On Fri, Feb 2, 2024 at 2:15 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > On Fri, Feb 02, 2024 at 01:02:47PM +0800, Efly Young <yangyifei03@xxxxxxxxxxxx> wrote: > > > Looking at the code, I'm not quite sure if this can be read this > > > literally. Efly might be able to elaborate, but we do a full loop of > > > all nodes and cgroups in the tree before checking nr_to_reclaimed, and > > > rely on priority level for granularity. So request size and complexity > > > of the cgroup tree play a role. I don't know where the exact factor > > > two would come from. > > > > I'm sorry that this conclusion may be arbitrary. It might just only suit > > for my case. In my case, I traced it loop twice every time before checking > > nr_reclaimed, and it reclaimed less than my request size(1G) every time. > > So I think the upper bound is 2 * request. But now it seems that this is > > related to cgroup tree I constucted and my system status and my request > > size(a relatively large chunk). So there are many influencing factors, > > a specific upper bound is not accurate. > > Alright, thanks for the background. > > > > IMO it's more accurate to phrase it like this: > > > > > > Reclaim tries to balance nr_to_reclaim fidelity with fairness across > > > nodes and cgroups over which the pages are spread. As such, the bigger > > > the request, the bigger the absolute overreclaim error. Historic > > > in-kernel users of reclaim have used fixed, small request batches to > > > approach an appropriate reclaim rate over time. When we reclaim a user > > > request of arbitrary size, use decaying batches to manage error while > > > maintaining reasonable throughput. > > Hm, decay... > So shouldn't the formula be > nr_pages = delta <= SWAP_CLUSTER_MAX ? delta : (delta + 3*SWAP_CLUSTER_MAX) / 4 > where > delta = nr_to_reclaim - nr_reclaimed > ? > (So that convergence for smaller deltas is same like original- and other > reclaims while conservative factor is applied for effectivity of higher > user requests.) Tapering out at 32 instead of 4 doesn't make much difference in practice because of how far off the actually reclaimed amount can be from the request size. We're talking thousands of pages of error for a request size of a few megs, and hundreds of pages of error for requests less than 100 pages. So all of these should be more or less equivalent: delta <= SWAP_CLUSTER_MAX ? delta : (delta + 3*SWAP_CLUSTER_MAX) / 4 max((nr_to_reclaim - nr_reclaimed) / 4, (nr_to_reclaim - nr_reclaimed) % 4) (nr_to_reclaim - nr_reclaimed) / 4 + 4 (nr_to_reclaim - nr_reclaimed) / 4 I was just trying to avoid putting in a 0 for the request size with the mod.
