* J. Bruce Fields (bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org) wrote:

On Mon, Oct 29, 2012 at 11:13:43AM -0400, Mathieu Desnoyers wrote:

quoted

* Linus Torvalds (torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org) wrote:

quoted

On Mon, Oct 29, 2012 at 5:42 AM, Mathieu Desnoyers
[off-list ref] wrote:

quoted

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.

FYIW: I've made a nice boo-boo above. It should have been:

#define DFR_HASH_BITS   (PAGE_SHIFT - ilog2(sizeof(struct hlist_head)))

Because we happen to have a memory indexed in bytes, not in bits. I
guess this goes a long way proving Linus' point about virtues of trivial
code. ;-)

Thanks,

Mathieu

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