Re: EXPLAIN (plan off, rewrite off) for benchmarking

Hi,

For benchmarking the parser I added the above options (dim suggested this on
irc) which proved to be rather useful for me.
I added the additional rewrite option because the overhead of copying the tree
around makes the profile significantly less expressive.
I would also like an option which would only do the actual parsing instead of
parse + parse analyse but that seemed a tad more complicated...

Is anybody else interested in such EXPLAIN options?

Andres

Andres Freund <andres(at)anarazel(dot)de> writes:
> Hi,
> For benchmarking the parser I added the above options (dim suggested this on
> irc) which proved to be rather useful for me.

What exactly is EXPLAIN printing, if you've not done planning? Also, I
believe the planner depends on the assumption that the rewriter has done
its work, so these seem to amount to EXPLAIN (break_it).

If you just want to benchmark parsing, perhaps CREATE RULE would be a
useful environment.

regards, tom lane

On Saturday, November 19, 2011 12:16:18 AM Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > Hi,
> > For benchmarking the parser I added the above options (dim suggested this
> > on irc) which proved to be rather useful for me.
>
> What exactly is EXPLAIN printing, if you've not done planning?
Nothing very interesting:

postgres=# EXPLAIN (rewrite off) SELECT 1, 'test', "pg_class"."oid",relname,
relkind FROM pg_class WHERE oid = 1000;
QUERY PLAN
------------------------------------------
not rewriting query because auf !rewrite
(1 row)

Explain is just a vehicle here, I admit that. But on what else should I bolt
it?
The only thing I could think of otherwise would be to do the parsing via from
a C func. But to simmulate a real scenario there would require too much
bootstrapping for my taste.

> Also, I
> believe the planner depends on the assumption that the rewriter has done
> its work, so these seem to amount to EXPLAIN (break_it).
"rewrite off" currently simply aborts before doing the rewriting and
copyObject(). copyObject is the expensive part there for simple queries.

rewriting happened to be the functional part where I wanted to stop - because
of the overhead of copyObject - instead of a goal in itself.

Btw, optimizing copyObject() memory usage wise would be another big
performance gain. But even murkier than the stuff over in the lists thread...

> If you just want to benchmark parsing, perhaps CREATE RULE would be a
> useful environment.
I don't really see how one can use that to benchmark parsing. CREATE OR
REPLACE VIEW is - not unexpectedly - far slower than EXPLAIN (rewrite off) or
EXPLAIN (plan off).

for the statement:

SELECT * FROM pg_class WHERE oid = 1000;

rewrite off:
tps = 16086.694353
plan off, no copyObject()
tps = 15663.280093
plan off:
tps = 13471.272551
explain:
tps = 6162.161946
explain analyze:
tps = 5744.172839
normal execution:
tps = 6991.398740
CREATE OR REPLACE VIEW (after changing the log level):
tps = 2550.246625

Greetings,

Andres

Andres Freund <andres(at)anarazel(dot)de> writes:
> Explain is just a vehicle here, I admit that. But on what else should I bolt
> it?

If you don't like CREATE RULE, try having your test program send just
Parse messages, and not Bind/Execute. I still dislike the idea of
exposing a fundamentally-broken-and-useless variant of EXPLAIN in order
to have a test harness for a variant of performance testing that hardly
anyone cares about. (There is no real-world case where the performance
of the parser matters in isolation.) If we do that, it will be a
feature that we have to support forever, and possibly fix bugs in ---
what if the system crashes because the rewriter wasn't invoked, for
example?

regards, tom lane

On Saturday, November 19, 2011 04:52:10 PM Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > Explain is just a vehicle here, I admit that. But on what else should I
> > bolt it?
>
> If you don't like CREATE RULE, try having your test program send just
> Parse messages, and not Bind/Execute.
That sounds like a plan. Except that I would prefer to use pgbench. To avoid
the planning overhead...
I see it correctly that I would need to

I tpgbench is a more appropriate place to add such an option...

> If we do that, it will be a
> feature that we have to support forever, and possibly fix bugs in ---
> what if the system crashes because the rewriter wasn't invoked, for
> example?
rewrite=off aborts before planning, so that specific problem I don't see. The
name is rather bad I admit. Its mostly there to avoid the copyObject() which
skews results considerably:

* Because the rewriter and planner tend to scribble on the input, we
make
* a preliminary copy of the source querytree. This prevents problems in
* the case that the EXPLAIN is in a portal or plpgsql function and is
* executed repeatedly. (See also the same hack in DECLARE CURSOR and
* PREPARE.) XXX FIXME someday.
*/
rewritten = QueryRewrite((Query *) copyObject(stmt->query));

> I still dislike the idea of
> exposing a fundamentally-broken-and-useless variant of EXPLAIN in order
> to have a test harness for a variant of performance testing that hardly
> anyone cares about. (There is no real-world case where the performance
> of the parser matters in isolation.)
I absolutely cannot agree on the fact that the speed parse-analyze is
irrelevant though. In several scenarios using prepared statements is not a
viable/simple option. Think transaction-level pooling for example. Or the
queries generated by all those ORMs out there.

When executing many small statments without prepared statments a rather large
portion of time is spent parsing.

Consider:
statement: EXPLAIN SELECT * FROM pg_class WHERE oid = 1000;

pgbench -M simple -f ~/.tmp/simple1.sql -T 3
tps = 16067.780407

pgbench -M prepared -f ~/.tmp/simple1.sql -T 3
tps = 24348.244630

In both variants the queries are fully planned as far as I can see. The only
difference there is parsing. I do find the difference in speed rather notable.

That does represent measurements from realworld profiles I gathered were
functions related to parsing + parse analysis contribute up to 1/3 of the
runtime.

Obviously nobody needs to benchmark the parser alone in a production scenario.
But if you want to improve the overall performance of some workload analysing
bits and pieces alone to get a useful, more detailed profile is a pretty sane
approach.

Why should that be a variant of performance testing that nobody cares about?

Andres

Andres Freund <andres(at)anarazel(dot)de> writes:
> On Saturday, November 19, 2011 04:52:10 PM Tom Lane wrote:
>> If you don't like CREATE RULE, try having your test program send just
>> Parse messages, and not Bind/Execute.

> That sounds like a plan. Except that I would prefer to use pgbench.

Well, how about plan C: write a small C function that consists of a loop
around calling just the part of the parser you want to test? I've done
that in the past when I wanted fine-grained profiles, and it works a
whole lot better than anything involving per-query messages from the
client side.

regards, tom lane

On Sat, Nov 19, 2011 at 11:47 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> I absolutely cannot agree on the fact that the speed parse-analyze is
> irrelevant though.

Tom may be right that the speed of the parser *in isolation* is
irrelevant, in the narrow sense that if we made the parser twice as
slow but somehow by that change made up the time in the executor,
nobody would care; in fact, it would be a net win for people using
prepared statements. But I completely agree that parsing speed is
something we need to worry about. Unfortunately, I don't have a lot
of good ideas for improving it. A while back I tried ripping out most
of the parser to see whether that would speed up performance parsing
very simple statements, but the improvement was pretty small. Maybe a
more thorough job than what I did is possible, but it didn't seem
promising. Maybe we could find a way to reduce the size of the parse
tree (i.e. fewer nodes), or the number of times that it has to be
walked/copied.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> ... Maybe we could find a way to reduce the size of the parse
> tree (i.e. fewer nodes), or the number of times that it has to be
> walked/copied.

We could eliminate some annoying tree-copy steps if we could institute
the policy that parse analysis doesn't scribble on the raw parse tree,
rewriter doesn't modify parse analysis output, and planner doesn't
modify rewriter output. However, it would be a lot of work, and I'm not
entirely sure that we'd end up with a significant speed benefit. In a
lot of places, the only way to not scribble on the input is to copy it
anyway ...

regards, tom lane

On Mon, Nov 21, 2011 at 8:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> ... Maybe we could find a way to reduce the size of the parse
>> tree (i.e. fewer nodes), or the number of times that it has to be
>> walked/copied.
>
> We could eliminate some annoying tree-copy steps if we could institute
> the policy that parse analysis doesn't scribble on the raw parse tree,
> rewriter doesn't modify parse analysis output, and planner doesn't
> modify rewriter output.  However, it would be a lot of work, and I'm not
> entirely sure that we'd end up with a significant speed benefit.  In a
> lot of places, the only way to not scribble on the input is to copy it
> anyway ...

This is probably a stupid question, but why does it matter if parse
analysis scribbles on the raw parse tree, or the rewriter on the parse
analysis output? I understand that we may sometimes need to replan
the output of the rewriter, so we'd better not modify it
destructively, but I would have thought that parse and parse analysis
would always be done together, in which case it doesn't obviously
matter. I'm probably missing something here...

Another thing we might want to consider doing is introducing some kind
of container data structure other than List. I think that trying to
change the semantics of the existing List datatype in any meaningful
way is probably doomed, but if we introduce a new abstraction and
gradually convert things over, we have a lot more flexibility. What
I'm imagining is something that is optimized for holding a small
number of pointers (say, 4) that are normally added in order, but
which can grow if needed, at some cost in performance. For example:

struct Thingy
{
unsigned short nused;
unsigned short nalloc;
struct Thingy *next; /* if we run out of elements in this
Thingy, we can link to another Thingy with more space */
void *item[FLEXIBLE_ARRAY_MEMBER]; /* really nalloc */
};

This would mean fewer palloc() calls and less overall memory usage
than a List, and might also improve memory access locality. Right
now, walking a three element list could involve pulling in one cache
line for the list, three more for the list cells, and then
(presumably) three more for the elements themselves. There's not much
help for the fact that the elements might end up in different cache
lines, but at least if we did something like this you wouldn't need to
bounce around to find the list cells. This is particularly obnoxious
for things like RangeVars, where we build up the list representation
mostly as an intermediate step and then flatten it again.

Now, one obvious problem with this line of attack is that each
individual patch in this area isn't likely to save anything noticeable
on real workloads. But I don't think that should stop us from trying.
With every performance patch that goes in, the number of workloads
that are dominated by the cost of backend-local memory allocation is
growing. The cost is extremely distributed and it's very hard to pin
down where it's all coming from, but we've gotta start somewhere.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Nov 21, 2011 at 8:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> We could eliminate some annoying tree-copy steps if we could institute
>> the policy that parse analysis doesn't scribble on the raw parse tree,
>> rewriter doesn't modify parse analysis output, and planner doesn't
>> modify rewriter output.

> This is probably a stupid question, but why does it matter if parse
> analysis scribbles on the raw parse tree, or the rewriter on the parse
> analysis output?

Because we frequently need to save the original tree for possible
re-analysis later. This doesn't matter in the simple-query protocol,
but it does matter in any code path that involves plancache.

> I understand that we may sometimes need to replan
> the output of the rewriter, so we'd better not modify it
> destructively, but I would have thought that parse and parse analysis
> would always be done together, in which case it doesn't obviously
> matter.

No, actually it's the raw grammar output tree that gets saved for
re-analysis in case we are told of a DDL change. (I've considered
having the code save only the original query string, but then you'd
have to repeat the flex/bison work, and those things show up high
enough on any profile to make it seem unlikely that this is cheaper
than copying the parse tree.)

It's possible that we don't need a read-only guarantee for the rewriter
versus parse analysis output, but I doubt that helps much.

regards, tom lane