On Thu, Mar 21, 2013 at 12:25:18PM -0400, Johannes Weiner wrote:
> On Sun, Mar 17, 2013 at 01:04:08PM +0000, Mel Gorman wrote:
> > Simplistically, the anon and file LRU lists are scanned proportionally
> > depending on the value of vm.swappiness although there are other factors
> > taken into account by get_scan_count(). The patch "mm: vmscan: Limit
> > the number of pages kswapd reclaims" limits the number of pages kswapd
> > reclaims but it breaks this proportional scanning and may evenly shrink
> > anon/file LRUs regardless of vm.swappiness.
> >
> > This patch preserves the proportional scanning and reclaim. It does mean
> > that kswapd will reclaim more than requested but the number of pages will
> > be related to the high watermark.
>
> Swappiness is about page types, but this implementation compares all
> LRUs against each other, and I'm not convinced that this makes sense
> as there is no guaranteed balance between the inactive and active
> lists. For example, the active file LRU could get knocked out when
> it's almost empty while the inactive file LRU has more easy cache than
> the anon lists combined.
>
Ok, I see your point. I think Michal was making the same point but I
failed to understand it the first time around.
> Would it be better to compare the sum of file pages with the sum of
> anon pages and then knock out the smaller pair?
Yes, it makes more sense but the issue then becomes how can we do that
sensibly, The following is straight-forward and roughly in line with your
suggestion but it does not preseve the scanning ratio between active and
inactive of the remaining LRU lists.
/*
* For kswapd and memcg, reclaim at least the number of pages
* requested. Ensure that the anon and file LRUs shrink
* proportionally what was requested by get_scan_count(). We
* stop reclaiming one LRU and reduce the amount scanning
* required on the other.
*/
nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
if (nr_file > nr_anon) {
nr[LRU_INACTIVE_FILE] -= min(nr_anon, nr[LRU_INACTIVE_FILE]);
nr[LRU_ACTIVE_FILE] -= min(nr_anon, nr[LRU_ACTIVE_FILE]);
nr[LRU_INACTIVE_ANON] = nr[LRU_ACTIVE_ANON] = 0;
} else {
nr[LRU_INACTIVE_ANON] -= min(nr_file, nr[LRU_INACTIVE_ANON]);
nr[LRU_ACTIVE_ANON] -= min(nr_file, nr[LRU_ACTIVE_ANON]);
nr[LRU_INACTIVE_FILE] = nr[LRU_ACTIVE_FILE] = 0;
}
scan_adjusted = true;
Preserving the ratio gets complicated and to avoid excessive branching,
it ends up looking like the following untested code.
/*
* For kswapd and memcg, reclaim at least the number of pages
* requested. Ensure that the anon and file LRUs shrink
* proportionally what was requested by get_scan_count(). We
* stop reclaiming one LRU and reduce the amount scanning
* required on the other preserving the ratio between the
* active/inactive lists.
*
* Start by preparing to shrink the larger of the LRUs by
* the size of the smaller list.
*/
nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];
nr_shrink = (nr_file > nr_anon) ? nr_anon : nr_file;
lru = (nr_file > nr_anon) ? LRU_FILE : 0;
/* Work out the ratio of the inactive/active list */
top = min(nr[LRU_ACTIVE + lru], nr[lru]);
bottom = max(nr[LRU_ACTIVE + lru], nr[lru]);
percentage = top * 100 / bottom;
nr_fraction = nr_shrink * percentage / 100;
nr_remaining = nr_anon - nr_fraction;
/* Reduce the remaining pages to scan proportionally */
if (nr[LRU_ACTIVE + lru] > nr[lru]) {
nr[LRU_ACTIVE + lru] -= min(nr_remaining, nr[LRU_ACTIVE + lru]);
nr[lru] -= min(nr_fraction, nr[lru]);
} else {
nr[LRU_ACTIVE + lru] -= min(nr_fraction, nr[LRU_ACTIVE + lru]);
nr[lru] -= min(nr_remaining, nr[lru]);
}
/* Stop scanning the smaller LRU */
lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
nr[LRU_ACTIVE + lru] = 0;
nr[lru] = 0;
Is this what you had in mind or had you something simplier in mind?
--
Mel Gorman
SUSE Labs
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