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Date: Mon, 21 Jul 2014 15:54:15 -0400
From: Neil Horman <nhorman@tuxdriver.com>
To: Chris Pappas <chrispappas12@gmail.com>
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Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] Random numbers at line-rate
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On Mon, Jul 21, 2014 at 09:24:36PM +0200, Chris Pappas wrote:
> Hi,
> 
> I need to generate a random number per packet and I used the rte_fast_rand
> function to do so. When I run the code for one port-core I get almost
> line-rate performance. However, running simultaneously on multiple cores
> degrades performance significantly. (in all cases I uses minimum-sized
> packets).
> 
> Shouldn't the implementation scale for multicore and not degrade
> performance or am I missing anything? Also, is there another recommendation
> for generating randomness at line-rate? (the cpu does not support rdrand).
> 
> Best regards,
> Chris
> 

thats an odd random number generator.  I think, without locking, its likely on a
multicore system to produce identical values on multiple cores operating in
parallel (since multiple cores can read rte_red_rand_seed at the same time).
That may well lead to multiple packets having the same nonce, which might cause
odd behavior.

If your cpu supports it, I'd suggest writing some inline assembly to use the
rdrand instruction instead.  I'm not sure about its performance relative to the
current implementation, but IIRC the instruction is handled internal to the
core, so it should scale with any number of cpus.

neil