linux-kernel - Re: Updated scalable urandom patchkit

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Date:	13 Oct 2015 17:10:30 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	ahferroin7@...il.com, andi@...stfloor.org, jepler@...ythonic.net,
	linux-kernel@...r.kernel.org, linux@...izon.com,
	linux@...musvillemoes.dk, tytso@....edu
Subject: Re: Updated scalable urandom patchkit

> Ted's patch peaks at 350K with four CPUs, but then quickly degrades to
> 50K ops/s at 20+ CPUs. At 144 CPUs it is slightly faster again at ~80K.

Good to know, thanks!  With its race conditions, it's basically a "best
case" for that particular design, which tells me that more significant
changes are required.

Off hand, do you know how large a read each operation is?  I want to
reduce mixback from once per 10 bytes to once per read, and the size
ratio will give me some idea of how large an improvement to expect.

> Spelvin's patch peaks at only 140K at 2 CPUs (so it's slower than base line),
> stays around 120K upto 20, then degrades quickly to 50K and then slowly
> improves again to ~80K.

Sorry to make you go to the trouble; I knew from discussions with Ted
that it wasn't going to work.  It was mostly just in the form of a patch
for the sake of a more concrete discussion.

I'll have a patch that I hope will do some good for testing in a couple
of hours.

> The duplicated pool patch is ~200K upto 20 CPus, 400K upto 40, 600K at
> slightly below 60 CPUs, and then very slowly degrades to 520K at 144. 

Shitty performance is practically a design goal of /dev/urandom.  You
are NOT supposed to hit it more than once per minute per thread.

But since we have a real-world problem with it, Ted's "abusse mitigation
mode" idea (where the kernel does what the app should do: seed a private
CPRNG and use that) will provide good security at extremely high access
rates.
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