On Tue 29-12-20 22:35:13, Feng Tang wrote: > When checking a memory cgroup related performance regression [1], > from the perf c2c profiling data, we found high false sharing for > accessing 'usage' and 'parent'. > > On 64 bit system, the 'usage' and 'parent' are close to each other, > and easy to be in one cacheline (for cacheline size == 64+ B). 'usage' > is usally written, while 'parent' is usually read as the cgroup's > hierarchical counting nature. > > So move the 'parent' to the end of the structure to make sure they > are in different cache lines. Yes, parent is write-once field so having it away from other heavy RW fields makes sense to me. > Following are some performance data with the patch, against > v5.11-rc1, on several generations of Xeon platforms. Most of the > results are improvements, with only one malloc case on one platform > shows a -4.0% regression. Each category below has several subcases > run on different platform, and only the worst and best scores are > listed: > > fio: +1.8% ~ +8.3% > will-it-scale/malloc1: -4.0% ~ +8.9% > will-it-scale/page_fault1: no change > will-it-scale/page_fault2: +2.4% ~ +20.2% What is the second number? Std? > [1].https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/ > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx> > Cc: Roman Gushchin <guro@xxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > --- > include/linux/page_counter.h | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h > index 85bd413..6795913 100644 > --- a/include/linux/page_counter.h > +++ b/include/linux/page_counter.h > @@ -12,7 +12,6 @@ struct page_counter { > unsigned long low; > unsigned long high; > unsigned long max; > - struct page_counter *parent; > > /* effective memory.min and memory.min usage tracking */ > unsigned long emin; > @@ -27,6 +26,14 @@ struct page_counter { > /* legacy */ > unsigned long watermark; > unsigned long failcnt; > + > + /* > + * 'parent' is placed here to be far from 'usage' to reduce > + * cache false sharing, as 'usage' is written mostly while > + * parent is frequently read for cgroup's hierarchical > + * counting nature. > + */ > + struct page_counter *parent; > }; > > #if BITS_PER_LONG == 32 > -- > 2.7.4 > -- Michal Hocko SUSE Labs
