On Tue, Dec 11, 2018 at 11:32:54AM -0700, Jens Axboe wrote: > Don't see any regressions. But if we're fiddling with it anyway, can't > we do something smarter? Make the fast path just index a table, and put > all the big hammers in setup/destroy. We're spending a non-substantial > amount of time doing lookups, that's really no different before and > after the patch. Thanks for checking it out. I think the fast path does just index a table. Until you have more than 64 pointers in the XArray, it's just xa->head->slots[i]. And then up to 4096 pointers, it's xa->head->slots[i >> 6]->slots[i]. It has the advantage that if you only have one kioctx (which is surely many programs using AIO), it's just xa->head, so even better than a table lookup. It'll start to deteriorate after 4096 kioctxs, with one extra indirection for every 6 bits, but by that point, we'd've been straining the memory allocator to allocate a large table anyway.
