linux-kernel - Re: Allow setting of number of raw devices as a module parameter

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Message-ID: <20110430154733.GA21893@suse.de>
Date:	Sat, 30 Apr 2011 08:47:33 -0700
From:	Greg KH <gregkh@...e.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Allow setting of number of raw devices as a module parameter

On Sat, Apr 30, 2011 at 04:41:41PM +0100, Alan Cox wrote:
> > > A large vmalloc array is very antisocial on a 32bit x86 box. It looks
> > > like almost all of it would become sane if there was an array of pointers
> > > to raw devices and the devices were initially allocated on need (even if
> > > for now only recovered on rmmod)
> > 
> > In practice, we've never seen a problem with this[1].  Machines that
> > want thousands of raw devices have plenty of memory to handle this
> > situation.
> > 
> > I doubt adding the complexity of dynamically allocating the devices
> > as-needed is worth it for the very few systems that ever use this
> > driver, compounded with the fact that we keep saying that this code
> > isn't to be used by "normal" people anyway.
> 
> That's no excuse for sloppy code. Making it an array populated on demand
> is an improvement for everyone, making it vmalloc hurts every access (in
> TLB misses for one as well as vmalloc overhead).
> 
> I note two of us immediately made the same observation. Doing it
> basically right (array of pointers) is easy. Doing the full works with a
> hash for the lookups is a bit harder and that I would agree is overkill.

Ah, ok, I was thinking of the latter, I'll work on implementing the
former next week, unless Jan wants to do it instead?

thanks for the review,

greg k-h
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