* Copyright (c) 2001-2006, PostgreSQL Global Development Group
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.17 2006/06/07 18:49:03 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/executor/instrument.c,v 1.18 2006/06/09 19:30:56 tgl Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
-#include <math.h>
#include <unistd.h>
#include "executor/instrument.h"
-/*
- * As of PostgreSQL 8.2, we try to reduce the overhead of EXPLAIN ANALYZE
- * by not calling INSTR_TIME_SET_CURRENT() for every single node execution.
- * (Because that requires a kernel call on most systems, it's expensive.)
- *
- * This macro determines the sampling interval: that is, after how many more
- * iterations will we take the next time sample, given that niters iterations
- * have occurred already. In general, if the function is f(x), then for N
- * iterations 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 iterations:
- *
- * 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.
- *
- * The actual sampling interval is randomized with the SampleFunc() value
- * as the mean; this hopefully will reduce any measurement bias due to
- * variation in the node execution time.
- */
-#ifdef HAVE_CBRT
-#define SampleFunc(niters) cbrt(niters)
-#else
-#define SampleFunc(niters) sqrt(niters)
-#endif
-
-#define SampleInterval(niters) \
- (SampleFunc(niters) * (double) random() / (double) (MAX_RANDOM_VALUE/2))
-
-/*
- * We sample at every node iteration until we've reached this threshold,
- * so that nodes not called a large number of times are completely accurate.
- * (Perhaps this should be a GUC variable?)
- */
-#define SAMPLE_THRESHOLD 50
-
-/*
- * Determine the sampling overhead. This only needs to be done once per
- * backend (actually, probably once per postmaster would do ...)
- */
-static double SampleOverhead;
-static bool SampleOverheadCalculated = false;
-
-static void
-CalculateSampleOverhead(void)
-{
- int i;
-
- /*
- * We could get a wrong result due to being interrupted by task switch.
- * To minimize the risk we do it a few times and take the lowest.
- */
- SampleOverhead = 1.0e6;
-
- for (i = 0; i < 5; i++)
- {
- Instrumentation timer;
- instr_time tmptime;
- int j;
- double overhead;
-
- memset(&timer, 0, sizeof(timer));
- InstrStartNode(&timer);
-#define TEST_COUNT 100
- for (j = 0; j < TEST_COUNT; j++)
- {
- INSTR_TIME_SET_CURRENT(tmptime);
- }
- InstrStopNode(&timer, 1);
- overhead = INSTR_TIME_GET_DOUBLE(timer.counter) / TEST_COUNT;
- if (overhead < SampleOverhead)
- SampleOverhead = overhead;
- }
-
- SampleOverheadCalculated = true;
-}
-
-
/* Allocate new instrumentation structure(s) */
Instrumentation *
InstrAlloc(int n)
{
- Instrumentation *instr;
-
- /* Calculate sampling overhead, if not done yet in this backend */
- if (!SampleOverheadCalculated)
- CalculateSampleOverhead();
-
- instr = palloc0(n * sizeof(Instrumentation));
+ Instrumentation *instr = palloc0(n * sizeof(Instrumentation));
/* we don't need to do any initialization except zero 'em */
InstrStartNode(Instrumentation *instr)
{
if (INSTR_TIME_IS_ZERO(instr->starttime))
- {
- /*
- * Always sample if not yet up to threshold, else check whether
- * next threshold has been reached
- */
- if (instr->itercount < SAMPLE_THRESHOLD)
- instr->sampling = true;
- else if (instr->itercount >= instr->nextsample)
- {
- instr->sampling = true;
- instr->nextsample =
- instr->itercount + SampleInterval(instr->itercount);
- }
- if (instr->sampling)
- INSTR_TIME_SET_CURRENT(instr->starttime);
- }
+ INSTR_TIME_SET_CURRENT(instr->starttime);
else
elog(DEBUG2, "InstrStartNode called twice in a row");
}
void
InstrStopNode(Instrumentation *instr, double nTuples)
{
- /* count the returned tuples and iterations */
+ instr_time endtime;
+
+ /* count the returned tuples */
instr->tuplecount += nTuples;
- instr->itercount += 1;
- /* measure runtime if appropriate */
- if (instr->sampling)
+ if (INSTR_TIME_IS_ZERO(instr->starttime))
{
- instr_time endtime;
-
- /*
- * To be sure that SampleOverhead accurately reflects the extra
- * overhead, we must do INSTR_TIME_SET_CURRENT() as the *first*
- * action that is different between the sampling and non-sampling
- * code paths.
- */
- INSTR_TIME_SET_CURRENT(endtime);
+ 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);
#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->samplecount += 1;
- instr->sampling = false;
- }
+ INSTR_TIME_SET_ZERO(instr->starttime);
/* Is this the first tuple of this cycle? */
if (!instr->running)
if (!INSTR_TIME_IS_ZERO(instr->starttime))
elog(DEBUG2, "InstrEndLoop called on running node");
- /* Compute time spent in node */
+ /* Accumulate per-cycle statistics into totals */
totaltime = INSTR_TIME_GET_DOUBLE(instr->counter);
- /*
- * If we didn't measure runtime on every iteration, then we must increase
- * the measured total to account for the other iterations. Naively
- * multiplying totaltime by itercount/samplecount would be wrong because
- * it effectively assumes the sampling overhead applies to all iterations,
- * even the ones we didn't measure. (Note that what we are trying to
- * estimate here is the actual time spent in the node, including the
- * actual measurement overhead; not the time exclusive of measurement
- * overhead.) We exclude the first iteration from the correction basis,
- * because it often takes longer than others.
- */
- if (instr->itercount > instr->samplecount)
- {
- double per_iter;
-
- per_iter = (totaltime - instr->firsttuple) / (instr->samplecount - 1)
- - SampleOverhead;
- if (per_iter > 0) /* sanity check */
- totaltime += per_iter * (instr->itercount - instr->samplecount);
- }
-
- /* Accumulate per-cycle statistics into totals */
instr->startup += instr->firsttuple;
instr->total += totaltime;
instr->ntuples += instr->tuplecount;
/* Reset for next cycle (if any) */
instr->running = false;
- instr->sampling = false;
INSTR_TIME_SET_ZERO(instr->starttime);
INSTR_TIME_SET_ZERO(instr->counter);
instr->firsttuple = 0;
instr->tuplecount = 0;
- instr->itercount = 0;
- instr->samplecount = 0;
- instr->nextsample = 0;
}
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