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SubjectRe: [PATCH RFC] mm readahead: Fix the readahead fail in case of empty numa node
On 12/05/2013 03:18 AM, Andrew Morton wrote:
> On Wed, 04 Dec 2013 14:38:11 +0530 Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
>
>> On 12/04/2013 02:11 PM, Andrew Morton wrote:
> :
> : This patch takes it all out and applies the same upper limit as is used in
> : sys_readahead() - half the inactive list.
> :
> : +/*
> : + * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
> : + * sensible upper limit.
> : + */
> : +unsigned long max_sane_readahead(unsigned long nr)
> : +{
> : + unsigned long active;
> : + unsigned long inactive;
> : +
> : + get_zone_counts(&active, &inactive);
> : + return min(nr, inactive / 2);
> : +}
>

Hi Andrew, Thanks for digging out. So it seems like earlier we had not
even considered free pages?

> And one would need to go back further still to understand the rationale
> for the sys_readahead() decision and that even predates the BK repo.
>
> iirc the thinking was that we need _some_ limit on readahead size so
> the user can't go and do ridiculously large amounts of readahead via
> sys_readahead(). But that doesn't make a lot of sense because the user
> could do the same thing with plain old read().
>

True.

> So for argument's sake I'm thinking we just kill it altogether and
> permit arbitrarily large readahead:
>
> --- a/mm/readahead.c~a
> +++ a/mm/readahead.c
> @@ -238,13 +238,12 @@ int force_page_cache_readahead(struct ad
> }
>
> /*
> - * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
> - * sensible upper limit.
> + * max_sane_readahead() is disabled. It can later be removed altogether, but
> + * let's keep a skeleton in place for now, in case disabling was the wrong call.
> */
> unsigned long max_sane_readahead(unsigned long nr)
> {
> - return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
> - + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
> + return nr;
> }
>

I had something like below in mind for posting. But it looks
simple now with your patch.


unsigned long max_sane_readahead(unsigned long nr)
{
int nid;
unsigned long free_page = 0;

for_each_node_state(nid, N_MEMORY)
free_page += node_page_state(nid, NR_INACTIVE_FILE)
+ node_page_state(nid, NR_FREE_PAGES);

/*
* Readahead onto remote memory is better than no readahead when local
* numa node does not have memory. We sanitize readahead size depending
* on potential free memory in the whole system.
*/
return min(nr, free_page / (2 * nr_node_ids));

Or if we wanted to avoid iteration on nodes simply returning

something like nr/8 or something like that for remote numa fault cases.




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