On Tue, 25 Nov 2014 13:23:50 -0500 Johannes Weiner <[email protected]> wrote:
> The slab shrinkers are currently invoked from the zonelist walkers in > kswapd, direct reclaim, and zone reclaim, all of which roughly gauge > the eligible LRU pages and assemble a nodemask to pass to NUMA-aware > shrinkers, which then again have to walk over the nodemask. This is > redundant code, extra runtime work, and fairly inaccurate when it > comes to the estimation of actually scannable LRU pages. The code > duplication will only get worse when making the shrinkers cgroup-aware > and requiring them to have out-of-band cgroup hierarchy walks as well. > > Instead, invoke the shrinkers from shrink_zone(), which is where all > reclaimers end up, to avoid this duplication. > > Take the count for eligible LRU pages out of get_scan_count(), which > considers many more factors than just the availability of swap space, > like zone_reclaimable_pages() currently does. Accumulate the number > over all visited lruvecs to get the per-zone value. > > Some nodes have multiple zones due to memory addressing restrictions. > To avoid putting too much pressure on the shrinkers, only invoke them > once for each such node, using the class zone of the allocation as the > pivot zone. > > For now, this integrates the slab shrinking better into the reclaim > logic and gets rid of duplicative invocations from kswapd, direct > reclaim, and zone reclaim. It also prepares for cgroup-awareness, > allowing memcg-capable shrinkers to be added at the lruvec level > without much duplication of both code and runtime work. > > This changes kswapd behavior, which used to invoke the shrinkers for > each zone, but with scan ratios gathered from the entire node, > resulting in meaningless pressure quantities on multi-zone nodes. It's a troublesome patch - we've been poking at this code for years and now it gets significantly upended. It all *seems* sensible, but any warts will take time to identify. > Zone reclaim behavior also changes. It used to shrink slabs until the > same amount of pages were shrunk as were reclaimed from the LRUs. Now > it merely invokes the shrinkers once with the zone's scan ratio, which > makes the shrinkers go easier on caches that implement aging and would > prefer feeding back pressure from recently used slab objects to unused > LRU pages. hm, "go easier on caches" means it changes reclaim balancing. Is the result better or worse? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
