Re: [PATCH] use posix_fadvise to increase readahead

[Eric Wong <normalperson@yhbt.net>]

Måns Rullgård <mans@mansr.com> wrote:
> Eric Wong <normalperson@yhbt.net> writes:
> 
> > Eric Wong <normalperson@yhbt.net> wrote:
> >> All relevant audio file formats store data sequentially, so
> >> give a hint to the kernel to perform more readahead.  In current
> >> Linux, the readahead hint doubles readahead pages and can help
> >> with playback on slow devices.
> >
> > Btw, I've been running this for a few years, too; but pretty
> > much all on FLAC.
> >
> > I don't know if there's audio formats we support which aren't
> > sequential.  Maybe there's some wacky audio format which
> > requires random read all over the place...
> 
> I can't think of any format that isn't mostly sequential, certainly not
> that's supported in SoX.  There might exist some format that separates
> channel data such that reading sequentially from multiple starting
> points is the best strategy.

Yeah, I was wondering about something along those lines,
especially if exceeding 2 channels...

> What sort of improvement do you get from this anyway?  I'm not opposed
> to the addition, just curious.

I didn't measure before :x  It's more of a "it couldn't hurt"
thing and let the kernel (and configured per-device readahead)
decide.  It's only intended as a hint for the kernel, after all.

My 7200 RPM HDD has 256 MB cache on it; I couldn't measure any
difference even with 'echo 3 >/proc/sys/vm/drop_caches' (Linux)
and reading ~300 MB of stuff before testing in hopes it'd clear
the internal HDD cache.  Maybe slower mounts (optical storage,
4200 RPM HDDs, or network FSes) will see the benefit.

I used to play audio on a laptop using an sshfs mount back in
the day (with FUSE readahead, too), but haven't had the need or
ability to do that for years, now.


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