On Fri, May 18, 2007 at 12:43:04AM -0700, Andrew Morton wrote:
> On Fri, 18 May 2007 09:32:23 +0200 Nick Piggin <[email protected]> wrote:
>
> > On Fri, May 18, 2007 at 12:19:05AM -0700, Andrew Morton wrote:
> > > On Fri, 18 May 2007 06:08:54 +0200 Nick Piggin <[email protected]> wrote:
> > >
> > > > Many batch operations on struct page are completely random,
> > >
> > > But they shouldn't be: we should aim to place physically contiguous pages
> > > into logically contiguous pagecache slots, for all the reasons we
> > > discussed.
> >
> > For big IO batch operations, pagecache would be more likely to be
> > physically contiguous, as would LRU, I suppose.
>
> read(), write(), truncate(), writeback, pagefault. Pretty common stuff.
Of course, but if you're doing them on random-ish ranges, or multiple
files, or continually on the same file while parts of it get reclaimed
and reinstantiated...
> > I'm more thinking of operations where things get reclaimed over time,
> > touched or dirtied in slightly different orderings, interleaved with
> > other allocations, etc.
>
> Yes, that can happen. But in such cases we by definition aren't touching
> the pageframes very often. I'd assert that when the kernel is really
> hitting those pageframes hard, it is commonly doing this in ascending
> pagecache order.
I'm not sure that I would always agree. Sure if there is random *IO*
involved, then it is going to be slow and we won't be hitting page frames
so hard. But if there is random pagecache access, it will hit them almost
as hard.
> > > If/when that happens, there will be a *lot* of locality of reference
> > > against the pageframes in a lot of important codepaths.
> >
> > And when it doesn't happen, we eat 75% more cache misses. And for that
> > matter we eat 75% more cache misses for non-batch operations like
> > allocating or freeing a page by slab, for example.
>
> "measure twice, cut once"
Definitely agree there.
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