Messages in this thread | | | Subject | Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure | From | John Hubbard <> | Date | Fri, 24 Mar 2017 11:15:49 -0700 |
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On 03/24/2017 09:52 AM, Tim Chen wrote: > On Fri, 2017-03-24 at 06:56 -0700, Dave Hansen wrote: >> On 03/24/2017 12:33 AM, John Hubbard wrote: >>> >>> There might be some additional information you are using to come up with >>> that conclusion, that is not obvious to me. Any thoughts there? These >>> calls use the same underlying page allocator (and I thought that both >>> were subject to the same constraints on defragmentation, as a result of >>> that). So I am not seeing any way that kmalloc could possibly be a >>> less-fragmenting call than vmalloc. >> You guys are having quite a discussion over a very small point. >> >> But, Ying is right. >> >> Let's say we have a two-page data structure. vmalloc() takes two >> effectively random order-0 pages, probably from two different 2M pages >> and pins them. That "kills" two 2M pages. >> >> kmalloc(), allocating two *contiguous* pages, is very unlikely to cross >> a 2M boundary (it theoretically could). That means it will only "kill" >> the possibility of a single 2M page. More 2M pages == less fragmentation. > > In vmalloc, it eventually calls __vmalloc_area_node that allocates the > page one at a time. There's no attempt there to make the pages contiguous > if I am reading the code correctly. So that will increase the memory > fragmentation as we will be piecing together pages from all over the places. > > Tim
OK. Thanks everyone for spelling it out for me, before I started doing larger projects, with an incorrect way of looking at the fragmentation behavior. :)
-- thanks, john h
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