linux-kernel - Re: [patch] mm, slab: avoid high-order slab pages when it does not reduce waste

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Message-ID: <0100016679e54c6e-67ca8716-c95e-427f-aec9-a5bee5e84792-000000@email.amazonses.com>
Date:   Mon, 15 Oct 2018 22:42:42 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     David Rientjes <rientjes@...gle.com>
cc:     Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [patch] mm, slab: avoid high-order slab pages when it does not
 reduce waste

On Fri, 12 Oct 2018, David Rientjes wrote:

> @@ -1803,6 +1804,20 @@ static size_t calculate_slab_order(struct kmem_cache *cachep,
>  		 */
>  		if (left_over * 8 <= (PAGE_SIZE << gfporder))
>  			break;
> +
> +		/*
> +		 * If a higher gfporder would not reduce internal fragmentation,
> +		 * no need to continue.  The preference is to keep gfporder as
> +		 * small as possible so slab allocations can be served from
> +		 * MIGRATE_UNMOVABLE pcp lists to avoid stranding.
> +		 */

I think either go for order 0 (because then you can use the pcp lists) or
go as high as possible (then you can allocator larger memory areas with a
single pass through the page allocator).

But then I am not sure that the whole approach will do any good.