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

[Steven Pratt <slpratt@austin.ibm.com>]

Oleg Nesterov wrote:

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

Yes, It is fine.

>From: Steven Pratt <slpratt@austin.ibm.com>
>
>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;
>  
>