Messages in this thread | | | Date | 21 Dec 2016 13:07:22 -0500 | From | "George Spelvin" <> | Subject | Re: HalfSipHash Acceptable Usage |
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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.
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