On Fri, Dec 01, 2023 at 10:33:01AM +0100, Michal Hocko wrote: > On Thu 30-11-23 11:56:42, Johannes Weiner wrote: > [...] > > So I wouldn't say it's merely a reclaim hint. It controls a very > > concrete and influential factor in VM decision making. And since the > > global swappiness is long-established ABI, I don't expect its meaning > > to change significantly any time soon. > > As I've said I am more worried about potential future changes which > would modify existing, reduce or add more corner cases which would be > seen as a change of behavior from the user space POV. That means that we > would have to be really explicit about the fact that the reclaim is free > to override the swappiness provided by user. So essentially a best > effort interface without any actual guarantees. That surely makes it > harder to use. Is it still useable? For our needs (limiting swapout and avoiding swap-depletion) we rely on two semantics of vm.swappiness. 1) Lower swappiness results in less swap-out, more swappiness results in more swap-out - for the same workload. Our proactive reclaimer monitors swap-out and lowers swappiness in response if we exceed our target swap-out rate. 2) swappiness = 0 results in no or very little swap-out. We rely on this to avoid exhausting swap due to proactive reclaim and triggering OOMs. We already depend on these semantics of vm.swappiness *today*. I think changing either of these would be seen as a behavior change from user space POV irrespective of this patch. The proposal in this patch only allows for vm.swappiness (whatever its semantics) to be configured separately for proactive reclaim.
