On Wed, Sep 29, 2021 at 4:40 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 29.09.21 16:30, Uladzislau Rezki wrote: > >> > >> So the idea is that once we run into a dead end because we took a left > >> subtree, we rollback to the next possible rigth subtree and try again. > >> If we run into another dead end, we repeat ... thus, this can now happen > >> more than once. > >> > >> I assume the only implication is that this can now be slower in some > >> corner cases with larger alignment, because it might take longer to find > >> something suitable. Fair enough. > >> > > Yep, your understanding is correct regarding the tree traversal. If no > > suitable block > > is found in left sub-tree we roll-back and check right one. So it can > > be(the scanning) > > more than one time. > > > > I did some performance analyzing using vmalloc test suite to figure > > out a performance > > loss for allocations with specific alignment. On that syntactic test i > > see approx. 30% > > of degradation: > > How realistic is that test case? I assume most alignment we're dealing > with is: > * 1/PAGE_SIZE > * huge page size (for automatic huge page placing) > Well that is synthetic test. Most of the alignments are 1 or PAGE_SIZE. There are users which use internal API where you can specify an alignment you want but those are mainly like KASAN, module alloc, etc. > > > > 2.225 microseconds vs 1.496 microseconds. That time includes both > > vmalloc() and vfree() > > calls. I do not consider it as a big degrade, but from the other hand > > we can still adjust the > > search length for alignments > one page: > > > > # add it on top of previous proposal and search length instead of size > > length = align > PAGE_SIZE ? size + align:size; > > That will not allow to place huge pages in the case of kasan. And I > consider that more important than optimizing a syntactic test :) My 2 cents. > Could you please to be more specific? I mean how is it connected with huge pages mappings? Huge-pages are which have order > 0. Or you mean that a special alignments are needed for mapping huge pages? -- Uladzislau Rezki
