
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{
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instr_time endtime;
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/* count the returned tuples */
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if (returnedTuple)
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instr->tuplecount += 1;
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instr->tuplecount += nTuples;
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if (INSTR_TIME_IS_ZERO(instr->starttime))
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if (instr->sampling)
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{
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elog(DEBUG2, "InstrStopNode called without start");
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return;
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}
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if (INSTR_TIME_IS_ZERO(instr->starttime))
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{
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elog(DEBUG2, "InstrStopNode called without start");
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return;
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}
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INSTR_TIME_SET_CURRENT(endtime);
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INSTR_TIME_SET_CURRENT(endtime);
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#ifndef WIN32
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instr->counter.tv_sec += endtime.tv_sec - instr->starttime.tv_sec;
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instr->counter.tv_usec += endtime.tv_usec - instr->starttime.tv_usec;
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instr->counter.tv_sec += endtime.tv_sec - instr->starttime.tv_sec;
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instr->counter.tv_usec += endtime.tv_usec - instr->starttime.tv_usec;
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/* Normalize after each add to avoid overflow/underflow of tv_usec */
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while (instr->counter.tv_usec < 0)
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{
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instr->counter.tv_usec += 1000000;
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instr->counter.tv_sec--;
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}
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while (instr->counter.tv_usec >= 1000000)
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{
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instr->counter.tv_usec -= 1000000;
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instr->counter.tv_sec++;
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}
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/* Normalize after each add to avoid overflow/underflow of tv_usec */
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while (instr->counter.tv_usec < 0)
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{
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instr->counter.tv_usec += 1000000;
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instr->counter.tv_sec--;
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}
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while (instr->counter.tv_usec >= 1000000)
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{
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instr->counter.tv_usec -= 1000000;
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instr->counter.tv_sec++;
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}
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#else /* WIN32 */
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instr->counter.QuadPart += (endtime.QuadPart - instr->starttime.QuadPart);
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instr->counter.QuadPart += (endtime.QuadPart - instr->starttime.QuadPart);
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#endif
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INSTR_TIME_SET_ZERO(instr->starttime);
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INSTR_TIME_SET_ZERO(instr->starttime);
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instr->samplecount += nTuples;
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instr->sampling = false;
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}
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/* Is this the first tuple of this cycle? */
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if (!instr->running)
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@ -86,17 +171,6 @@ InstrStopNode(Instrumentation *instr, bool returnedTuple)
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}
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}
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/* As above, but count multiple tuples returned at once */
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void
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InstrStopNodeMulti(Instrumentation *instr, double nTuples)
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{
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/* count the returned tuples */
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instr->tuplecount += nTuples;
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/* delegate the rest */
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InstrStopNode(instr, false);
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}
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/* Finish a run cycle for a plan node */
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void
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InstrEndLoop(Instrumentation *instr)
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@ -114,8 +188,24 @@ InstrEndLoop(Instrumentation *instr)
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totaltime = INSTR_TIME_GET_DOUBLE(instr->counter);
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instr->startup += instr->firsttuple;
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instr->total += totaltime;
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/* Here we take into account sampling effects. Doing it naively ends
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* up assuming the sampling overhead applies to all tuples, even the
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* ones we didn't measure. We've calculated an overhead, so we
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* subtract that for all samples we didn't measure. The first tuple
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* is also special cased, because it usually takes longer. */
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if( instr->samplecount < instr->tuplecount )
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{
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double pertuple = (totaltime - instr->firsttuple) / (instr->samplecount - 1);
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instr->total += instr->firsttuple + (pertuple * (instr->samplecount - 1))
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+ ((pertuple - SampleOverhead) * (instr->tuplecount - instr->samplecount));
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}
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else
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instr->total += totaltime;
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instr->ntuples += instr->tuplecount;
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instr->nsamples += instr->samplecount;
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instr->nloops += 1;
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/* Reset for next cycle (if any) */
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@ -123,5 +213,6 @@ InstrEndLoop(Instrumentation *instr)
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INSTR_TIME_SET_ZERO(instr->starttime);
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INSTR_TIME_SET_ZERO(instr->counter);
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instr->firsttuple = 0;
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instr->samplecount = 0;
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instr->tuplecount = 0;
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}
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapAnd.c,v 1.7 2006/05/23 15:21:52 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapAnd.c,v 1.8 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -161,7 +161,7 @@ MultiExecBitmapAnd(BitmapAndState *node)
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/* must provide our own instrumentation support */
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if (node->ps.instrument)
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InstrStopNodeMulti(node->ps.instrument, 0 /* XXX */ );
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InstrStopNode(node->ps.instrument, 0 /* XXX */ );
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return (Node *) result;
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}
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapIndexscan.c,v 1.18 2006/05/23 15:21:52 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapIndexscan.c,v 1.19 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -112,7 +112,7 @@ MultiExecBitmapIndexScan(BitmapIndexScanState *node)
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/* must provide our own instrumentation support */
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if (node->ss.ps.instrument)
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InstrStopNodeMulti(node->ss.ps.instrument, nTuples);
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InstrStopNode(node->ss.ps.instrument, nTuples);
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return (Node *) tbm;
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}
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapOr.c,v 1.6 2006/05/23 15:21:52 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/executor/nodeBitmapOr.c,v 1.7 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -177,7 +177,7 @@ MultiExecBitmapOr(BitmapOrState *node)
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/* must provide our own instrumentation support */
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if (node->ps.instrument)
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InstrStopNodeMulti(node->ps.instrument, 0 /* XXX */ );
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InstrStopNode(node->ps.instrument, 0 /* XXX */ );
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return (Node *) result;
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}
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@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.102 2006/05/23 15:21:52 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.103 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -97,7 +97,7 @@ MultiExecHash(HashState *node)
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/* must provide our own instrumentation support */
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if (node->ps.instrument)
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InstrStopNodeMulti(node->ps.instrument, hashtable->totalTuples);
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InstrStopNode(node->ps.instrument, hashtable->totalTuples);
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/*
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* We do not return the hash table directly because it's not a subtype of
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@ -6,7 +6,7 @@
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*
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* Copyright (c) 2001-2006, PostgreSQL Global Development Group
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*
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* $PostgreSQL: pgsql/src/include/executor/instrument.h,v 1.13 2006/03/05 15:58:56 momjian Exp $
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* $PostgreSQL: pgsql/src/include/executor/instrument.h,v 1.14 2006/05/30 14:01:58 momjian Exp $
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*
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*-------------------------------------------------------------------------
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*/
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@ -61,17 +61,21 @@ typedef struct Instrumentation
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instr_time counter; /* Accumulated runtime for this node */
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double firsttuple; /* Time for first tuple of this cycle */
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double tuplecount; /* Tuples emitted so far this cycle */
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double samplecount; /* Samples collected this cycle */
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/* Accumulated statistics across all completed cycles: */
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double startup; /* Total startup time (in seconds) */
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double total; /* Total total time (in seconds) */
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double ntuples; /* Total tuples produced */
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double nloops; /* # of run cycles for this node */
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double nsamples; /* # of samples taken */
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/* Tracking for sampling */
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bool sampling; /* Are we sampling this iteration */
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double nextsample; /* The next tuplecount we're going to sample */
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} Instrumentation;
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extern Instrumentation *InstrAlloc(int n);
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extern void InstrStartNode(Instrumentation *instr);
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extern void InstrStopNode(Instrumentation *instr, bool returnedTuple);
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extern void InstrStopNodeMulti(Instrumentation *instr, double nTuples);
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extern void InstrStopNode(Instrumentation *instr, double nTuples);
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extern void InstrEndLoop(Instrumentation *instr);
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#endif /* INSTRUMENT_H */
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