Date: Mon, 21 Nov 2005 08:49:50 -0200
From: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
To: Matt Mackall <mpm@selenic.com>
Cc: Rob Landley <rob@landley.net>, akpm@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] skip initramfs check
Message-ID: <20051121104950.GA26480@logos.cnet>
References: <20051117141432.GD9753@logos.cnet> <200511210130.55774.rob@landley.net> <20051121062350.GA24381@logos.cnet> <200511210904.46495.rob@landley.net> <20051121155050.GK31287@waste.org>
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On Mon, Nov 21, 2005 at 07:50:50AM -0800, Matt Mackall wrote:
> On Mon, Nov 21, 2005 at 09:04:46AM -0600, Rob Landley wrote:
> > On Monday 21 November 2005 00:23, Marcelo Tosatti wrote:
> > > Hi Rob,
> > > > Query: is the problem that a big initramfs image is being unpacked more
> > > > than once, or is unpacking an empty initramfs image (134 bytes) causing a
> > > > significant delay?
> > >
> > > The problem is a big non-initramfs RAMDISK image (used for root mountpoint
> > > on this particular embedded platform), that is decompressed more than once:
> > >
> > > - during the initramfs check, which fails because it is not initramfs.
> > > - during the real RAMDISK decompression to memory.
> > >
> > > > I'm fairly certain that back in 1990 I could unzip 134 bytes on my 33 mhz
> > > > 386 running dos in a fraction of a second.  What's the use case here?
> > >
> > > So the issue is not the empty initramfs image (which BTW could probably
> > > be made unecessary?), but a 10Mb RAMDISK image being decompressed by a
> > > 48Mhz PPC, which takes quite a few seconds.
> > >
> > > Need to rework the patch to use a __setup option as Andrew suggested.
> > 
> > It sounds to me like is the initial check (which is just giving a thumbs 
> > up/thumbs down "is this an initramfs", correct?) only needs to decompress the 
> > first page or so of data to make this determination.  A quick glance at the 
> > code seems to imply it's just checking the header and the first entry, so 4k 
> > should be plenty for that.
> > 
> > Some variant of lib/zlib_inflate...  Ouch, bit of a mess there.  Hey Matt: you 
> > know this area.  Is it feasible to do some kind of:
> > 
> > deflate_init(whatever)
> > deflate_next_x_bytes(source *, dest *, length)
> > 
> > To grab a the first X bytes from the initramfs image?
> 
> Not at the moment. But I think it is feasible to simply move the ramdisk
> detection and unpacking inside the ramfs state machine. In other
> words, add two new states:
> 
> - detecting type
> - unpacking ramdisk
> 
> When the first few bytes are fed from the decompressor to the state
> machine, we either transition to the normal ramfs unpacking or we
> treat it as a ramdisk.

Yes that would be ideal. 
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