On 10 Sep 2020, at 10:34, David Hildenbrand wrote: >>> As long as we stay in safe zone boundaries you get a benefit in most >>> scenarios. As soon as we would have a (temporary) workload that would >>> require more unmovable allocations we would fallback to polluting some >>> pageblocks only. >> >> The idea would work well until unmoveable pages begin to overflow into >> ZONE_PREFER_MOVABLE or we move the boundary of ZONE_PREFER_MOVABLE to >> avoid unmoveable page overflow. The issue comes from the lifetime of >> the unmoveable pages. Since some long-live ones can be around the boundary, >> there is no guarantee that ZONE_PREFER_MOVABLE cannot grow back >> even if other unmoveable pages are deallocated. Ultimately, >> ZONE_PREFER_MOVABLE would be shrink to a small size and the situation is >> back to what we have now. > > As discussed this would not happen in the usual case in case we size it > reasonable. Of course, if you push it to the extreme (which was never > suggested!), you would create mess. There is always a way to create a > mess if you abuse such mechanism. Also see Rik's reply regarding reclaim. > >> >> OK. I have a stupid question here. Why not just grow pageblock to a larger >> size, like 1GB? So the fragmentation of unmoveable pages will be at larger >> granularity. But it is less likely unmoveable pages will be allocated at >> a movable pageblock, since the kernel has 1GB pageblock for them after >> a pageblock stealing. If other kinds of pageblocks run out, moveable and >> reclaimable pages can fall back to unmoveable pageblocks. >> What am I missing here? > > Oh no. For example pageblocks have to completely fit into a single > section (that's where metadata is maintained). Please refrain from > suggesting to increase the section size ;) Thank you for the explanation. I have no idea about the restrictions on pageblock and section. Out of curiosity, what prevents the growth of the section size? — Best Regards, Yan Zi
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