netdev - Re: [PATCH v7 09/16] SUNRPC/cache: use new hashtable implementation

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Message-ID: <20121029151343.GA17722@Krystal>
Date:	Mon, 29 Oct 2012 11:13:43 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <levinsasha928@...il.com>, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v7 09/16] SUNRPC/cache: use new hashtable implementation

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> On Mon, Oct 29, 2012 at 5:42 AM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > So defining e.g.:
> >
> > #include <linux/log2.h>
> >
> > #define DFR_HASH_BITS  (PAGE_SHIFT - ilog2(BITS_PER_LONG))
> >
> > would keep the intended behavior in all cases: use one page for the hash
> > array.
> 
> Well, since that wasn't true before either because of the long-time
> bug you point out, clearly the page size isn't all that important. I
> think it's more important to have small and simple code, and "9" is
> certainly that, compared to playing ilog2 games with not-so-obvious
> things.
> 
> Because there's no reason to believe that '9' is in any way a worse
> random number than something page-shift-related, is there? And getting
> away from *previous* overly-complicated size calculations that had
> been broken because they were too complicated and random, sounds like
> a good idea.

Good point. I agree that unless we really care about the precise number
of TLB entries and cache lines used by this hash table, we might want to
stay away from page-size and pointer-size based calculation.

It might not hurt to explain this in the patch changelog though.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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