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From: "George Spelvin" <linux@sciencehorizons.net>
To: linux@sciencehorizons.net, torvalds@linux-foundation.org
Cc: ak@linux.intel.com, davem@davemloft.net, David.Laight@aculab.com,
	djb@cr.yp.to, ebiggers3@gmail.com, eric.dumazet@gmail.com,
	hannes@stressinduktion.org, Jason@zx2c4.com,
	jeanphilippe.aumasson@gmail.com,
	kernel-hardening@lists.openwall.com,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
	luto@amacapital.net, netdev@vger.kernel.org, tom@herbertland.com,
	tytso@mit.edu, vegard.nossum@gmail.com
Subject: Re: HalfSipHash Acceptable Usage
Date: 21 Dec 2016 13:07:22 -0500	[thread overview]
Message-ID: <20161221180722.564.qmail@ns.sciencehorizons.net> (raw)
In-Reply-To: <CA+55aFy8fNOxw3bnwkX1S46jKnW6i26mueaiuOsScyN3kFJp+A@mail.gmail.com>

Linus wrote:
>> How much does kernel_fpu_begin()/kernel_fpu_end() cost?
>
> It's now better than it used to be, but it's absolutely disastrous
> still. We're talking easily many hundreds of cycles. Under some loads,
> thousands.

I think I've been thoroughly dissuaded, but just to clarify one thing
that resembles a misunderstanding:

> In contrast, in reality, especially with things like "do it once or
> twice per incoming packet", you'll easily hit the absolute worst
> cases, where not only does it take a few hundred cycles to save the FP
> state, you'll then return to user space in between packets, which
> triggers the slow-path return code and reloads the FP state, which is
> another few hundred cycles plus.

Everything being discussed is per-TCP-connection overhead, *not* per
packet.  (Twice for outgoing connections, because one is to generate
the ephemeral port number.)

I know you know this, but I don't want anyone spectating to be confused
about it.

  reply	other threads:[~2016-12-21 18:07 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-19 17:32 Jason A. Donenfeld
     [not found] ` <CAGiyFdduUNSGq24zfsk0ZU=hnOCmewAw8vw6XvDoS-3f+3UPKQ@mail.gmail.com>
2016-12-19 21:00   ` Jason A. Donenfeld
2016-12-20 21:36 ` Theodore Ts'o
2016-12-20 23:07   ` George Spelvin
2016-12-20 23:55   ` Eric Dumazet
2016-12-21  3:28     ` George Spelvin
2016-12-21  5:29       ` Eric Dumazet
2016-12-21  6:34         ` George Spelvin
2016-12-21 14:24           ` Jason A. Donenfeld
2016-12-21 15:55             ` George Spelvin
2016-12-21 16:37               ` Jason A. Donenfeld
2016-12-21 16:41               ` [kernel-hardening] " Rik van Riel
2016-12-21 17:25               ` Linus Torvalds
2016-12-21 18:07                 ` George Spelvin [this message]
2016-12-22  1:54                 ` Andy Lutomirski
2016-12-21 14:42         ` Jason A. Donenfeld
2016-12-21 15:56           ` Eric Dumazet
2016-12-21 16:33             ` Jason A. Donenfeld
2016-12-21 16:39             ` [kernel-hardening] " Rik van Riel
2016-12-21 17:08               ` Eric Dumazet
2016-12-21 18:37             ` George Spelvin
2016-12-21 18:40               ` Jason A. Donenfeld
2016-12-21 22:27               ` Theodore Ts'o
2016-12-22  0:18                 ` George Spelvin
2016-12-22  1:13                 ` George Spelvin

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