On 1/24/2018 9:25 PM, Vinayak Menon wrote: > 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 Ping for comments, thanks. -- 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>
