linux-kernel - Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable

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Date:	Fri, 3 Aug 2012 17:05:31 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <levinsasha928@...il.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	paul.gortmaker@...driver.com, davem@...emloft.net,
	rostedt@...dmis.org, mingo@...e.hu, ebiederm@...ssion.com,
	aarcange@...hat.com, ericvh@...il.com, netdev@...r.kernel.org
Subject: Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable

Hello,

On Fri, Aug 03, 2012 at 04:47:47PM -0700, Linus Torvalds wrote:
> On Fri, Aug 3, 2012 at 3:36 PM, Tejun Heo <tj@...nel.org> wrote:
> >
> > I suppose you mean unsized.  I remember this working.  Maybe I'm
> > confusing it with zero-sized array.  Hmm... gcc doesn't complain about
> > the following.  --std=c99 seems happy too.
> 
> Ok, I'm surprised, but maybe it's supposed to work if you do it inside
> another struct like that, exactly so that you can preallocate things..

Yeah, I think the rule is var array should be the last member of any
given struct definition.  Once a struct is defined, its alignment and
size are fixed and it behaves like any other struct.

> Or maybe it's just a gcc bug. I do think this all is way hackier than
> Sasha's original simple code that didn't need these kinds of games,
> and didn't need a size member at all.
> 
> I really think all the extra complexity and overhead is just *bad*.
> The first simple version was much nicer and likely generated better
> code too.

The size member could have performance impact in extreme cases.  If
we're looking for something simple & fast, maybe just pass in @size as
argument and be done with it?

Thanks.

-- 
tejun
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