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qsort again (was Re: [PERFORM] Strange Create Index behaviour)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gary Doades <gpd(at)gpdnet(dot)co(dot)uk>
Cc: pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: qsort again (was Re: [PERFORM] Strange Create Index behaviour)
Date: 2006-02-15 23:28:29
Message-ID: 20781.1140046109@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Gary Doades <gpd(at)gpdnet(dot)co(dot)uk> writes:
> If I run the script again, it is not always the first case that is slow, 
> it varies from run to run, which is why I repeated it quite a few times 
> for the test.

For some reason I hadn't immediately twigged to the fact that your test
script is just N repetitions of the exact same structure with random data.
So it's not so surprising that you get random variations in behavior
with different test data sets.

I did some experimentation comparing the qsort from Fedora Core 4
(glibc-2.3.5-10.3) with our src/port/qsort.c.  For those who weren't
following the pgsql-performance thread, the test case is just this
repeated a lot of times:

create table atest(i int4, r int4);
insert into atest (i,r) select generate_series(1,100000), 0;
insert into atest (i,r) select generate_series(1,100000), random()*100000;
\timing
create index idx on atest(r);
\timing
drop table atest;

I did this 100 times and sorted the reported runtimes.  (Investigation
with trace_sort = on confirms that the runtime is almost entirely spent
in qsort() called from our performsort --- the Postgres overhead is
about 100msec on this machine.)  Results are below.

It seems clear that our qsort.c is doing a pretty awful job of picking
qsort pivots, while glibc is mostly managing not to make that mistake.
I haven't looked at the glibc code yet to see what they are doing
differently.

I'd say this puts a considerable damper on my enthusiasm for using our
qsort all the time, as was recently debated in this thread:
http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.php
We need to fix our qsort.c before pushing ahead with that idea.

			regards, tom lane


100 runtimes for glibc qsort, sorted ascending:

Time: 459.860 ms
Time: 460.209 ms
Time: 460.704 ms
Time: 461.317 ms
Time: 461.538 ms
Time: 461.652 ms
Time: 461.988 ms
Time: 462.573 ms
Time: 462.638 ms
Time: 462.716 ms
Time: 462.917 ms
Time: 463.219 ms
Time: 463.455 ms
Time: 463.650 ms
Time: 463.723 ms
Time: 463.737 ms
Time: 463.750 ms
Time: 463.852 ms
Time: 463.964 ms
Time: 463.988 ms
Time: 464.003 ms
Time: 464.135 ms
Time: 464.372 ms
Time: 464.458 ms
Time: 464.496 ms
Time: 464.551 ms
Time: 464.599 ms
Time: 464.655 ms
Time: 464.656 ms
Time: 464.722 ms
Time: 464.814 ms
Time: 464.827 ms
Time: 464.878 ms
Time: 464.899 ms
Time: 464.905 ms
Time: 464.987 ms
Time: 465.055 ms
Time: 465.138 ms
Time: 465.159 ms
Time: 465.194 ms
Time: 465.310 ms
Time: 465.316 ms
Time: 465.375 ms
Time: 465.450 ms
Time: 465.535 ms
Time: 465.595 ms
Time: 465.680 ms
Time: 465.769 ms
Time: 465.865 ms
Time: 465.892 ms
Time: 465.903 ms
Time: 466.003 ms
Time: 466.154 ms
Time: 466.164 ms
Time: 466.203 ms
Time: 466.305 ms
Time: 466.344 ms
Time: 466.364 ms
Time: 466.388 ms
Time: 466.502 ms
Time: 466.593 ms
Time: 466.725 ms
Time: 466.794 ms
Time: 466.798 ms
Time: 466.904 ms
Time: 466.971 ms
Time: 466.997 ms
Time: 467.122 ms
Time: 467.146 ms
Time: 467.221 ms
Time: 467.224 ms
Time: 467.244 ms
Time: 467.277 ms
Time: 467.587 ms
Time: 468.142 ms
Time: 468.207 ms
Time: 468.237 ms
Time: 468.471 ms
Time: 468.663 ms
Time: 468.700 ms
Time: 469.235 ms
Time: 469.840 ms
Time: 470.472 ms
Time: 471.140 ms
Time: 472.811 ms
Time: 472.959 ms
Time: 474.858 ms
Time: 477.210 ms
Time: 479.571 ms
Time: 479.671 ms
Time: 482.797 ms
Time: 488.852 ms
Time: 514.639 ms
Time: 529.287 ms
Time: 612.185 ms
Time: 660.748 ms
Time: 742.227 ms
Time: 866.814 ms
Time: 1234.848 ms
Time: 1267.398 ms


100 runtimes for port/qsort.c, sorted ascending:

Time: 418.905 ms
Time: 420.611 ms
Time: 420.764 ms
Time: 420.904 ms
Time: 421.706 ms
Time: 422.466 ms
Time: 422.627 ms
Time: 423.189 ms
Time: 423.302 ms
Time: 425.096 ms
Time: 425.731 ms
Time: 425.851 ms
Time: 427.253 ms
Time: 430.113 ms
Time: 432.756 ms
Time: 432.963 ms
Time: 440.502 ms
Time: 440.640 ms
Time: 450.452 ms
Time: 458.143 ms
Time: 459.212 ms
Time: 467.706 ms
Time: 468.006 ms
Time: 468.574 ms
Time: 470.003 ms
Time: 472.313 ms
Time: 483.622 ms
Time: 492.395 ms
Time: 509.564 ms
Time: 531.037 ms
Time: 533.366 ms
Time: 535.610 ms
Time: 575.523 ms
Time: 582.688 ms
Time: 593.545 ms
Time: 647.364 ms
Time: 660.612 ms
Time: 677.312 ms
Time: 680.288 ms
Time: 697.626 ms
Time: 833.066 ms
Time: 834.511 ms
Time: 851.819 ms
Time: 920.443 ms
Time: 926.731 ms
Time: 954.289 ms
Time: 1045.214 ms
Time: 1059.200 ms
Time: 1062.328 ms
Time: 1136.018 ms
Time: 1260.091 ms
Time: 1276.883 ms
Time: 1319.351 ms
Time: 1438.854 ms
Time: 1475.457 ms
Time: 1538.211 ms
Time: 1549.004 ms
Time: 1744.642 ms
Time: 1771.258 ms
Time: 1959.530 ms
Time: 2300.140 ms
Time: 2589.641 ms
Time: 2612.780 ms
Time: 3100.024 ms
Time: 3284.125 ms
Time: 3379.792 ms
Time: 3750.278 ms
Time: 4302.278 ms
Time: 4780.624 ms
Time: 5000.056 ms
Time: 5092.604 ms
Time: 5168.722 ms
Time: 5292.941 ms
Time: 5895.964 ms
Time: 7003.164 ms
Time: 7099.449 ms
Time: 7115.083 ms
Time: 7384.940 ms
Time: 8214.010 ms
Time: 8700.771 ms
Time: 9331.225 ms
Time: 10503.360 ms
Time: 12496.026 ms
Time: 12982.474 ms
Time: 15192.390 ms
Time: 15392.161 ms
Time: 15958.295 ms
Time: 18375.693 ms
Time: 18617.706 ms
Time: 18927.515 ms
Time: 19898.018 ms
Time: 20865.979 ms
Time: 21000.907 ms
Time: 21297.585 ms
Time: 21714.518 ms
Time: 25423.235 ms
Time: 27543.052 ms
Time: 28314.182 ms
Time: 29400.278 ms
Time: 34142.534 ms

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