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Date: Thu, 10 Apr 2008 03:38:13 +0200 From: Patrick McHardy <kaber@...sh.net> To: Andi Kleen <andi@...stfloor.org> CC: Juliusz Chroboczek <Juliusz.Chroboczek@....jussieu.fr>, netdev@...r.kernel.org Subject: Re: [PATCH] Stochastic Fair Blue queue discipline Patrick McHardy wrote: > Andi Kleen wrote: >>> 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. >> >> They are not necessarily uncorreleated, especially on platforms >> which do have poor entropy support and when your initialization happens >> at boot time. Take a look at how the random pool starts in random.c. >> >>> 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. >> >> A lot of people get this wrong, but this doesn't mean that the >> problem should be readded in new code again. > > > Well, if I'm not mistaken net_random() used to be a function > (in net/core/utils.c) that didn't have this problem. So these > problems seem to have been introduced by the conversion to > srandom(). Two more noteworthy things: - net_random() was intended to provide *mediocre* random, cheap to compute, not perfect. Good enough for many networking related things. - traffic schedulers shouldn't depend on perfect random, its more about statistical multiplexing. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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