netdev - Re: Strange CPU load when flushing route cache (kernel 2.6.31.6)

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Date:	Mon, 23 Nov 2009 11:29:14 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <jdb@...x.dk>
CC:	Linux Kernel Network Hackers <netdev@...r.kernel.org>,
	Robert Olsson <robert@...julf.net>
Subject: Re: Strange CPU load when flushing route cache (kernel 2.6.31.6)

Jesper Dangaard Brouer a écrit :
> Hi Eric and netdev,
> 
> I have observed a strange route cache behaviour when I upgraded some
> of my production Linux routers (1Gbit/s tg3) to kernel 2.6.31.6 (from
> kernel 2.6.25.7).
> 
> Every time the route cache is flushed I get a CPU spike (in softirq)
> with a tail.  I have attached some graphs that illustrate the issue
> (hope vger.kernel.org will allow these attachments...)
> 
> 
> I have done some tuning of the route cache:
> 
>  # From /etc/sysctl.conf
>  #
>  # Adjusting the route cache flush interval
>  net/ipv4/route/secret_interval = 1200
> 
>  # Limiting the route cache size
>  # ip_dst_cache slab objects is 256 bytes.
>  # 2000000 * 256 bytes = 512 MB
>  net/ipv4/route/max_size = 2000000
> 
> Boot parameters: "rhash_entries=262143 vmalloc=256M"
> 
> The rhash_entries is for the route cache hash size.  The vmalloc is
> needed because I have _very_ large iptables rulesets (and is running
> on a 32-bit kernel, due to old hardware).
> 
> Any thoughs on how to avoid these CPU spikes?
> Or where the issue occurs in the code?
> 

Sure, after a flush, we have to rebuild the cache, so extra work is expected.

(We receive a packet, notice the cached entry is obsolete, free it, allocate a new one
and inert it into cache)

If you dont want these spikes, just dont flush cache :)

Do you run a 2G/2G User/Kernel split kernel ?

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