[PATCH 3/3] Stop maximizing the bitmap buffer to reduce the risk of OOM.

[Baoquan He]

On 06/10/14 at 04:54am, Atsushi Kumagai wrote:
> We tried to maximize the bitmap buffer to get the best performance,
> but the performance degradation caused by multi-cycle processing
> looks very small according to the benchmark on 2TB memory:
> 
>   https://lkml.org/lkml/2013/3/26/914
> 
> This result means we don't need to make an effort to maximize the
> bitmap buffer, it will just increase the risk of OOM.
> 
> This patch sets a small fixed value (4MB) as a safety limit,
> it may be safer and enough in most cases.
> 
> Signed-off-by: Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp>
> ---
>  makedumpfile.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/makedumpfile.c b/makedumpfile.c
> index b8f1ec4..c23667b 100644
> --- a/makedumpfile.c
> +++ b/makedumpfile.c
> @@ -8946,13 +8946,15 @@ out:
>  
>  
>  /*
> - * Choose the lesser value of the two below as the size of cyclic buffer.
> - *  - the size enough for storing the 1st/2nd bitmap for the whole of vmcore
> - *  - 80% of free memory (as safety limit)
> + * Choose the lesser value of the three below as the size of cyclic buffer.
                 ~~~~~ typo, it should be less.
> + *  - the size enough for storing the 1st or 2nd bitmap for the whole of vmcore
> + *  - 4MB as sufficient value
> + *  - 60% of free memory as safety limit
>   */
>  int
>  calculate_cyclic_buffer_size(void) {
>  	unsigned long long limit_size, bitmap_size;
> +	const unsigned long long maximum_size = 4 * 1024 * 1024;
>  
>  	if (info->max_mapnr <= 0) {
>  		ERRMSG("Invalid max_mapnr(%llu).\n", info->max_mapnr);
> @@ -8960,17 +8962,18 @@ calculate_cyclic_buffer_size(void) {
>  	}
>  
>  	/*
> -	 *  We should keep the size of cyclic buffer within 80% of free memory
> -	 *  for safety.
> +	 *  At least, we should keep the size of cyclic buffer within 60% of
> +	 *  free memory for safety.
>  	 */
> -	limit_size = get_free_memory_size() * 0.8;
> +	limit_size = get_free_memory_size() * 0.6;
>  	bitmap_size = info->max_mapnr / BITPERBYTE;
>  
>  	/* if --split was specified cyclic buffer allocated per dump file */
>  	if (info->num_dumpfile > 1)
>  		bitmap_size /= info->num_dumpfile;
>  
> -	info->bufsize_cyclic = MIN(limit_size, bitmap_size);
> +	/* 4MB will be enought for performance according to benchmarks. */
                      ~~~~~~~~   
There's a typo, here should be enough.

> +	info->bufsize_cyclic = MIN(MIN(limit_size, maximum_size), bitmap_size);
>  
>  	return TRUE;
>  }
> -- 
> 1.9.0
> 
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