linux-kernel - Re: [RFC PATCH V3] mm readahead: Fix the readahead fail in case of empty numa node

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Date:	Mon, 6 Jan 2014 11:56:20 +0100
From:	Jan Kara <jack@...e.cz>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>, jack@...e.cz,
	Linus <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V3] mm readahead: Fix the readahead fail in case of
 empty numa node

On Mon 06-01-14 15:51:55, Raghavendra K T wrote:
> Currently, max_sane_readahead returns zero on the cpu with empty numa node,
> fix this by checking for potential empty numa node case during calculation.
> We also limit the number of readahead pages to 4k.
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> ---
> The current patch limits the readahead into 4k pages (16MB was suggested
> by Linus).  and also handles the case of memoryless cpu issuing readahead
> failures.  We still do not consider [fm]advise() specific calculations
> here.  I have dropped the iterating over numa node to calculate free page
> idea.  I do not have much idea whether there is any impact on big
> streaming apps..  Comments/suggestions ?
  As you say I would be also interested what impact this has on a streaming
application. It should be rather easy to check - create 1 GB file, drop
caches. Then measure how long does it take to open the file, call fadvise
FADV_WILLNEED, read the whole file (for a kernel with and without your
patch). Do several measurements so that we get some meaningful statistics.
Resulting numbers can then be part of the changelog. Thanks!

								Honza

>  mm/readahead.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 7cdbb44..be4d205 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -237,14 +237,25 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
>  	return ret;
>  }
>  
> +#define MAX_REMOTE_READAHEAD   4096UL
>  /*
>   * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
>   * sensible upper limit.
>   */
>  unsigned long max_sane_readahead(unsigned long nr)
>  {
> -	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
> -		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
> +	unsigned long local_free_page;
> +	unsigned long sane_nr = min(nr, MAX_REMOTE_READAHEAD);
> +
> +	local_free_page = node_page_state(numa_node_id(), NR_INACTIVE_FILE)
> +			  + node_page_state(numa_node_id(), NR_FREE_PAGES);
> +
> +	/*
> +	 * Readahead onto remote memory is better than no readahead when local
> +	 * numa node does not have memory. We sanitize readahead size depending
> +	 * on free memory in the local node but limiting to 4k pages.
> +	 */
> +	return local_free_page ? min(sane_nr, local_free_page / 2) : sane_nr;
>  }
>  
>  /*
> -- 
> 1.7.11.7
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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