linux-kernel - Re: [PATCH v3 10/11] null_blk: allow non power of 2 zoned devices

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Message-ID: <b14775a9-da39-f26a-fa46-b0b1b789c30e@acm.org>
Date:   Thu, 12 May 2022 10:22:23 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Pankaj Raghav <p.raghav@...sung.com>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        jaegeuk@...nel.org, hare@...e.de, dsterba@...e.com,
        axboe@...nel.dk, hch@....de, snitzer@...nel.org
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        linux-fsdevel@...r.kernel.org, matias.bjorling@....com,
        Jens Axboe <axboe@...com>, gost.dev@...sung.com,
        jonathan.derrick@...ux.dev, jiangbo.365@...edance.com,
        linux-nvme@...ts.infradead.org, dm-devel@...hat.com,
        Naohiro Aota <naohiro.aota@....com>,
        linux-kernel@...r.kernel.org, Johannes Thumshirn <jth@...nel.org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Alasdair Kergon <agk@...hat.com>, linux-block@...r.kernel.org,
        Chaitanya Kulkarni <kch@...dia.com>,
        Keith Busch <kbusch@...nel.org>, linux-btrfs@...r.kernel.org,
        Luis Chamberlain <mcgrof@...nel.org>
Subject: Re: [PATCH v3 10/11] null_blk: allow non power of 2 zoned devices

On 5/9/22 04:56, Pankaj Raghav wrote:
> Even though I am not sure if this optimization will directly add value
> looking at my experiments with the current change, I can fold this in
> with a comment on top of zone_size_sect_shifts variable stating that
> size can be npo2 and this variable is only meaningful for the po2 size
> scenario.

Have these experiments perhaps been run on an x86_64 CPU? These CPUs 
only need a single instruction to calculate ilog2(). No equivalent of 
that instruction is available on ARM CPUs as far as I know. I think the 
optimization Damien proposed will help on ARM CPUs.

Thanks,

Bart.