netdev - Re: 0% cpu usasge after fresh boot or net restart but 10% CPU if kernel flush route cache

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Date:	Wed, 27 Jan 2010 16:26:16 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	cold cold <nedkonedev@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: 0% cpu usasge after fresh boot or net restart but 10% CPU if
 kernel  flush route cache

Le mardi 26 janvier 2010 à 18:58 +0200, cold cold a écrit :
> HI,
> 
> 
> i have expiriance some CPU usage spikes up to 10% on each four cpus
> after the first kernel route cache flush.
> After mashine start first 20 min CPU is 0%si  300Mbits/s full duplex
> and arount 100k pps forwarded traffic, without any firewall, just
> plain routing.
> 
> route -n |wc -l
> 34
> 
> ip route show cache | wc -l
> 2140842
> 
> cat /proc/sys/net/ipv4/route/secret_interval
> 600
> cat /proc/sys/net/ipv4/route/max_size
> 33554432
> 
> after kernel  flush i got 10% on all CPUs for 5-6 mins.  It's not from
> rebuilding route cashe becouse after
> fresh boot or network restrat there is no CPU usage until kernel flush
> route cache.
> I try to play with rhash_entries= 300000 to 2000000 same result.

If you have one million dst entries to flush, it takes some time.

You could try to not increase the rhash_entries 
(or keep it low, say 131072)
but tune /proc/sys/net/ipv4/route settings.

Try to reduce gc_elasticity from 8 to 2
Try to reduce gc_interval from 60 to 1

Important thing to consider is to irq affinities (so that one cpu
handles network interrupts, to minimize cache ping poings )



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