Hi, It is observed that watermark_scale_factor when used to reduce thundering herds in direct reclaim, reduces the direct reclaims, but results in unnecessary reclaim due to kswapd running for long after being woken up. The tests are done with 4 GB of RAM and the tests done are multibuild and another which opens a set of apps sequentially on Android and repeating the sequence N times. The tests are done on 4.9 kernel. The issue seems to be because of watermark_scale_factor creating larger gap between low and high watermarks. The following results are with watermark_scale_factor of 120 and the other with watermark_scale_factor 120 with a reduced gap between low and high watermarks. The patch used to reduce the gap is given below. The min-low gap is untouched. It can be seen that with the reduced low-high gap, the direct reclaims are almost same as base, but with 45% less pgpgin. Reduced low-high gap improves the latency by around 11% in the sequential app test due to lesser IO and kswapd activity. wsf-120-default wsf-120-reduced-low-high-gap workingset_activate 15120206 8319182 pgpgin 269795482 147928581 allocstall 1406 1498 pgsteal_kswapd 68676960 38105142 slabs_scanned 94181738 49085755 This is the diff of wsf-120-reduced-low-high-gap for comments. The patch considers low-high gap as a fraction of min-low gap, and the fraction a function of managed pages, increasing non-linearly. The multiplier 4 is was chosen as a reasonable value which does not alter the low-high gap much from the base for large machines. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 3a11a50..749d1eb 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6898,7 +6898,11 @@ static void __setup_per_zone_wmarks(void) watermark_scale_factor, 10000)); zone->watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp; - zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2; + + tmp = clamp_t(u64, mult_frac(tmp, int_sqrt(4 * zone->managed_pages), + 10000), min_wmark_pages(zone) >> 2 , tmp); + + zone->watermark[WMARK_HIGH] = low_wmark_pages(zone) + tmp; spin_unlock_irqrestore(&zone->lock, flags); } With the patch, With watermark_scale_factor as default 10, the low-high gap: unchanged for 140G at 143M, for 65G, reduces from 65M to 53M for 4GB, reduces from 4M to 1M With watermark_scale_factor 120, the low-high gap: unchanged for 140G for 65G, reduces from 786M to 644M for 4GB, reduces from 49M to 10M Thanks, Vinayak -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
