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FLAC I/O less efficient than STDIN, Direct file access can be almost twice slower than STDIN
post Nov 26 2012, 18:26
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I was optimizing caudec when I came across this oddity. Basically, letting /usr/bin/flac access .flac files on a slowish HDD directly for decoding ('flac -d file.flac') was in one particular case almost twice slower than piping the files to /usr/bin/flac via STDIN ('cat file.flac | flac -d -').

I used a double album for testing, made of 37 tracks for a total of about 1 GiB, located on a HDD that tops out at about 70 MB/s. Incidentally, flac decodes on my machine at a similar rate.

I ran caudec twice (figuratively - I repeated the tests many times) with 8 concurrent processes, for decoding those FLAC files to WAV on a ramdisk. I made sure to drop all caches between each run. First run was with direct file access, and completed in 40 seconds. Second run was with piping to STDIN, and completed in 25 seconds.

The difference was much less pronounced, surprisingly, on a USB flash drive that tops out at 35 MB/s, 34 seconds vs. 30 seconds, and non-existant on a RAID 1 array that tops out at 130 MB/s and on a SSD that tops out at 500 MB/s. I experienced similar differences with WavPack.

Does anyone have any idea of what's going on?

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post Nov 26 2012, 23:28
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I tested this on my machine with mostly insignificant differences.

On a 171MB FLAC -8 encoded file.
I ran one decode to allow Linux to cache the FLAC file in RAM and then discarded the results.

I did 3 runs...

A proper process efficient redirection to standard input:
flac -o test.wav -d - < 01-A\ Change\ Of\ Seasons.flac

real 0m3.740s
user 0m3.432s
sys 0m0.300s

A process inefficient pipe from cat:
cat 01-A\ Change\ Of\ Seasons.flac | flac -o test.wav -d -

real 0m3.869s
user 0m3.428s
sys 0m0.720s

Allowing FLAC to read the file itself:
flac -o test.wav -d 01-A\ Change\ Of\ Seasons.flac

real 0m3.765s
user 0m3.392s
sys 0m0.336s

I ran 3 runs of each test and while the numbers fluctuated slightly, the time spread remained similar on all runs.

In the given example run:
The difference between the best run (process efficient redirect) and the worst (pipe from cat) is 129ms
The difference between the process efficient redirect and directly reading is only 25ms.

Given this admittedly abysmally inadequate sample size it would appear that shell STDIN redirection provides the fastest decode, but the difference between redirection and directly reading the file is small enough to basically dismiss as noise. It would appear that in all cases the context switches involved with invoking cat yield the slowest results by a significant margin.

This post has been edited by yourlord: Nov 26 2012, 23:29
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