linux-kernel - Re: Idea for reducing sysfs memory usage

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Message-ID: <56C3CC43.8060706@cantab.net>
Date:	Wed, 17 Feb 2016 01:26:27 +0000
From:	Edward Cree <ec429@...tab.net>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, David Ahern <dsa@...ulusnetworks.com>
Subject: Re: Idea for reducing sysfs memory usage

On 17/02/16 00:47, Greg Kroah-Hartman wrote:
> How many sysfs entries are you creating for that 20kb?  And how did you
> measure it?  If you don't access the files, the backing store is not
> allocated, saving you a lot of memory.
Thinking about this some more, could we not do the same thing with the
struct kernfs_nodes, i.e. only allocate them when someone first accesses
the file?  Or simpler, defer allocation of all the files in the dir until
someone first touches the directory?  Of course it would add a little
latency to that first access, but (barring differences in cache warmth) it
would subtract the same amount of time from the initial dir creation, and
in the case where no-one ever looks at the directory, it would save the
memory.
I did find a patch series from 2009 doing something vaguely similar[1], but
a) it looks like it wasn't applied and b) it appears to involve a function
pointer in struct sysfs_elem_dir to say how to populate the directory.
All we need here is to get our kobj (which we have in priv member of struct
kernfs_node) and call populate_dir().  (And remove whatever flag or mark we
used to say "not populated yet".  And some locking will be needed.)

Again, no hard numbers on how much memory this would save, nor evidence
that the "no-one ever touches the dir" case happens in practice...

[1] http://thread.gmane.org/gmane.linux.kernel/904418
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