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Date: Thu, 22 Jan 2015 16:45:03 +0100
From: Vlastimil Babka <vbabka@suse.cz>
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To: WANG Chao <chaowang@redhat.com>, Andrew Morton <akpm@linux-foundation.org>
CC: Davidlohr Bueso <dave@stgolabs.net>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michel Lespinasse <walken@google.com>, Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm, vmacache: Add kconfig VMACACHE_SHIFT
References: <1421908189-18938-1-git-send-email-chaowang@redhat.com>
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On 01/22/2015 07:29 AM, WANG Chao wrote:
> Add a new kconfig option VMACACHE_SHIFT (as a power of 2) to specify the
> number of slots vma cache has for each thread. Range is chosen 0-4 (1-16
> slots) to consider both overhead and performance penalty. Default is 2

One could say that overhead and performance penalty is the same thing. 
Please elaborate?

Also, got any performance numbers to share for workloads benefiting from 
more/less than the default?

> (4 slots) as it originally is, which provides good enough balance.
>
> Signed-off-by: WANG Chao <chaowang@redhat.com>
> ---
>   include/linux/sched.h | 2 +-
>   mm/Kconfig            | 7 +++++++
>   2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8db31ef..56fd96d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -134,7 +134,7 @@ struct perf_event_context;
>   struct blk_plug;
>   struct filename;
>
> -#define VMACACHE_BITS 2
> +#define VMACACHE_BITS CONFIG_VMACACHE_SHIFT
>   #define VMACACHE_SIZE (1U << VMACACHE_BITS)
>   #define VMACACHE_MASK (VMACACHE_SIZE - 1)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 1d1ae6b..7b82a52 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -618,3 +618,10 @@ config MAX_STACK_SIZE_MB
>   	  changed to a smaller value in which case that is used.
>
>   	  A sane initial value is 80 MB.
> +
> +config VMACACHE_SHIFT
> +	int "Number of slots in per-thread VMA cache (as a power of 2)"
> +	range 0 4
> +	default 2
> +	help
> +	  This is the number of slots VMA cache has for each thread.

As a user, I wouldn't find this informative enough to make the decision.