Re: [00/17] Large Blocksize Support V3

Andrew Morton
2007-04-28T18:27:03Z

On Sat, 28 Apr 2007 07:09:07 -0700 William Lee Irwin III <wli@holomorphy.com> wrote:

> On Sat, 28 Apr 2007 10:04:08 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >> only 4.4 times faster, and more scalable, since we don't bounce the
> >> upper level locks around.
>
> On Sat, Apr 28, 2007 at 01:22:51AM -0700, Andrew Morton wrote:
> > I'm not sure what we're looking at here. radix-tree changes? Locking
> > changes? Both?
> > If we have a whole pile of pages to insert then there are obvious gains
> > from not taking the lock once per page (gang insert). But I expect there
> > will also be gains from not walking down the radix tree once per page too:
> > walk all the way down and populate all the way to the end of the node.
>
> The gang allocation affair would may also want to make the calls into
> the page allocator batched. For instance, grab enough compound pages to
> build the gang under the lock, since we're going to blow the per-cpu
> lists with so many pages, then break the compound pages up outside the
> zone->lock.

Sure, but...

Allocating a single order-3 (say) page _is_ a form of batching

We don't want compound pages here: just higher-order ones

Higher-order allocations bypass the per-cpu lists

> I think it'd be good to have some corresponding tactics for freeing as
> well.

hm, hadn't thought about that - would need to peek at contiguous pages in
the pagecache and see if we can gang-free them as higher-order pages.

The place to do that is perhaps inside the per-cpu magazines: it's more
general. Dunno if it would net advantageous though.