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

[Mathieu Desnoyers <mathieu desnoyers efficios com>]

* J. Bruce Fields (bfields fieldses org) wrote:
> 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.

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