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
