netdev - Re: [PATCH] Stochastic Fair Blue queue discipline

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Date:	Wed, 09 Apr 2008 17:45:34 +0200
From:	Juliusz Chroboczek <Juliusz.Chroboczek@....jussieu.fr>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] Stochastic Fair Blue queue discipline

>> > no net_random/srandom32 is per CPU and likely does the wrong thing for you.

>> Why is that?  I'm only using this to initialise a global data
>> structure, why should it matter that I use per-cpu state?

> Because they're independent and there is no guarantee you always 
> run on the same CPU and sampling them randomly will not necessarily
> give you a good random number sequence.

I'm sorry to be such a pain, but I still don't understand.

Random32 is initialised from get_random_bytes; so the per-cpu
pseudo-random sequences should be uncorrelated.  I fail to see how an
arbitrary interleaving of uncorrelated good pseudo-random sequences
can fail to be good.

Looking at line 448 of sch_sfq.c in Linus' current HEAD, I see that
somebody else thinks the same as I do.  So please let me know if sfq
needs fixed, or whether I can use net_random in sfb.

Thanks,

                                        Juliusz
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