On Tue, Jan 26, 2016 at 04:00:05PM -0500, Johannes Weiner wrote: > For per-cgroup thrash detection, we need to store the memcg ID inside > the radix tree cookie as well. However, on 32 bit that doesn't leave > enough bits for the eviction timestamp to cover the necessary range of > recently evicted pages. The radix tree entry would look like this: > > [ RADIX_TREE_EXCEPTIONAL(2) | ZONEID(2) | MEMCGID(16) | EVICTION(12) ] > > 12 bits means 4096 pages, means 16M worth of recently evicted pages. > But refaults are actionable up to distances covering half of memory. > To not miss refaults, we have to stretch out the range at the cost of > how precisely we can tell when a page was evicted. This way we can > shave off lower bits from the eviction timestamp until the necessary > range is covered. E.g. grouping evictions into 1M buckets (256 pages) > will stretch the longest representable refault distance to 4G. > > This patch implements eviction buckets that are automatically sized > according to the available bits and the necessary refault range, in > preparation for per-cgroup thrash detection. > > The maximum actionable distance is currently half of memory, but to > support memory hotplug of up to 200% of boot-time memory, we size the > buckets to cover double the distance. Beyond that, thrashing won't be > detectable anymore. > > During boot, the kernel will print out the exact parameters, like so: > > [ 0.113929] workingset: timestamp_bits=12 max_order=18 bucket_order=6 > > In this example, there are 12 radix entry bits available for the > eviction timestamp, to cover a maximum distance of 2^18 pages (this is > a 1G machine). Consequently, evictions must be grouped into buckets of > 2^6 pages, or 256K. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> One nit below. > +/* > + * Eviction timestamps need to be able to cover the full range of > + * actionable refaults. However, bits are tight in the radix tree > + * entry, and after storing the identifier for the lruvec there might > + * not be enough left to represent every single actionable refault. In > + * that case, we have to sacrifice granularity for distance, and group > + * evictions into coarser buckets by shaving off lower timestamp bits. > + */ > +static unsigned int bucket_order; __read_mostly? Thanks, Vladimir -- 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>