On Fri, Feb 02, 2024 at 01:02:47PM +0800, Efly Young [off-list ref] wrote:

quoted

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.

quoted

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.)

Thanks,
Michal