Re: [VM PATCH] rotate_reclaimable_page fails frequently

[Nick Piggin <nickpiggin@yahoo.com.au>]

Marcelo Tosatti wrote:

> Marking PG_writeback pages as PG_rotated once they're chosen candidates
> for eviction increases the number of rotated pages dramatically, but
> that does not necessarily increase performance (I was unable to see any
> performance increase under the limited testing I've done, even though
> the pgrotated numbers were _way_ higher).
> 

Just FYI, this change can end up leaking the PageReclaim bit
which IIRC can make bad noises in the free pages check, and
is also a tiny bit sloppy unless we also do a precautionary
ClearPageReclaim in writeback paths.

However I don't think it is a bad idea in theory.

> Another issue is that increasing the number of rotated pages increases
> lru_lock contention, which might not be an advantage for certain
> workloads.
> 
> So, any change in this area needs careful study under a varied,
> meaningful set of workloads and configurations (which has not been
> happening very often).
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5a61080..26319eb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -447,8 +447,14 @@ static int shrink_list(struct list_head 
>  		if (page_mapped(page) || PageSwapCache(page))
>  			sc->nr_scanned++;
>  
> -		if (PageWriteback(page))
> +		if (PageWriteback(page)) {
> +			/* mark writeback, candidate for eviction pages as 
> +			 * PG_reclaim to free them immediately once they're 
> +			 * laundered.
> +			 */
> +			SetPageReclaim(page);
>  			goto keep_locked;
> +		}
>  
>  		referenced = page_referenced(page, 1);
>  		/* In active use or really unfreeable?  Activate it. */
> 

-- 
SUSE Labs, Novell Inc.
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