netdev - Re: [PATCH] Limit size of route cache hash table

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Date:	Mon, 27 Apr 2009 07:17:42 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Anton Blanchard <anton@...ba.org>
CC:	netdev@...r.kernel.org
Subject: Re: [PATCH] Limit size of route cache hash table

Anton Blanchard a écrit :
> Right now we have no upper limit on the size of the route cache hash table.
> On a 128GB POWER6 box it ends up as 32MB:
> 
>     IP route cache hash table entries: 4194304 (order: 9, 33554432 bytes)
> 
> It would be nice to cap this just for memory consumption reasons, but this
> massive hashtable also causes a significant spike when measuring OS
> jitter.
> 
> With a 32MB hashtable and 4 million entries, rt_worker_func is taking
> 5 ms to complete. On another system with more memory it's taking 14 ms.
> Even though rt_worker_func does call cond_sched() to limit its impact,
> in an HPC environment we want to keep all sources of OS jitter to a minimum.

Then boot with rhash_entries = 8000 ?
or 
echo 1 >/proc/sys/net/ipv4/route/gc_interval
> 
> With the patch applied we limit the number of entries to 64k which
> can still be overriden by using the rt_entries boot option:
> 
>     IP route cache hash table entries: 65536 (order: 3, 524288 bytes)
> 
> With this patch rt_worker_func takes 0.060 ms on the same system.
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> ---
> 
> Is 64k a reasonable default for the limit?
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index c40debe..5064c26 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -3397,7 +3397,7 @@ int __init ip_rt_init(void)
>  					0,
>  					&rt_hash_log,
>  					&rt_hash_mask,
> -					0);
> +					rhash_entries ? 0 : 64 * 1024);
>  	memset(rt_hash_table, 0, (rt_hash_mask + 1) * sizeof(struct rt_hash_bucket));
>  	rt_hash_lock_init();
>  
> 


Sorry this limit is too small. Many of my customer machines would collapse.

It would be smart to eventually change ip_rt_gc_interval from 60 
to 1 second for such machines ? Dividing 5 ms per 60 gives 83 us, which
is correct. 


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