On Fri, Apr 13 2018 at 5:22am -0400, Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 03/21/2018 07:36 PM, Mikulas Patocka wrote: > > > > > > On Wed, 21 Mar 2018, Christopher Lameter wrote: > > > >> On Wed, 21 Mar 2018, Mikulas Patocka wrote: > >> > >>>> You should not be using the slab allocators for these. Allocate higher > >>>> order pages or numbers of consecutive smaller pagess from the page > >>>> allocator. The slab allocators are written for objects smaller than page > >>>> size. > >>> > >>> So, do you argue that I need to write my own slab cache functionality > >>> instead of using the existing slab code? > >> > >> Just use the existing page allocator calls to allocate and free the > >> memory you need. > >> > >>> I can do it - but duplicating code is bad thing. > >> > >> There is no need to duplicate anything. There is lots of infrastructure > >> already in the kernel. You just need to use the right allocation / freeing > >> calls. > > > > So, what would you recommend for allocating 640KB objects while minimizing > > wasted space? > > * alloc_pages - rounds up to the next power of two > > * kmalloc - rounds up to the next power of two > > * alloc_pages_exact - O(n*log n) complexity; and causes memory > > fragmentation if used excesivelly > > * vmalloc - horrible performance (modifies page tables and that causes > > synchronization across all CPUs) > > > > anything else? > > > > The slab cache with large order seems as a best choice for this. > > Sorry for being late, I just read this thread and tend to agree with > Mikulas, that this is a good use case for SL*B. If we extend the > use-case from "space-efficient allocator of objects smaller than page > size" to "space-efficient allocator of objects that are not power-of-two > pages" then IMHO it turns out the implementation would be almost the > same. All other variants listed above would lead to waste of memory or > fragmentation. > > Would this perhaps be a good LSF/MM discussion topic? Mikulas, are you > attending, or anyone else that can vouch for your usecase? Any further discussion on SLAB_MINIMIZE_WASTE should continue on list. Mikulas won't be at LSF/MM. But I included Mikulas' dm-bufio changes that no longer depend on this proposed SLAB_MINIMIZE_WASTE (as part of the 4.17 merge window). Mike
