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Make EXPLAIN sampling smarter, to avoid excessive sampling delay.

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Martijn van Oosterhout
This commit is contained in:
Bruce Momjian
2006-05-30 14:01:58 +00:00
parent 53d669e5c6
commit 87bd07d979
8 changed files with 149 additions and 54 deletions
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167
src/backend/executor/instrument.c
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@ -7,16 +7,78 @@
* Copyright (c) 2001-2006, PostgreSQL Global Development Group
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.14 2006/03/05 15:58:26 momjian Exp $
* $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.15 2006/05/30 14:01:58 momjian Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <unistd.h>
#include <math.h>
#include "executor/instrument.h"
/* This is the function that is used to determine the sampling intervals. In
* general, if the function is f(x), then for N tuples we will take on the
* order of integral(1/f(x), x=0..N) samples. Some examples follow, with the
* number of samples that would be collected over 1,000,000 tuples.
f(x) = x => log2(N) 20
f(x) = x^(1/2) => 2 * N^(1/2) 2000
f(x) = x^(1/3) => 1.5 * N^(2/3) 15000
* I've chosen the last one as it seems to provide a good compromise between
* low overhead but still getting a meaningful number of samples. However,
* not all machines have the cbrt() function so on those we substitute
* sqrt(). The difference is not very significant in the tests I made.
*/
#ifdef HAVE_CBRT
#define SampleFunc cbrt
#else
#define SampleFunc sqrt
#endif
#define SAMPLE_THRESHOLD 50
static double SampleOverhead;
static bool SampleOverheadCalculated;
static void
CalculateSampleOverhead()
{
Instrumentation instr;
int i;
/* We want to determine the sampling overhead, to correct
* calculations later. This only needs to be done once per backend.
* Is this the place? A wrong value here (due to a mistimed
* task-switch) will cause bad calculations later.
*
* To minimize the risk we do it a few times and take the lowest.
*/
SampleOverhead = 1.0e6;
for( i = 0; i<5; i++ )
{
int j;
double overhead;
memset( &instr, 0, sizeof(instr) );
/* Loop SAMPLE_THRESHOLD times or 100 microseconds, whichever is faster */
for( j=0; j<SAMPLE_THRESHOLD && INSTR_TIME_GET_DOUBLE(instr.counter) < 100e-6; i++ )
{
InstrStartNode( &instr );
InstrStopNode( &instr, 1 );
}
overhead = INSTR_TIME_GET_DOUBLE(instr.counter) / instr.samplecount;
if( overhead < SampleOverhead )
SampleOverhead = overhead;
}
SampleOverheadCalculated = true;
}
/* Allocate new instrumentation structure(s) */
Instrumentation *
@ -25,7 +87,10 @@ InstrAlloc(int n)
Instrumentation *instr = palloc0(n * sizeof(Instrumentation));
/* we don't need to do any initialization except zero 'em */
/* Calculate overhead, if not done yet */
if( !SampleOverheadCalculated )
CalculateSampleOverhead();
return instr;
}
@ -34,49 +99,69 @@ void
InstrStartNode(Instrumentation *instr)
{
if (INSTR_TIME_IS_ZERO(instr->starttime))
INSTR_TIME_SET_CURRENT(instr->starttime);
{
/* We always sample the first SAMPLE_THRESHOLD tuples, so small nodes are always accurate */
if (instr->tuplecount < SAMPLE_THRESHOLD)
instr->sampling = true;
else
{
/* Otherwise we go to sampling, see the comments on SampleFunc at the top of the file */
if( instr->tuplecount > instr->nextsample )
{
instr->sampling = true;
/* The doubling is so the random will average 1 over time */
instr->nextsample += 2.0 * SampleFunc(instr->tuplecount) * (double)rand() / (double)RAND_MAX;
}
}
if (instr->sampling)
INSTR_TIME_SET_CURRENT(instr->starttime);
}
else
elog(DEBUG2, "InstrStartNode called twice in a row");
}
/* Exit from a plan node */
void
InstrStopNode(Instrumentation *instr, bool returnedTuple)
InstrStopNode(Instrumentation *instr, double nTuples)
{
instr_time endtime;
/* count the returned tuples */
if (returnedTuple)
instr->tuplecount += 1;
instr->tuplecount += nTuples;
if (INSTR_TIME_IS_ZERO(instr->starttime))
if (instr->sampling)
{
elog(DEBUG2, "InstrStopNode called without start");
return;
}
if (INSTR_TIME_IS_ZERO(instr->starttime))
{
elog(DEBUG2, "InstrStopNode called without start");
return;
}
INSTR_TIME_SET_CURRENT(endtime);
INSTR_TIME_SET_CURRENT(endtime);
#ifndef WIN32
instr->counter.tv_sec += endtime.tv_sec - instr->starttime.tv_sec;
instr->counter.tv_usec += endtime.tv_usec - instr->starttime.tv_usec;
instr->counter.tv_sec += endtime.tv_sec - instr->starttime.tv_sec;
instr->counter.tv_usec += endtime.tv_usec - instr->starttime.tv_usec;
/* Normalize after each add to avoid overflow/underflow of tv_usec */
while (instr->counter.tv_usec < 0)
{
instr->counter.tv_usec += 1000000;
instr->counter.tv_sec--;
}
while (instr->counter.tv_usec >= 1000000)
{
instr->counter.tv_usec -= 1000000;
instr->counter.tv_sec++;
}
/* Normalize after each add to avoid overflow/underflow of tv_usec */
while (instr->counter.tv_usec < 0)
{
instr->counter.tv_usec += 1000000;
instr->counter.tv_sec--;
}
while (instr->counter.tv_usec >= 1000000)
{
instr->counter.tv_usec -= 1000000;
instr->counter.tv_sec++;
}
#else /* WIN32 */
instr->counter.QuadPart += (endtime.QuadPart - instr->starttime.QuadPart);
instr->counter.QuadPart += (endtime.QuadPart - instr->starttime.QuadPart);
#endif
INSTR_TIME_SET_ZERO(instr->starttime);
INSTR_TIME_SET_ZERO(instr->starttime);
instr->samplecount += nTuples;
instr->sampling = false;
}
/* Is this the first tuple of this cycle? */
if (!instr->running)
@ -86,17 +171,6 @@ InstrStopNode(Instrumentation *instr, bool returnedTuple)
}
}
/* As above, but count multiple tuples returned at once */
void
InstrStopNodeMulti(Instrumentation *instr, double nTuples)
{
/* count the returned tuples */
instr->tuplecount += nTuples;
/* delegate the rest */
InstrStopNode(instr, false);
}
/* Finish a run cycle for a plan node */
void
InstrEndLoop(Instrumentation *instr)
@ -114,8 +188,24 @@ InstrEndLoop(Instrumentation *instr)
totaltime = INSTR_TIME_GET_DOUBLE(instr->counter);
instr->startup += instr->firsttuple;
instr->total += totaltime;
/* Here we take into account sampling effects. Doing it naively ends
* up assuming the sampling overhead applies to all tuples, even the
* ones we didn't measure. We've calculated an overhead, so we
* subtract that for all samples we didn't measure. The first tuple
* is also special cased, because it usually takes longer. */
if( instr->samplecount < instr->tuplecount )
{
double pertuple = (totaltime - instr->firsttuple) / (instr->samplecount - 1);
instr->total += instr->firsttuple + (pertuple * (instr->samplecount - 1))
+ ((pertuple - SampleOverhead) * (instr->tuplecount - instr->samplecount));
}
else
instr->total += totaltime;
instr->ntuples += instr->tuplecount;
instr->nsamples += instr->samplecount;
instr->nloops += 1;
/* Reset for next cycle (if any) */
@ -123,5 +213,6 @@ InstrEndLoop(Instrumentation *instr)
INSTR_TIME_SET_ZERO(instr->starttime);
INSTR_TIME_SET_ZERO(instr->counter);
instr->firsttuple = 0;
instr->samplecount = 0;
instr->tuplecount = 0;
}