Re: copy_page_range()

[William Lee Irwin III <wli@holomorphy.com>]

On Mon, 9 Aug 2004, William Lee Irwin III wrote:
>> These results are actually consistent with large-memory ia32.
>> Instruction-level profiles showed that the largest overhead in
>> copy_page_range() on such ia32 boxen appeared to be mm->rss++.

On Mon, Aug 09, 2004 at 10:08:05AM -0700, Linus Torvalds wrote:
> That sounds unlikely. Most ia32 instruction profiles will give high 
> profile counts to instructions _following_ the one that was expensive, and 
> in this case I'd strongyl suspect that the real expense on x86 is the 
> "get_page(page)" thing. 
> Which is an atomic increment, and thus very expensive.

But it was real. The theory is that mm->rss++; was an off-node memory
access, where struct page (due to boot-time remapping voodoo) and pmd's
(thanks to my patchwerk) were node-local, and the 40:1 off-node memory
access latency for a remote cache miss (i.e. ZONE_NORMAL) killed it all.

Thankfully Oracle has me parked on 64-bit machines with cache
directories and vaguely speedy interconnects for this kind of work.


-- wli