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Date:   Fri, 20 May 2022 17:55:05 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Theodore Ts'o <tytso@mit.edu>, Christoph Hellwig <hch@lst.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v4 0/3] random: convert to using iters, for Al Viro
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Hi Jens,

On Fri, May 20, 2022 at 09:44:25AM -0600, Jens Axboe wrote:
> Ran 32, 1k, 4k here and it does seem to be down aboout 3%. Which is
> definitely bigger than I expected, particularly for larger reads. If
> anything, the 32b read seems comparably better than eg 1k or 4k, which
> is also unexpected. Let me do a bit of profiling to see what is up.

Something to keep in mind wrt 32b is that for complicated crypto
reasons, the function has this logic:

- If len <= 32, generate one 64 byte block and give <= 32 bytes of it to
  the caller.

- If len > 32, generate one 64 byte block, but give 0 of it to the
  caller. Then generate ⌈len/64⌉ blocks for the caller.

Put together, this means:

- 1..32, 1 block
- 33..64, 2 blocks
- 65..128, 3 blocks
- 129..196, 4 blocks

So you get this sort of shelf where the amortization benefits don't
really kick in until after 3 blocks.

> If you're worried about it, I'd just keep the read/write and add the
> iter variants on the side.
 
Not a chance of that. These functions are already finicky as-is; I would
really hate to have to duplicate all of these paths.

Jason