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Re: [v3 0/9] parallelized "struct page" zeroing

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From: Matthew Wilcox <willy@infradead.org>
Date: Wed, 10 May 2017 10:17:03 -0700

> On Wed, May 10, 2017 at 11:19:43AM -0400, David Miller wrote:
>> From: Michal Hocko <mhocko@kernel.org>
>> Date: Wed, 10 May 2017 16:57:26 +0200
>> 
>> > Have you measured that? I do not think it would be super hard to
>> > measure. I would be quite surprised if this added much if anything at
>> > all as the whole struct page should be in the cache line already. We do
>> > set reference count and other struct members. Almost nobody should be
>> > looking at our page at this time and stealing the cache line. On the
>> > other hand a large memcpy will basically wipe everything away from the
>> > cpu cache. Or am I missing something?
>> 
>> I guess it might be clearer if you understand what the block
>> initializing stores do on sparc64.  There are no memory accesses at
>> all.
>> 
>> The cpu just zeros out the cache line, that's it.
>> 
>> No L3 cache line is allocated.  So this "wipe everything" behavior
>> will not happen in the L3.
> 
> There's either something wrong with your explanation or my reading
> skills :-)
> 
> "There are no memory accesses"
> "No L3 cache line is allocated"
> 
> You can have one or the other ... either the CPU sends a cacheline-sized
> write of zeroes to memory without allocating an L3 cache line (maybe
> using the store buffer?), or the CPU allocates an L3 cache line and sets
> its contents to zeroes, probably putting it in the last way of the set
> so it's the first thing to be evicted if not touched.

There is no conflict in what I said.

Only an L2 cache line is allocated and cleared.  L3 is left alone.

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[v3 0/9] parallelized "struct page" zeroing Pavel Tatashin <pasha.tatashin@oracle.com> - 2017-05-05 19:10 +0200
  [v3 8/9] powerpc: teach platforms not to zero struct pages memory Pavel Tatashin <pasha.tatashin@oracle.com> - 2017-05-05 19:10 +0200
  [v3 9/9] s390: teach platforms not to zero struct pages memory Pavel Tatashin <pasha.tatashin@oracle.com> - 2017-05-05 19:10 +0200
    Re: [v3 9/9] s390: teach platforms not to zero struct pages memory Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-05-08 13:40 +0200
  [v3 1/9] sparc64: simplify vmemmap_populate Pavel Tatashin <pasha.tatashin@oracle.com> - 2017-05-05 19:10 +0200
  Re: [v3 0/9] parallelized "struct page" zeroing Michal Hocko <mhocko@kernel.org> - 2017-05-09 20:20 +0200
    Re: [v3 0/9] parallelized "struct page" zeroing Pasha Tatashin <pasha.tatashin@oracle.com> - 2017-05-09 21:00 +0200
      Re: [v3 0/9] parallelized "struct page" zeroing Michal Hocko <mhocko@kernel.org> - 2017-05-10 09:30 +0200
        Re: [v3 0/9] parallelized "struct page" zeroing Pasha Tatashin <pasha.tatashin@oracle.com> - 2017-05-10 15:50 +0200
          Re: [v3 0/9] parallelized "struct page" zeroing Michal Hocko <mhocko@kernel.org> - 2017-05-10 17:00 +0200
            Re: [v3 0/9] parallelized "struct page" zeroing Pasha Tatashin <pasha.tatashin@oracle.com> - 2017-05-10 17:10 +0200
              Re: [v3 0/9] parallelized "struct page" zeroing David Miller <davem@davemloft.net> - 2017-05-10 17:30 +0200
            Re: [v3 0/9] parallelized "struct page" zeroing David Miller <davem@davemloft.net> - 2017-05-10 17:20 +0200
              Re: [v3 0/9] parallelized "struct page" zeroing Matthew Wilcox <willy@infradead.org> - 2017-05-10 19:20 +0200
                Re: [v3 0/9] parallelized "struct page" zeroing David Miller <davem@davemloft.net> - 2017-05-10 20:10 +0200
                Re: [v3 0/9] parallelized "struct page" zeroing Matthew Wilcox <willy@infradead.org> - 2017-05-10 23:20 +0200
              Re: [v3 0/9] parallelized "struct page" zeroing Michal Hocko <mhocko@kernel.org> - 2017-05-11 10:10 +0200
                Re: [v3 0/9] parallelized "struct page" zeroing David Miller <davem@davemloft.net> - 2017-05-11 16:40 +0200

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