Date: Mon, 29 Oct 2012 11:16:54 -0400
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <levinsasha928@gmail.com>, tj@kernel.org,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, paul.gortmaker@windriver.com, davem@davemloft.net,
        rostedt@goodmis.org, mingo@elte.hu, ebiederm@xmission.com,
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        agk@redhat.com, dm-devel@redhat.com, neilb@suse.de,
        ccaulfie@redhat.com, teigland@redhat.com, Trond.Myklebust@netapp.com,
        fweisbec@gmail.com, jesse@nicira.com, venkat.x.venkatsubra@oracle.com,
        ejt@redhat.com, snitzer@redhat.com, edumazet@google.com,
        linux-nfs@vger.kernel.org, dev@openvswitch.org,
        rds-devel@oss.oracle.com, lw@cn.fujitsu.com
Subject: Re: [PATCH v7 09/16] SUNRPC/cache: use new hashtable implementation
Message-ID: <20121029151653.GC9502@fieldses.org>
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 <20121029151343.GA17722@Krystal>
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On Mon, Oct 29, 2012 at 11:13:43AM -0400, Mathieu Desnoyers wrote:
> * Linus Torvalds (torvalds@linux-foundation.org) wrote:
> > On Mon, Oct 29, 2012 at 5:42 AM, Mathieu Desnoyers
> > <mathieu.desnoyers@efficios.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.

I'd also be happy to take that as a separate patch now.

--b.