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Date:   Wed, 23 Mar 2022 08:10:50 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Christoph Hellwig <hch@lst.de>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, Qu Wenruo <wqu@suse.com>,
        Naohiro Aota <naohiro.aota@wdc.com>,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 28/40] btrfs: do not allocate a btrfs_io_context in
 btrfs_map_bio
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References: <20220322155606.1267165-1-hch@lst.de> <20220322155606.1267165-29-hch@lst.de> <d9062a7d-c83c-06d7-50ac-272ffc0788f1@gmx.com> <20220323061339.GJ24302@lst.de> <ebb61597-dc43-2e81-2106-e448145f212c@gmx.com>
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On Wed, Mar 23, 2022 at 02:59:10PM +0800, Qu Wenruo wrote:
>> How do we waste memory?  We stop allocating the btrfs_io_context now
>> which can be quite big.
>
> Doesn't we waste the embedded __stripe if we choose to use the pointer one?
> And vice versus.
>
> And for SINGLE profile, we don't really need btrfs_bio_stripe at all, we
> can fast-path just setting bdev and bi_sector, and submit without even
> overriding its endio/private.

Yes, the 16 byes of the embedded stripe is wasted for I/O that doesn't
use it.  But all reads use it, which is typically the majority of all
I/O and the most performance critical one.  If the "waste" is a concern
we can split out a separate btrfs_read_bio.  Chance are that it will
just use slack by the time this all settles with a bunch of other
members that can be removed or unioned.