Messages in this thread |  | | Subject | Re: [v3 0/9] parallelized "struct page" zeroing | From | Pasha Tatashin <> | Date | Wed, 10 May 2017 11:01:40 -0400 |
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On 05/10/2017 10:57 AM, Michal Hocko wrote: > On Wed 10-05-17 09:42:22, Pasha Tatashin wrote: >>> >>> Well, I didn't object to this particular part. I was mostly concerned >>> about >>> http://lkml.kernel.org/r/1494003796-748672-4-git-send-email-pasha.tatashin@oracle.com >>> and the "zero" argument for other functions. I guess we can do without >>> that. I _think_ that we should simply _always_ initialize the page at the >>> __init_single_page time rather than during the allocation. That would >>> require dropping __GFP_ZERO for non-memblock allocations. Or do you >>> think we could regress for single threaded initialization? >>> >> >> Hi Michal, >> >> Thats exactly right, I am worried that we will regress when there is no >> parallelized initialization of "struct pages" if we force unconditionally do >> memset() in __init_single_page(). The overhead of calling memset() on a >> smaller chunks (64-bytes) may cause the regression, this is why I opted only >> for parallelized case to zero this metadata. This way, we are guaranteed to >> see great improvements from this change without having regressions on >> platforms and builds that do not support parallelized initialization of >> "struct pages". > > 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? >
Perhaps you are right, and I will measure on x86. But, I suspect hit can become unacceptable on some platfoms: there is an overhead of calling a function, even if it is leaf-optimized, and there is an overhead in memset() to check for alignments of size and address, types of setting (zeroing vs. non-zeroing), etc., that adds up quickly.
Pasha
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