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FLAC I/O less efficient than STDIN, Direct file access can be almost twice slower than STDIN
skamp
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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greensdrive
post Nov 26 2012, 23:16
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try using just one thread for cat and the same for flac -d. perhaps one binary is optimized for multi-core, and one is not.

seems to me that flac would use ram to decode. the less ram (because of the ramdisk) may be limiting the decode ability of flac. but I think that maybe the same could be said for cat. that's assuming you used actual ram and not swap or other temporary hard drive space for the ramdisk.

also... /usr/bin/flac seems like a binary provided by your distribution. maybe try using a more optimized one that you compiled, or even one from rarewares (if they have it) since caudec supports wine anyway.
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