Help optimizing sort of large files Post: 302925515

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Top Forums UNIX for Advanced & Expert Users Help optimizing sort of large files Post 302925515 by kogorman3 on Monday 17th of November 2014 03:33:24 PM

11-17-2014

Registered User

Quote:

Originally Posted by Corona688

ext4 partitions are relatively easy to defrag, being designed with runtime defragmentation in mind (yes, runtime -- no need to unmount) via the e4defrag utility. There's no point defragging an empty partition, but check that your input and output partitions aren't a mess after all this testing.

Nice to know. I tried e4defrag and it showed a fragmentation score of 0 on all directories.

Quote:

The process of merge-sorting doesn't work that way. No matter how big your buffers are, it has to do the same number of merges on the same number of elements of the same sizes, nearly all of them tiny... Starting with billions of 2-element merges, half the number of 4-element merges, etc, etc, etc. (A little oversimplification, but the merging options don't substantially change this.) That's why pushing buffers to ridiculous sizes is so little help -- they're nearly always dead weight except for the final merge, when it's never going to be big enough to matter anyway.

I'm still a bit new to this, even after peeking at the source code. But it seems to me that there are two distinct phases to GNU sort. Both are merge sorts, but there's a big difference between merging in RAM and merging disk files. The main point of my optimizing effort turned out to be minimizing the number of file merges. Here's how I think of it now:

If all of the data can fit in the specified buffer, no temporary files will be created, and the output of the single in-RAM merge will go to the output file.

Otherwise, buffer-loads of data will be merged in RAM, and output to temporary files. There will be at least 2 of these, and perhaps a great many. These will be merged <batch-size> files at a time, with all the I/O time you'd expect. It's therefore desirable to ensure that no more than <batch-size> temporary files are created. If that's impossible, then limit it to the square, or even the cube of that number. Both buffer-size and batch-size affect the achievement of these goals.

Of course, even this is an oversimplification. It ignores effects of the TLB, of the L1 and L2 caches, and the kernel's buffer cache. Thus my interest in measuring actual times.

This is not going well, and I don't know why. Successive runs report quite different times. It's true the system is used for other things, but not heavily -- it's all my personal use. This is showing up in elapsed time, but not much in either system or user time. Odd. Moreover, the results of the second runs were consistently worse than the first. I'm trying third runs now.

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LEARN ABOUT DEBIAN

avimerge

avimerge(1)						      General Commands Manual						       avimerge(1)

NAME

       avimerge - merge several AVI-files into one

SYNOPSIS

       avimerge -o name -i file1 [ file2 [ ... ] ] [ -p file ] [ -a num ] [ -A num ] [ -b num ] [ -c ] [ -f commentfile ] [ -x indexfile ]

COPYRIGHT

       avimerge is Copyright (C) by Thomas Oestreich.

DESCRIPTION

       avimerge is a  versatile tool.  It can contatenate several AVI files into one. It can also be used to fix an index of a broken file and can
       also replace audio tracks or muxes new ones. It can read raw AC3 and MP3 files for multplexing.

OPTIONS

       -o name
	      Specify the name of the output file.

       -i file
	      Specify the name(s) of the input file(s) to merge into the output file.

       -p file
	      Specify the name of the audio file to multiplex into the output file. The type of file can be either another AVI file or an MP3 or
	      AC3 file.

       -b num Specify if avimerge should write an VBR mp3 header into the AVI file. Default is dependent on the input file (and usually correct).
	      num is either 1 or 0.

       -c     Drop video frames in case audio is missing [off]
	      Only when merging multiple AVI files. Some AVI files run a little bit (usually for one or two video frames) short on audio. This
	      means avimerge cannot keep up sync when concatinating them. The files play fine when played individually but not when merged because
	      audio from the new file gets played back with video from the old file.  avimerge will print a message like

		No audiodata left for track 0->0 (59950.25=59950.25) continuing ..

	      When you turn on the -c option, the video which is too much will be dropped.

       -f commentfile
	      Read AVI tombstone data for header comments from commentfile. See /docs/avi_comments.txt for a sample.

       -x indexfile
	      Read the AVI index from indexfile.  See aviindex(1) for information on how to create such a file.

       -a num Specify the number of the audio track you want to use from the input file.

       -A num Specify the number of the audio track you want to use in the output file. If you specify an existing track number, the track will be
	      replaced. If omitted, the next free slot will be used.

EXAMPLES

       The command

       avimerge -o big.avi -i my_file1.avi my_file2.avi my_file3.avi

       merges the three input files my_file[123].avi into one big AVI-file big.avi.

       avimerge -o out.avi -i in.avi -p audio2.avi -a 1

       merges track number 1 form in.avi to the next free track number in out.avi. You can create audio-only AVI-files using

       transcode -i song.mp3 -x null,mp3 -g 0x0 -y raw -a 1 -o audio2.avi -u 50

       The command

       avimerge -o out.avi -i in.avi -p sound.mp3

       merges the file sound.mp3 as an additional audio track into out.avi.

AUTHORS

       avimerge was written by Thomas Oestreich
       <ostreich@theorie.physik.uni-goettingen.de> with contributions from many others.  See AUTHORS for details.

SEE ALSO

       aviindex(1), avifix(1), avisplit(1), tccat(1), tcdecode(1), tcdemux(1), tcextract(1), tcprobe(1), tcscan(1), transcode(1)

avimerge(1)							 26th January 2004						       avimerge(1)

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