On Thu, Nov 30, 2017 at 10:35:03PM +0900, Tetsuo Handa wrote: > According to xb_set_bit(), it seems to me that we are trying to avoid memory allocation > for "struct ida_bitmap" when all set bits within a 1024-bits bitmap reside in the first > 61 bits. > > But does such saving help? Is there characteristic bias that majority of set bits resides > in the first 61 bits, for "bit" is "unsigned long" which holds a page number (isn't it)? > If no such bias, wouldn't eliminating radix_tree_exception() case and always storing > "struct ida_bitmap" simplifies the code (and make the processing faster)? It happens all the time. The vast majority of users of the IDA set low bits. Also, it's the first 62 bits -- going up to 63 bits with the XArray rewrite. I do plan to redo the xbitmap on top of the XArray; I'm just trying to get the XArray merged first. The IDA and xbitmap code will share much more code when that happens.
