Re: [PATCH 09/19] list_lru: per-node list infrastructure

[Glauber Costa]

On 11/27/2012 03:14 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Now that we have an LRU list API, we can start to enhance the
> implementation.  This splits the single LRU list into per-node lists
> and locks to enhance scalability. Items are placed on lists
> according to the node the memory belongs to. To make scanning the
> lists efficient, also track whether the per-node lists have entries
> in them in a active nodemask.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  include/linux/list_lru.h |   14 ++--
>  lib/list_lru.c           |  160 +++++++++++++++++++++++++++++++++++-----------
>  2 files changed, 129 insertions(+), 45 deletions(-)
> 
> diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> index 3423949..b0e3ba2 100644
> --- a/include/linux/list_lru.h
> +++ b/include/linux/list_lru.h
> @@ -8,21 +8,23 @@
>  #define _LRU_LIST_H 0
>  
>  #include <linux/list.h>
> +#include <linux/nodemask.h>
>  
> -struct list_lru {
> +struct list_lru_node {
>  	spinlock_t		lock;
>  	struct list_head	list;
>  	long			nr_items;
> +} ____cacheline_aligned_in_smp;
> +
> +struct list_lru {
> +	struct list_lru_node	node[MAX_NUMNODES];
> +	nodemask_t		active_nodes;
>  };
>  
MAX_NUMNODES will default to 1 << 9, if I'm not mistaken. Your
list_lru_node seems to be around 32 bytes on 64-bit systems (128 with
debug). So we're talking about 16k per lru.
The superblocks only, are present by the dozens even in a small system,
and I believe the whole goal of this API is to get more users to switch
to it. This can easily use up a respectable bunch of megs.

Isn't it a bit too much ?

I am wondering if we can't do better in here and at least allocate+grow
according to the actual number of nodes.

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