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

[Ming Lei <ming.lei@redhat.com>]

On Sun, Jan 09, 2022 at 07:43:26PM -0700, Jens Axboe wrote:
> 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.

OK, got it, it is just sort of code style thing, and not any functional
change.

Will include it in V2.


Thanks,
Ming