Messages in this thread | | | Subject | Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1 | From | David Hildenbrand <> | Date | Wed, 29 Sep 2021 16:40:00 +0200 |
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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)
> > 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.
-- Thanks,
David / dhildenb
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