Messages in this thread | | | Date | Tue, 13 Mar 2007 22:30:19 +1100 | From | Nick Piggin <> | Subject | Re: [QUICKLIST 0/4] Arch independent quicklists V2 |
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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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