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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
>>On Tue, 13 Mar 2007 22:06:46 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>>Andrew Morton wrote:
>>
>>>>On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@yahoo.com.au> 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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