11-21-2014
I'm not going to load a database, because the results of the sort will be used just once, and as a practical matter may be passed in a pipe without ever hitting the filesystem. For testing, there's an output file, but just for testing, and to make the results more generally relevant to anyone else who might read this.
My sort times are generally within a factor of 2 of the cost of copying the file to temp and then to the output. So thrashing and computing are not horrible and I'm not going to write a separate sort or any part of it because it will take too long to get it right, and I'm not going to use multiple invocations of sort(1) because the disk I/O will clearly eat any benefits.
I've re-run my timing scripts on the small test file, and some comments I made earlier have to be corrected. The differences between runs are not that alarming after all, and are easily explained by differences in other competing activities on the same machine. My speedups so far are more modest than I thought, but --parallel=4 really does give me 20%, and there's about another 20% available from jiggering parameters.
Running after a fresh boot, I noticed some things that surprised me, though perhaps they should not have. By the time testing is done, the kernel has filled 64GB of memory, mostly with "cached" blocks, and has swapped out a little over 3 MB of memory. I presume it's swapping idle daemons. So these results will not scale up for files large enough to do a complete cache wipe.
I've pretty much determined that the main thing to avoid is getting more than one merge pass on the temporaries. I think it's time to try just a few things with my TB-sized things, because I know those were doing at least 2 extra passes with the default parameters. It took forever. I think the defaults are 4GB buffers (1/8 real memory) and merges of 16 files. The buffers seem to have a lot of overhead, so the temp files are smaller than you might expect. On a 1TB file, that will be roughly 500 2-GB temp files, and 3 levels of merge. The question is: given a choice, is it better to use a bigger buffer, or a wider merge? I'm betting 4GB buffers are too big, but I need to do some testing.
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SORT(1) User Commands SORT(1)
NAME
sort - sort lines of text files
SYNOPSIS
sort [OPTION]... [FILE]...
DESCRIPTION
Write sorted concatenation of all FILE(s) to standard output.
Ordering options:
Mandatory arguments to long options are mandatory for short options too.
-b, --ignore-leading-blanks ignore leading blanks
-d, --dictionary-order
consider only blanks and alphanumeric characters
-f, --ignore-case
fold lower case to upper case characters
-g, --general-numeric-sort
compare according to general numerical value
-i, --ignore-nonprinting
consider only printable characters
-M, --month-sort
compare (unknown) < `JAN' < ... < `DEC'
-n, --numeric-sort
compare according to string numerical value
-r, --reverse
reverse the result of comparisons
Other options:
-c, --check
check whether input is sorted; do not sort
-k, --key=POS1[,POS2]
start a key at POS1, end it at POS 2 (origin 1)
-m, --merge
merge already sorted files; do not sort
-o, --output=FILE
write result to FILE instead of standard output
-s, --stable
stabilize sort by disabling last-resort comparison
-S, --buffer-size=SIZE
use SIZE for main memory buffer
-t, --field-separator=SEP use SEP instead of non- to whitespace transition
-T, --temporary-directory=DIR
use DIR for temporaries, not $TMPDIR or /tmp multiple options specify multiple directories
-u, --unique
with -c: check for strict ordering
otherwise: output only the first of an equal run
-z, --zero-terminated
end lines with 0 byte, not newline
--help display this help and exit
--version
output version information and exit
POS is F[.C][OPTS], where F is the field number and C the character position in the field. OPTS is one or more single-letter ordering
options, which override global ordering options for that key. If no key is given, use the entire line as the key.
SIZE may be followed by the following multiplicative suffixes: % 1% of memory, b 1, K 1024 (default), and so on for M, G, T, P, E, Z, Y.
With no FILE, or when FILE is -, read standard input.
*** WARNING *** The locale specified by the environment affects sort order. Set LC_ALL=C to get the traditional sort order that uses
native byte values.
AUTHOR
Written by Mike Haertel and Paul Eggert.
REPORTING BUGS
Report bugs to <bug-coreutils@gnu.org>.
COPYRIGHT
Copyright (C) 2002 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
LAR PURPOSE.
SEE ALSO
The full documentation for sort is maintained as a Texinfo manual. If the info and sort programs are properly installed at your site, the
command
info sort
should give you access to the complete manual.
sort (coreutils) 4.5.3 October 2002 SORT(1)