On Tue, 30 Oct 2012 14:42:04 -0400
Rik van Riel <riel@xxxxxxxxxx> wrote:
> If we have more inactive file pages than active file pages, we
> skip scanning the active file pages alltogether, with the idea
> that we do not want to evict the working set when there is
> plenty of streaming IO in the cache.
Yes, I've never liked that. The "(active > inactive)" thing is a magic
number. And suddenly causing a complete cessation of vm scanning at a
particular magic threshold seems rather crude, compared to some complex
graduated thing which will also always do the wrong thing, only more
obscurely ;)
Ho hum, in the absence of observed problems, I guess we don't muck with
it.
> However, the code forgot to also skip scanning anonymous pages
> in that situation. That lead to the curious situation of keeping
> the active file pages protected from being paged out when there
> are lots of inactive file pages, while still scanning and evicting
> anonymous pages.
>
> This patch fixes that situation, by only evicting file pages
> when we have plenty of them and most are inactive.
>
Any observed runtime effects from this? If so, were they good?
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1686,6 +1686,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> fraction[1] = 0;
> denominator = 1;
> goto out;
> + } else if (!inactive_file_is_low_global(zone)) {
> + /*
> + * There is enough inactive page cache, do not
> + * reclaim anything from the working set right now.
> + */
> + fraction[0] = 0;
> + fraction[1] = 1;
> + denominator = 1;
> + goto out;
> }
> }
Let's make the commenting look logical:
--- a/mm/vmscan.c~mmvmscan-only-evict-file-pages-when-we-have-plenty-fix
+++ a/mm/vmscan.c
@@ -1679,9 +1679,11 @@ static void get_scan_count(struct lruvec
if (global_reclaim(sc)) {
free = zone_page_state(zone, NR_FREE_PAGES);
- /* If we have very few page cache pages,
- force-scan anon pages. */
if (unlikely(file + free <= high_wmark_pages(zone))) {
+ /*
+ * If we have very few page cache pages, force-scan
+ * anon pages.
+ */
fraction[0] = 1;
fraction[1] = 0;
denominator = 1;
_
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