Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: 21 Dec 2016 13:07:22 -0500
From: "George Spelvin" <>
Subject: Re: HalfSipHash Acceptable Usage

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.

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.