linux-kernel - Re: [PATCH v4 0/3] random: convert to using iters, for Al Viro

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Date:   Fri, 20 May 2022 09:58:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Theodore Ts'o <tytso@....edu>, Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 0/3] random: convert to using iters, for Al Viro

On 5/20/22 9:55 AM, Jason A. Donenfeld wrote:
> 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.

Ah I see, I can see if 64b is closer to the change for eg 1k.

>> 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.

Then I'd say there are only two options:

- Add a helper that provides splice for something that only has
  read/write set.

- Just accept that we're 3% slower reading from /dev/urandom for now,
  and maybe 1-2% for small reads. Can't really imagine this being a huge
  issue, how many high throughput /dev/urandom read situations exist in
  the real world?

-- 
Jens Axboe