On 08/24/2017 06:39 PM, josef@xxxxxxxxxxxxxx wrote: > From: Josef Bacik <jbacik@xxxxxx> > > Previously we were using the ratio of the number of lru pages scanned to > the number of eligible lru pages to determine the number of slab objects > to scan. The problem with this is that these two things have nothing to > do with each other, so in slab heavy work loads where there is little to > no page cache we can end up with the pages scanned being a very low > number. This means that we reclaim next to no slab pages and waste a > lot of time reclaiming small amounts of space. > > Consider the following scenario, where we have the following values and > the rest of the memory usage is in slab > > Active: 58840 kB > Inactive: 46860 kB > > Every time we do a get_scan_count() we do this > > scan = size >> sc->priority > > where sc->priority starts at DEF_PRIORITY, which is 12. The first loop > through reclaim would result in a scan target of 2 pages to 11715 total > inactive pages, and 3 pages to 14710 total active pages. This is a > really really small target for a system that is entirely slab pages. > And this is super optimistic, this assumes we even get to scan these > pages. We don't increment sc->nr_scanned unless we 1) isolate the page, > which assumes it's not in use, and 2) can lock the page. Under > pressure these numbers could probably go down, I'm sure there's some > random pages from daemons that aren't actually in use, so the targets > get even smaller. > > Instead use sc->priority in the same way we use it to determine scan > amounts for the lru's. This generally equates to pages. Consider the > following > > slab_pages = (nr_objects * object_size) / PAGE_SIZE > > What we would like to do is > > scan = slab_pages >> sc->priority > > but we don't know the number of slab pages each shrinker controls, only > the objects. However say that theoretically we knew how many pages a > shrinker controlled, we'd still have to convert this to objects, which > would look like the following > > scan = shrinker_pages >> sc->priority > scan_objects = (PAGE_SIZE / object_size) * scan > > or written another way > > scan_objects = (shrinker_pages >> sc->priority) * > (PAGE_SIZE / object_size) > > which can thus be written > > scan_objects = ((shrinker_pages * PAGE_SIZE) / object_size) >> > sc->priority > > which is just > > scan_objects = nr_objects >> sc->priority > > We don't need to know exactly how many pages each shrinker represents, > it's objects are all the information we need. Making this change allows > us to place an appropriate amount of pressure on the shrinker pools for > their relative size. > > Signed-off-by: Josef Bacik <jbacik@xxxxxx> > Acked-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> -- 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>
