Re: clear_user_highpage()

[Andrew Morton <akpm@osdl.org>]

William Lee Irwin III <wli@holomorphy.com> wrote:
>
> On Wed, 11 Aug 2004, David S. Miller wrote:
>  >> We have that cold/hot page thing in the current 2.6.x
>  >> tree, or are you talking about something else?
> 
>  On Wed, Aug 11, 2004 at 09:19:32PM -0700, Linus Torvalds wrote:
>  > You're right. It ended up never having problems (or they were worked out
>  > in the -mm tree), so I forgot all about it ;)
>  > How effective is it? Maybe the numbers that were done in 2001 aren't 
>  > relevant any more?
> 
>  For lock amortization it's extremely effective. Its effects on caching
>  have never been properly instrumented that I know of.

No, we (me, mbligh) instrumented the crap out of it.  It turned out that
the cache affinity was of very marginal benefit, if any.

I cooked up an artificial benchmark which consisted of writing 32k to a
file, then truncating it back to zero, then repeating.  Four instances of
that, against four separate files on 4-way showed a large speedup - 2x or
3x, from memory.  But for real-world workloads you really needed to squint
to see anything at all.

Which is why I dithered without sending it to Linus for a couple of months.
Ended up merging it anyway because of some lock contention benefits, and
because someone mught have a workload which involves repeated
write/truncate looping ;)