Re: A qsort template - Mailing list pgsql-hackers

I wrote:

> 0010 - Thresholds on my TODO list.

I did some basic tests on the insertion sort thresholds, and it looks
like we could safely and profitably increase the current value from 7
to 20 or so, in line with other more recent implementations. I've
attached an addendum on top of 0012 and the full test results on an
Intel Coffee Lake machine with gcc 11.1. I found that the object test
setup in 0012 had some kind of bug that was comparing the pointer of
the object array. Rather than fix that, I decided to use Datums, but
with the two extremes in comparator: simple branching with machine
instructions vs. a SQL-callable function. The papers I've read
indicate the results for Datum sizes would not be much different for
small structs. The largest existing sort element is SortTuple, but
that's only 24 bytes and has a bulky comparator as well.

The first thing to note is that I rejected outright any testing of a
"middle value" where the pivot is simply the middle of the array. Even
the Bently and McIlroy paper which is the reference for our
implementation says "The range that consists of the single integer 7
could be eliminated, but has been left adjustable because on some
machines larger ranges are a few percent better".

I tested thresholds up to 64, which is where I guessed results to get
worse (most implementations are smaller than that). Here are the best
thresholds at a quick glance:

- elementary comparator:

random: 16 or greater
decreasing, rotate: get noticeably better all the way up to 64
organ: little difference, but seems to get better all the way up to 64
0/1: seems to get worse above 20

- SQL-callable comparator:

random: between 12 and 20, but slight differences until 32
decreasing, rotate: get noticeably better all the way up to 64
organ: seems best at 12, but slight differences until 32
0/1: slight differences

Based on these tests and this machine, it seems 20 is a good default
value. I'll repeat this test on one older Intel and one non-Intel
platform with older compilers.

--
Running tally of patchset:

0001 - bsearch and unique is good to have, and we can keep the return
type pending further tests -- if none happen this cycle, suggest
committing this without the return type symbol.
0002/3 - I've yet to see a case where branchless comparators win, but
other than that, these are good. Notational improvement and not
performance sensitive.

0004/5 - Computing the arguments slows it down, but accessing the
underlying int16s gives an improvement. [1] Haven't done an in-situ
test on VACUUM. Could be worth it for pg15, since I imagine the
proposals for dead tuple storage won't be ready this cycle.
0006 - I expect this to be slower too. I also wonder if this could
also use the global function in 0004 once it's improved.

0007 - untested

0008 - Good performance in microbenchmarks, no in-situ testing.
Inlined reversal is not worth the binary space or notational overhead.

0009 - Based on 0004, I would guess that computing the arguments is
too slow. Not sure how to test in-situ to see if specializing helps.

0010 - Suggest leaving out the middle threshold and setting the
insertion sort threshold to ~20. Might also name them
ST_INSERTION_SORT_THRESHOLD and ST_NINTHER_THRESHOLD. (TODO: test on
other platforms)

0011 - Committed.

v3-0001 comparators for abbreviated keys - Clearly a win in this state
already, especially
for the "unsigned" case [2]. (gist untested) There are additional
possible improvements mentioned,
but they seem like a PG16 project(s).

[1] https://www.postgresql.org/message-id/CA%2BhUKG%2BS5SMoG8Z2PHj0bsK70CxVLgqQR1orQJq6Cjgibu26vA%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAFBsxsEFGAJ9eBpQVb5a86BE93WER3497zn2OT5wbjm1HHcqgA%40mail.gmail.com
(TODO: refine test)

-- 
John Naylor
EDB: http://www.enterprisedb.com