linux-kernel - Re: [PATCH V2] xfs: use roundup_pow_of_two instead of ffs during xlog_find

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Message-ID: <ZQDp1fNUgmJevXLr@dread.disaster.area>
Date:   Wed, 13 Sep 2023 08:44:37 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Wang Jianchao <jianchwa@...look.com>
Cc:     djwong@...nel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] xfs: use roundup_pow_of_two instead of ffs during
 xlog_find_tail

On Tue, Sep 12, 2023 at 03:20:56PM +0800, Wang Jianchao wrote:
> 
> In our production environment, we find that mounting a 500M /boot
> which is umount cleanly needs ~6s. One cause is that ffs() is
> used by xlog_write_log_records() to decide the buffer size. It
> can cause a lot of small IO easily when xlog_clear_stale_blocks()
> needs to wrap around the end of log area and log head block is
> not power of two. Things are similar in xlog_find_verify_cycle().
> 
> The code is able to handed bigger buffer very well, we can use
> roundup_pow_of_two() to replace ffs() directly to avoid small
> and sychronous IOs.
> 
> Changes in V1:
>  - Also replace the ffs in xlog_find_verify_cycle()

Change logs go either below the --- line or in the cover letter,
not the commit itself.

Other than that, the change looks ok. The use of ffs() was added in
2002 simply to make buffers a power-of-2 size. I don't think it had
anything to do with trying to maximise the actual buffer size at
all, otherwise it would have made to use fls() like
roundup_pow_of_two() does...

Reviewed-by: Dave Chinner <dchinner@...hat.com>

-- 
Dave Chinner
david@...morbit.com