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Re: sudden drop in statement turnaround latency -- yay!.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com>
Cc: "pgsqlperform" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: sudden drop in statement turnaround latency -- yay!.
Date: 2004-12-31 05:30:56
Message-ID: 7017.1104471056@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
"Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com> writes:
> I took advantage of the holidays to update a production server (dual
> Opteron on win2k) from an 11/16 build (about beta5 or so) to the latest
> release candidate.  No configuration changes were made, just a binary
> swap and a server stop/start.  

> I was shocked to see that statement latency dropped by a fairly large
> margin.

Hmm ... I trawled through the CVS logs since 11/16, and did not see very
many changes that looked like they might improve performance (list
attached) --- and even of those, hardly any looked like the change would
be significant.  Do you know whether the query plans changed?  Are you
running few enough queries per connection that backend startup overhead
might be an issue?

			regards, tom lane


2004-12-15 14:16  tgl

	* src/backend/access/nbtree/nbtutils.c: Calculation of
	keys_are_unique flag was wrong for cases involving redundant
	cross-datatype comparisons.  Per example from Merlin Moncure.

2004-12-02 10:32  momjian

	* configure, configure.in, doc/src/sgml/libpq.sgml,
	doc/src/sgml/ref/copy.sgml, src/interfaces/libpq/fe-connect.c,
	src/interfaces/libpq/fe-print.c, src/interfaces/libpq/fe-secure.c,
	src/interfaces/libpq/libpq-fe.h, src/interfaces/libpq/libpq-int.h:
	Rework libpq threaded SIGPIPE handling to avoid interference with
	calling applications.  This is done by blocking sigpipe in the
	libpq thread and using sigpending/sigwait to possibily discard any
	sigpipe we generated.

2004-12-01 20:34  tgl

	* src/: backend/optimizer/path/costsize.c,
	backend/optimizer/util/plancat.c,
	test/regress/expected/geometry.out,
	test/regress/expected/geometry_1.out,
	test/regress/expected/geometry_2.out,
	test/regress/expected/inherit.out, test/regress/expected/join.out,
	test/regress/sql/inherit.sql, test/regress/sql/join.sql: Make some
	adjustments to reduce platform dependencies in plan selection.	In
	particular, there was a mathematical tie between the two possible
	nestloop-with-materialized-inner-scan plans for a join (ie, we
	computed the same cost with either input on the inside), resulting
	in a roundoff error driven choice, if the relations were both small
	enough to fit in sort_mem.  Add a small cost factor to ensure we
	prefer materializing the smaller input.  This changes several
	regression test plans, but with any luck we will now have more
	stability across platforms.

2004-12-01 14:00  tgl

	* doc/src/sgml/catalogs.sgml, doc/src/sgml/diskusage.sgml,
	doc/src/sgml/perform.sgml, doc/src/sgml/release.sgml,
	src/backend/access/nbtree/nbtree.c, src/backend/catalog/heap.c,
	src/backend/catalog/index.c, src/backend/commands/vacuum.c,
	src/backend/commands/vacuumlazy.c,
	src/backend/optimizer/util/plancat.c,
	src/backend/optimizer/util/relnode.c, src/include/access/genam.h,
	src/include/nodes/relation.h, src/test/regress/expected/case.out,
	src/test/regress/expected/inherit.out,
	src/test/regress/expected/join.out,
	src/test/regress/expected/join_1.out,
	src/test/regress/expected/polymorphism.out: Change planner to use
	the current true disk file size as its estimate of a relation's
	number of blocks, rather than the possibly-obsolete value in
	pg_class.relpages.  Scale the value in pg_class.reltuples
	correspondingly to arrive at a hopefully more accurate number of
	rows.  When pg_class contains 0/0, estimate a tuple width from the
	column datatypes and divide that into current file size to estimate
	number of rows.  This improved methodology allows us to jettison
	the ancient hacks that put bogus default values into pg_class when
	a table is first created.  Also, per a suggestion from Simon, make
	VACUUM (but not VACUUM FULL or ANALYZE) adjust the value it puts
	into pg_class.reltuples to try to represent the mean tuple density
	instead of the minimal density that actually prevails just after
	VACUUM.  These changes alter the plans selected for certain
	regression tests, so update the expected files accordingly.  (I
	removed join_1.out because it's not clear if it still applies; we
	can add back any variant versions as they are shown to be needed.)

2004-11-21 17:57  tgl

	* src/backend/utils/hash/dynahash.c: Fix rounding problem in
	dynahash.c's decision about when the target fill factor has been
	exceeded.  We usually run with ffactor == 1, but the way the test
	was coded, it wouldn't split a bucket until the actual fill factor
	reached 2.0, because of use of integer division.  Change from > to
	>= so that it will split more aggressively when the table starts to
	get full.

2004-11-21 17:48  tgl

	* src/backend/utils/mmgr/portalmem.c: Reduce the default size of
	the PortalHashTable in order to save a few cycles during
	transaction exit.  A typical session probably wouldn't have as many
	as half a dozen portals open at once, so the original value of 64
	seems far larger than needed.

2004-11-20 15:19  tgl

	* src/backend/utils/cache/relcache.c: Avoid scanning the relcache
	during AtEOSubXact_RelationCache when there is nothing to do, which
	is most of the time.  This is another simple improvement to cut
	subtransaction entry/exit overhead.

2004-11-20 15:16  tgl

	* src/backend/storage/lmgr/lock.c: Reduce the default size of the
	local lock hash table.	There's usually no need for it to be nearly
	as big as the global hash table, and since it's not in shared
	memory it can grow if it does need to be bigger.  By reducing the
	size, we speed up hash_seq_search(), which saves a significant
	fraction of subtransaction entry/exit overhead.

2004-11-19 19:48  tgl

	* src/backend/tcop/postgres.c: Move pgstat_report_tabstat() call so
	that stats are not reported to the collector until the transaction
	commits.  Per recent discussion, this should avoid confusing
	autovacuum when an updating transaction runs for a long time.

2004-11-16 22:13  neilc

	* src/backend/access/: hash/hash.c, nbtree/nbtree.c:
	Micro-optimization of markpos() and restrpos() in btree and hash
	indexes.  Rather than using ReadBuffer() to increment the reference
	count on an already-pinned buffer, we should use
	IncrBufferRefCount() as it is faster and does not require acquiring
	the BufMgrLock.

2004-11-16 19:14  tgl

	* src/: backend/main/main.c, backend/port/win32/signal.c,
	backend/postmaster/pgstat.c, backend/postmaster/postmaster.c,
	include/port/win32.h: Fix Win32 problems with signals and sockets,
	by making the forkexec code even uglier than it was already :-(. 
	Also, on Windows only, use temporary shared memory segments instead
	of ordinary files to pass over critical variable values from
	postmaster to child processes.	Magnus Hagander

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Subject: Optimization while compiling
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Subject: sudden drop in statement turnaround latency -- yay!.

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