Re: [HACKERS] Polyphase merge is obsolete

[Tomas Vondra]

Hi,

I planned to do some benchmarking on this patch, but apparently the
patch no longer applies. Rebase please?

regards

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Re: [HACKERS] Polyphase merge is obsolete

[Peter Geoghegan]

On Wed, Aug 30, 2017 at 12:48 PM, Robert Haas  wrote:
> These are separate topics.  They should each be discussed on their own
> thread.  Please don't hijack this thread to talk about something else.

I don't think that that is a fair summary.

Heikki has done a number of things in this area, of which this is only
the latest. I'm saying "hey, have you thought about RS too?". Whether
or not I'm "hijacking" this thread is, at best, ambiguous.

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Re: [HACKERS] Polyphase merge is obsolete

[Robert Haas]

On Wed, Aug 30, 2017 at 2:54 PM, Peter Geoghegan  wrote:
> I noticed that this is in the upcoming CF 1 for v11. I'm signed up to review.
>
> I'd like to point out that replacement selection is also obsolete,
> which is something I brought up recently [1]. I don't actually have
> any feature-driven reason to want to kill replacement selection - it's
> just an annoyance at this point. I do think that RS is more deserving
> of being killed than Polyphase merge, because it actually costs users
> something to continue to support it. The replacement_sort_tuples GUC
> particularly deserves to be removed.
>
> It would be nice if killing RS was put in scope here. I'd appreciate
> it, at least, since it would simplify the heap routines noticeably.
> The original analysis that led to adding replacement_sort_tuples was
> based on certain performance characteristics of merging that have
> since changed by quite a bit, due to our work for v10.

These are separate topics.  They should each be discussed on their own
thread.  Please don't hijack this thread to talk about something else.

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Robert Haas
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Re: [HACKERS] Polyphase merge is obsolete

[Peter Geoghegan]

On Mon, Feb 27, 2017 at 2:45 PM, Peter Geoghegan  wrote:
> Since we have an awful lot of stuff in the last CF, and this patch
> doesn't seem particularly strategic, I've marked it "Returned with
> Feedback".

I noticed that this is in the upcoming CF 1 for v11. I'm signed up to review.

I'd like to point out that replacement selection is also obsolete,
which is something I brought up recently [1]. I don't actually have
any feature-driven reason to want to kill replacement selection - it's
just an annoyance at this point. I do think that RS is more deserving
of being killed than Polyphase merge, because it actually costs users
something to continue to support it. The replacement_sort_tuples GUC
particularly deserves to be removed.

It would be nice if killing RS was put in scope here. I'd appreciate
it, at least, since it would simplify the heap routines noticeably.
The original analysis that led to adding replacement_sort_tuples was
based on certain performance characteristics of merging that have
since changed by quite a bit, due to our work for v10.

[1] 
postgr.es/m/cah2-wzmmnjg_k0r9nqywmq3zjyjjk+hcbizynghay-zyjs6...@mail.gmail.com
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Re: [HACKERS] Polyphase merge is obsolete

[Peter Geoghegan]

On Mon, Jan 16, 2017 at 5:56 PM, Peter Geoghegan  wrote:
> On Wed, Oct 12, 2016 at 10:16 AM, Heikki Linnakangas  wrote:
>> The number of *input* tapes we can use in each merge pass is still limited,
>> by the memory needed for the tape buffers and the merge heap, but only one
>> output tape is active at any time. The inactive output tapes consume very
>> few resources. So the whole idea of trying to efficiently reuse input tapes
>> as output tapes is pointless
>
> I picked this up again. The patch won't apply cleanly. Can you rebase?

Since we have an awful lot of stuff in the last CF, and this patch
doesn't seem particularly strategic, I've marked it "Returned with
Feedback".

Thanks
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Re: [HACKERS] Polyphase merge is obsolete

[Peter Geoghegan]

On Wed, Oct 12, 2016 at 10:16 AM, Heikki Linnakangas  wrote:
> The number of *input* tapes we can use in each merge pass is still limited,
> by the memory needed for the tape buffers and the merge heap, but only one
> output tape is active at any time. The inactive output tapes consume very
> few resources. So the whole idea of trying to efficiently reuse input tapes
> as output tapes is pointless

I picked this up again. The patch won't apply cleanly. Can you rebase?
Also, please look at my bugfix for logtape.c free block management [1]
before doing so, as that might be prerequisite. Finally, I don't think
that the Logical tape pause/resume idea is compelling. Is it hard to
not do that, but still do everything else that you propose here?
That's what I lean towards doing right now.

Anyway, efficient use of tapes certainly mattered a lot more when
rewinding meant sitting around for a magnetic tape deck to physically
rewind. There is another algorithm in AoCP Vol III that lets us write
to tapes backwards, actually, which is motivated by similar obsolete
considerations about hardware. Why not write while we rewind, to avoid
doing nothing else while rewinding?!

Perhaps this patch should make a clean break from the "rewinding"
terminology. Perhaps you should rename LogicalTapeRewindForRead() to
LogicalTapePrepareForRead(), and so on. It's already a bit awkward
that that routine is called LogicalTapeRewindForRead(), because it
behaves significantly differently when a tape is frozen, and because
the whole point of logtape.c is space reuse that is completely
dissimilar to rewinding. (Space reuse is thus totally unlike how
polyphase merge is supposed to reuse space, which is all about
rewinding, and isn't nearly as eager. Same applies to K-way balanced
merge, of course.)

I think that the "rewinding" terminology does more harm than good, now
that it doesn't even help the Knuth reader to match Knuth's remarks to
what's going on in tuplesort.c. Just a thought.

> Let's switch over to a simple k-way balanced merge. Because it's simpler. If
> you're severely limited on memory, like when sorting 1GB of data with
> work_mem='1MB' or less, it's also slightly faster. I'm not too excited about
> the performance aspect, because in the typical case of a single-pass merge,
> there's no difference. But it would be worth changing on simplicity grounds,
> since we're mucking around in tuplesort.c anyway.

I actually think that the discontinuities in the merge scheduling are
worse than you suggest here. There doesn't have to be as extreme a
difference between work_mem and the size of input as you describe
here. As an example:

create table seq_tab(t int);
insert into seq_tab select generate_series(1, 1000);
set work_mem = '4MB';
select count(distinct t) from seq_tab;

The trace_sort output ends like this:

30119/2017-01-16 17:17:05 PST LOG:  begin datum sort: workMem = 4096,
randomAccess = f
30119/2017-01-16 17:17:05 PST LOG:  switching to external sort with 16
tapes: CPU: user: 0.07 s, system: 0.00 s, elapsed: 0.06 s
30119/2017-01-16 17:17:05 PST LOG:  starting quicksort of run 1: CPU:
user: 0.07 s, system: 0.00 s, elapsed: 0.06 s
30119/2017-01-16 17:17:05 PST LOG:  finished quicksort of run 1: CPU:
user: 0.07 s, system: 0.00 s, elapsed: 0.07 s
*** SNIP ***
30119/2017-01-16 17:17:08 PST LOG:  finished writing run 58 to tape 0:
CPU: user: 2.50 s, system: 0.27 s, elapsed: 2.78 s
30119/2017-01-16 17:17:08 PST LOG:  using 4095 KB of memory for read
buffers among 15 input tapes
30119/2017-01-16 17:17:08 PST LOG:  finished 1-way merge step: CPU:
user: 2.52 s, system: 0.28 s, elapsed: 2.80 s
30119/2017-01-16 17:17:08 PST LOG:  finished 4-way merge step: CPU:
user: 2.58 s, system: 0.30 s, elapsed: 2.89 s
30119/2017-01-16 17:17:08 PST LOG:  finished 14-way merge step: CPU:
user: 2.86 s, system: 0.34 s, elapsed: 3.20 s
30119/2017-01-16 17:17:08 PST LOG:  finished 14-way merge step: CPU:
user: 3.09 s, system: 0.41 s, elapsed: 3.51 s
30119/2017-01-16 17:17:09 PST LOG:  finished 15-way merge step: CPU:
user: 3.61 s, system: 0.52 s, elapsed: 4.14 s
30119/2017-01-16 17:17:09 PST LOG:  performsort done (except 15-way
final merge): CPU: user: 3.61 s, system: 0.52 s, elapsed: 4.14 s
30119/2017-01-16 17:17:10 PST LOG:  external sort ended, 14678 disk
blocks used: CPU: user: 4.93 s, system: 0.57 s, elapsed: 5.51 s

(This is the test case that Cy posted earlier today, for the bug that
was just fixed in master.)

The first 1-way merge step is clearly kind of a waste of time. We
incur no actual comparisons during this "merge", since there is only
one real run from each input tape (all other active tapes contain only
dummy runs). We are, in effect, just shoveling the tuples from that
single run from one tape to another (from one range in the underlying
logtape.c BufFile space to another). I've seen this quite a lot
before, over the years, while working on sorting. It's not that big of
a deal, but it's a degenerate case that 

Re: [HACKERS] Polyphase merge is obsolete

[Peter Geoghegan]

On Wed, Oct 12, 2016 at 10:16 AM, Heikki Linnakangas  wrote:
> Let's switch over to a simple k-way balanced merge. Because it's simpler. If
> you're severely limited on memory, like when sorting 1GB of data with
> work_mem='1MB' or less, it's also slightly faster. I'm not too excited about
> the performance aspect, because in the typical case of a single-pass merge,
> there's no difference. But it would be worth changing on simplicity grounds,
> since we're mucking around in tuplesort.c anyway.

This analysis seems sound. I suppose we might as well simplify things
while we're at it.


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Re: [HACKERS] Polyphase merge is obsolete

[Heikki Linnakangas]

On 10/12/2016 08:27 PM, Tom Lane wrote:

Heikki Linnakangas  writes:

The beauty of the polyphase merge algorithm is that it allows reusing
input tapes as output tapes efficiently ... So the whole idea of trying to
efficiently reuse input tapes as output tapes is pointless.


It's been awhile since I looked at that code, but I'm quite certain that
it *never* thought it was dealing with actual tapes.  Rather, the point of
sticking with polyphase merge was that it allowed efficient incremental
re-use of temporary disk files, so that the maximum on-disk footprint was
only about equal to the volume of data to be sorted, rather than being a
multiple of that.  Have we thrown that property away?


No, there's no difference to that behavior. logtape.c takes care of 
incremental re-use of disk space, regardless of the merging pattern.


- Heikki



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Re: [HACKERS] Polyphase merge is obsolete

[Tom Lane]

Heikki Linnakangas  writes:
> The beauty of the polyphase merge algorithm is that it allows reusing 
> input tapes as output tapes efficiently ... So the whole idea of trying to 
> efficiently reuse input tapes as output tapes is pointless.

It's been awhile since I looked at that code, but I'm quite certain that
it *never* thought it was dealing with actual tapes.  Rather, the point of
sticking with polyphase merge was that it allowed efficient incremental
re-use of temporary disk files, so that the maximum on-disk footprint was
only about equal to the volume of data to be sorted, rather than being a
multiple of that.  Have we thrown that property away?

regards, tom lane


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[HACKERS] Polyphase merge is obsolete

[Heikki Linnakangas]

The beauty of the polyphase merge algorithm is that it allows reusing 
input tapes as output tapes efficiently. So if you have N tape drives, 
you can keep them all busy throughout the merge.


That doesn't matter, when we can easily have as many "tape drives" as we 
want. In PostgreSQL, a tape drive consumes just a few kB of memory, for 
the buffers. With the patch being discussed to allow a tape to be 
"paused" between write passes [1], we don't even keep the tape buffers 
around, when a tape is not actively read written to, so all it consumes 
is the memory needed for the LogicalTape struct.


The number of *input* tapes we can use in each merge pass is still 
limited, by the memory needed for the tape buffers and the merge heap, 
but only one output tape is active at any time. The inactive output 
tapes consume very few resources. So the whole idea of trying to 
efficiently reuse input tapes as output tapes is pointless.


Let's switch over to a simple k-way balanced merge. Because it's 
simpler. If you're severely limited on memory, like when sorting 1GB of 
data with work_mem='1MB' or less, it's also slightly faster. I'm not too 
excited about the performance aspect, because in the typical case of a 
single-pass merge, there's no difference. But it would be worth changing 
on simplicity grounds, since we're mucking around in tuplesort.c anyway.


I came up with the attached patch to do that. This patch applies on top 
of my latest "Logical tape pause/resume" patches [1]. It includes 
changes to the logtape.c interface, that make it possible to create and 
destroy LogicalTapes in a tapeset on the fly. I believe this will also 
come handy for Peter's parallel tuplesort patch set.


[1] Logical tape pause/resume, 
https://www.postgresql.org/message-id/55b3b7ae-8dec-b188-b8eb-e07604052351%40iki.fi


PS. I finally bit the bullet and got self a copy of The Art of Computer 
Programming, Vol 3, 2nd edition. In section 5.4 on External Sorting, 
Knuth says:


"
When this book was first written, magnetic tapes were abundant and disk 
drives were expensive. But disks became enormously better during the 
1980s, and by the late 1990s they had almost completely replaced 
magnetic tape units on most of the world's computer systems. Therefore 
the once-crucial topic of tape merging has become of limited relevance 
to current needs.


Yet many of the patterns are quite beautiful, and the associated 
algorithms reflect some of the best research done in computer science 
during its early days; the techniques are just too nice to be discarded 
abruptly onto the rubbish heap of history. Indeed, the ways in which 
these methods blend theory with practice are especially instructive. 
Therefore merging patterns are discussed carefully and completely below, 
in what may be their last grand appearance before they accept the final 
curtain call.

"

Yep, the polyphase and other merge patterns are beautiful. I enjoyed 
reading through those sections. Now let's put them to rest in PostgreSQL.


- Heikki
>From 15169f32f99a69401d3565c8d0ff0d532d6b6638 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas 
Date: Wed, 12 Oct 2016 20:04:17 +0300
Subject: [PATCH 1/1] Replace polyphase merge algorithm with a simple balanced
 k-way merge.

The advantage of polyphase merge is that it can reuse the input tapes as
output tapes efficiently, but that is irrelevant on modern hardware, when
we can easily emulate any number of tape drives. The number of input tapes
we can/should use during merging is limited by work_mem, but output tapes
that we are not currently writing to only cost a little bit of memory, so
there is no need to skimp on them.

Refactor LogicalTapeSet/LogicalTape interface. All the tape functions,
like LogicalTapeRead and LogicalTapeWrite, take a LogicalTape as argument,
instead of LogicalTapeSet+tape number. You can create any number of
LogicalTapes in a single LogicalTapeSet, and you don't need to decide the
number upfront, when you create the tape set.
---
 src/backend/utils/sort/logtape.c   | 223 +
 src/backend/utils/sort/tuplesort.c | 665 +++--
 src/include/utils/logtape.h|  37 ++-
 3 files changed, 366 insertions(+), 559 deletions(-)

diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
index 1f540f0..2370fa5 100644
--- a/src/backend/utils/sort/logtape.c
+++ b/src/backend/utils/sort/logtape.c
@@ -118,6 +118,8 @@ typedef struct TapeBlockTrailer
  */
 typedef struct LogicalTape
 {
+	LogicalTapeSet *tapeSet;	/* tape set this tape is part of */
+
 	bool		writing;		/* T while in write phase */
 	bool		paused;			/* T if the tape is paused */
 	bool		frozen;			/* T if blocks should not be freed when read */
@@ -173,10 +175,6 @@ struct LogicalTapeSet
 	long	   *freeBlocks;		/* resizable array */
 	int			nFreeBlocks;	/* # of currently free blocks */
 	int			freeBlocksLen;	/* current allocated length of freeBlocks[] */
-
-	/* The