netdev - Re: [PATCH] net: implement emergency route cache rebulds when gc

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Date:	Mon, 06 Oct 2008 23:21:38 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	Bill Fink <billfink@...dspring.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kuznet@....inr.ac.ru, pekkas@...core.fi, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	Evgeniy Polyakov <johnpol@....mipt.ru>
Subject: Re: [PATCH] net: implement emergency route cache rebulds when	gc_elasticity
 is exceeded

Neil Horman a écrit :
> So, I've been playing with this patch, and I've not figured out eactly whats
> bothering me yet, since the math seems right, but something doesn't seem right
> about the outcome of this algorithm.  I've tested with my local system, and all
> works well, because the route cache is well behaved, and the sd value always
> works out to be very small, so ip_rt_gc_elasticity is used.  So I've been
> working through some scenarios by hand to see what this looks like using larger
> numbers.  If i assume ip_rt_gc_interval is 60, and rt_hash_log is 17, my sample
> count here is 7864320 samples per run.  If within that sample 393216 (about 4%)
> of the buckets have one entry on the chain, and all the rest are zeros, my hand
> calculations result in a standard deviation of approximately 140 and an average
> of .4.  That imples that in that sample set any one chain could be almost 500
> entires long before it triggered a cache rebuld.  Does that seem reasonable?
> 

if rt_hash_log is 17, and interval is 60, then you should scan 
(60 << 17)/300 slots. That's 26214 slots. (ie 20% of the 2^17 slots)

I have no idea how you can have sd = 140, even if scaled by (1 << 3)
with slots being empty or with one entry only...

If 4% of your slots have one element, then average length is 0.04 :)





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