Re: [PATCH] readahead: fix initial window size calculation

[Steven Pratt]

Oleg Nesterov wrote:

> (I hope it is ok to send this patch without Steven's reply)
>  

Yes, It is fine.

> From: Steven Pratt <slpratt@xxxxxxxxxxxxxx>
>
> The current current get_init_ra_size is not optimal across different IO
> sizes and max_readahead values.  Here is a quick summary of sizes
> computed under current design and under the attached patch.  All of
> these assume 1st IO at offset 0, or 1st detected sequential IO.
>
> 32k max, 4k request
>
> old         new
> -----------------
> 8k        8k
> 16k       16k
> 32k       32k
>
> 128k max, 4k request
> old         new
> -----------------
> 32k         16k
> 64k         32k
> 128k        64k
> 128k       128k
>
> 128k max, 32k request
> old         new
> -----------------
> 32k         64k    <-----
> 64k        128k
> 128k       128k
>
> 512k max, 4k request
> old         new
> -----------------
> 4k         32k     <----
> 16k        64k
> 64k       128k
> 128k      256k
> 512k      512k
>
> --- 2.6.16-rc3/mm/readahead.c~	2006-02-27 00:53:17.881019192 +0300
> +++ 2.6.16-rc3/mm/readahead.c	2006-02-27 01:10:39.172718792 +0300
> @@ -83,10 +83,10 @@ static unsigned long get_init_ra_size(un
> {
> 	unsigned long newsize = roundup_pow_of_two(size);
>
> -	if (newsize <= max / 64)
> -		newsize = newsize * newsize;
> +	if (newsize <= max / 32)
> +		newsize = newsize * 4;
> 	else if (newsize <= max / 4)
> -		newsize = max / 4;
> +		newsize = newsize * 2;
> 	else
> 		newsize = max;
> 	return newsize;
>  

-
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