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Date:   Mon, 10 Jan 2022 10:38:41 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, Martin Wilck <martin.wilck@suse.com>
Subject: Re: [PATCH] lib/sbitmap: kill 'depth' from sbitmap_word
Message-ID: <YducMfW4vhk15CMq@T590>
References: <20220110015007.326561-1-ming.lei@redhat.com>
 <020ba538-bb41-c827-1290-c2939bf8940c@kernel.dk>
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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.

Thanks,
Ming