netdev - Re: [PATCH] ipv4: Remove unnecessary code from rt_check

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Date:	Tue, 26 Jun 2012 10:23:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] ipv4: Remove unnecessary code from rt_check_expire().

On Tue, 2012-06-26 at 00:46 -0700, David Miller wrote:

> And for legitimate traffic it's completely the wrong thing to do.
> 
> There is absolutely zero reason to pure valid entries when hash chains
> average length of one.
> 
> I've been monitoring routing cache activity, and it's the height of
> stupidity.  Every 5 minutes we pure, and then they all get regenerated
> again.
> 

Thats because gc_interval (60) is big compared to ip_rt_gc_timeout
(300)

So each time rt_check_expire() triggers, we handle a big part of the
cache. On big servers I had to lower gc_interval to smooth things.

Garbage collect is needed to not waste kernel memory, even on legitimate
traffic on a typical web server.

Taken from my 8GB machine : 

# cat /proc/sys/net/ipv4/route/gc_thresh
262144

320 bytes per dst : 262144*320 = 83886080 bytes to store one dst per hash chain.

Also, why keeping a dst in cache if no traffic uses it in a 5 minutes period ?

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