Re: [QUICKLIST 0/4] Arch independent quicklists V2

[Nick Piggin]

Andrew Morton wrote:
> > On Tue, 13 Mar 2007 22:06:46 +1100 Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> > Andrew Morton wrote:
> >
> > > > On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> >
> > ...
> >
> >
> > > > Page allocator still requires interrupts to be disabled, which this doesn't.
>
>
> > > > it is worthwhile.
> > >
> > >
> > > If you want a zeroed page for pagecache and someone has just stuffed a
> > > known-zero, cache-hot page into the pagetable quicklists, you have good
> > > reason to be upset.
> >
> > The thing is, pagetable pages are the one really good exception to the
> > rule that we should keep cache hot and initialise-on-demand. They
> > typically are fairly sparsely populated and sparsely accessed. Even
> > for last level page tables, I think it is reasonable to assume they will
> > usually be pretty cold.
>
>
> eh?  I'd have thought that a pte page which has just gone through
> zap_pte_range() will very often have a _lot_ of hot cachelines, and
> that's a common case.
>
> Still.   It's pretty easy to test.

Well I guess that would be the case if you had just unmapped a 4MB
chunk that was pretty dense with pages.

My malloc seems to allocate and free in blocks of 128K, so that's
only going to give us 3% of the last level pte being cache hot when
it gets freed. Not sure what common mmap(file) access patterns
look like.

The majority of programs I run have a smattering of llpt pages
pretty sparsely populated, covering text, libraries, heap, stack,
vdso.

We don't actually have to zap_pte_range the entire page table in
order to free it (IIRC we used to have to, before the 4lpt patches).

But yeah let's see some tests. I would definitely want to avoid this
extra layer of complexity if it is just as good to return the pages
to the pcp lists.

> > > Maybe, dunno.  It was apparently a win on powerpc many years ago.  I had a
> > > fiddle with it 5-6 years ago on x86 using a cache-disabled mapping of the
> > > page.  But it needed too much support in core VM to bother.  Since then
> > > we've grown per-cpu page magazines and __GFP_ZERO.  Plus I'm not aware of
> > > anyone having tried doing it on x86 with non-temporal stores.
> >
> > You can win on specifically constructed benchmarks, easily.
> >
> > But considering all the other problems you're going to introduce, we'd need
> > a significant win on a significant something, IMO.
> >
> > You waste memory bandwidth. You also use more CPU and memory cycles
> > speculatively, ergo you waste more power.
>
>
> Yeah, prezeroing in idle is probably pointless.  But I'm not aware of
> anyone having tried it properly...

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