linux-kernel - Re: initramfs optimization suggestions

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Message-ID: <20080805083946.GC3165@traven>
Date:	Tue, 5 Aug 2008 10:39:46 +0200
From:	Matthias Kaehlcke <matthias@...hlcke.net>
To:	Frans Meulenbroeks <fransmeulenbroeks@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: initramfs optimization suggestions

Hi Frans

El Tue, Aug 05, 2008 at 09:42:11AM +0200 Frans Meulenbroeks ha dit:

> [Upfront apologies if this is the wrong place to post this to.Just
> give me a gentle redirect to the right place in that case.]
> 
> I've been looking into improving the boot time of an embedded linux
> system (monta vista based).
> While doing so I detected that initramfs initialisation was one of the
> places where a lot of time was spent, so I looked into it in some more
> detail and came up with the following findings & suggestions.
> 
> First proposal:
> ==========
> 
> initramfs is build from a compressed cpio archive.
> Proposal is to introduce a build option to make the compression and
> decompression optional.
> Rationale 1: could be faster as it trades off I/O time (to read the
> image) against decompression time
> Rationale 2: for architectures that use compressed images (bzImage)
> actually we compress twice, which is not really efficient.
> 
> I can implement this, but before spending time on it I would like to know if
> a) people consider this a good idea
> b) no one else already has doen this.
> 
> 
> Second proposal:
> ============
> 
> after decompressing the cpio archive all files are made using
> sys_open/sys_write/sys_close and friends.
> This implies that a lot of system calls and data copying is done.
> It would be nice if that could be avoided.
> I'm not fully into all details of how ramfs is implemented, but would
> it be possible to e.g. dump all blocks of a tmp ram fs into a data
> structure (e.g.an array of blocks) while making the kernel, and while
> booting the kernel initialise the fs cache with these data? (I guess
> this would be around fs/dcache.c; I understand the data here is
> kmalloc-ed, but it might be possible to initialise the cache with
> pointers to that data structure; due to the nature of ramfs they won't
> be deallocated anyway I assume).
> Does this sound feasible? Hidden snags? Appreciate your
> opinion/feedback/suggestions.
> 
> Best regards, Frans.
> 
> PS: if anyone has a pointer to a place which explains in full detail
> how linux fs internally works (apart from the source :-) ) I
> appreciate a reference. FM

you might want to get in touch with Robert P. J. Day
(rpjday@...shcourse.ca>) who is also working on initramfs
optimizations. see a recent thread on the kernelnewbies mailing list: 
http://article.gmane.org/gmane.linux.kernel.kernelnewbies/26978

-- 
Matthias Kaehlcke
Embedded Linux Engineer
Barcelona

            You can chain me, you can torture me, you can even
          destroy this body, but you will never imprison my mind
                            (Mahatma Gandhi)
                                                                 .''`.
    using free software / Debian GNU/Linux | http://debian.org  : :'  :
                                                                `. `'`
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