Re: [PATCH] lib/sbitmap: kill 'depth' from sbitmap_word

Jens Axboe
2022-01-10T02:43:32Z

On 1/9/22 7:38 PM, Ming Lei wrote:
> On Sun, Jan 09, 2022 at 06:54:21PM -0700, Jens Axboe wrote:
>> On 1/9/22 6:50 PM, Ming Lei wrote:
>>> Only the last sbitmap_word can have different depth, and all the others
>>> must have same depth of 1U << sb->shift, so not necessary to store it in
>>> sbitmap_word, and it can be retrieved easily and efficiently by adding
>>> one internal helper of __map_depth(sb, index).
>>>
>>> Not see performance effect when running iops test on null_blk.
>>>
>>> This way saves us one cacheline(usually 64 words) per each sbitmap_word.
>>
>> We probably want to kill the ____cacheline_aligned_in_smp from 'word' as
>> well.
>
> But sbitmap_deferred_clear_bit() is called in put fast path, then the
> cacheline becomes shared with get path, and I guess this way isn't
> expected.

Just from 'word', not from 'cleared'. They will still be in separate
cache lines, but usually doesn't make sense to have the leading member
marked as cacheline aligned, that's a whole struct property at that
point.

--
Jens Axboe