Re: Doc for your Adaptive ReadAhead patch

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Wed, Apr 11, 2007 at 04:44:34AM +0000, Jin Steve wrote:
> Just like the function check_ra_success: (Does it mean: last time 
> RA>VM_MAX_CACHE_HIT, so next time no need readahead?)

The cache hit logic is to prevent doing pointless readahead for
an already cached file: the pages are already there.

VM_MAX_CACHE_HIT is the threshold to disable readahead in this case.

> static inline int check_ra_success(struct file_ra_state *ra,
> 			unsigned long nr_to_read, unsigned long actual)
> {
> 	if (actual == 0) {
> 		ra->cache_hit += nr_to_read;
> 		if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
> 			ra_off(ra);
> 			ra->flags |= RA_FLAG_INCACHE;
> 			return 0;
> 		}
> 	} else {
> 		ra->cache_hit=0;
> 	}
> 	return 1;
> }
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