Re: [RFC PATCH V4] mm readahead: Fix readahead fail for no local memory and limit readahead pages

[Jan Kara <jack@suse.cz>]

On Fri 10-01-14 00:54:50, Raghavendra K T wrote:
> We limit the number of readahead pages to 4k.
> 
> max_sane_readahead returns zero on the cpu having no local memory
> node. Fix that by returning a sanitized number of pages viz.,
> minimum of (requested pages, 4k, number of local free pages)
> 
> Result:
> fadvise experiment with FADV_WILLNEED on a x240 machine with 1GB testfile
> 32GB* 4G RAM  numa machine ( 12 iterations) yielded
> 
> kernel       Avg        Stddev
> base         7.264      0.56%
> patched      7.285      1.14%
  OK, looks good to me. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> ---
>  mm/readahead.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> V4:  incorporated 16MB limit suggested by Linus for readahead and
> fixed transitioning to large readahead anomaly pointed by Andrew Morton with
> Honza's suggestion.
> 
> Test results shows no significant overhead with the current changes.
> 
> (Do I have to break patches into two??)
> 
> Suggestions/Comments please let me know.
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 7cdbb44..2f561a0 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -237,14 +237,30 @@ 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;
> +	int nid;
> +
> +	nid = numa_node_id();
> +	sane_nr = min(nr, MAX_REMOTE_READAHEAD);
> +
> +	local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
> +			  + node_page_state(nid, 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 node_present_pages(nid) ?
> +				min(sane_nr, local_free_page / 2) : sane_nr;
>  }
>  
>  /*
> -- 
> 1.7.11.7
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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