I would suggest a different approach. Estimate the number of pages freed by vm_pageout_lowmem() by computing the difference in vmd_free_count before and after calling vm_pageout_lowmem(), and reduce shortage by that estimate so that vm_pageout_scan_inactive() frees fewer pages.

Here is the intuition behind this suggestion. Suppose that the machine is reclaiming and allocating pages at a steady rate. The PID controller's integral term will dominate the output value, and the error term will be near zero on every invocation. Essentially, the PID controller's integral term divided by KID will be the average number of page allocations (and reclamations) per interval. Your approach is going to reduce that integral term even though the average rate of allocation hasn't really changed. So, on the next PID controller invocation, fewer pages will be freed.

At the moment, I think that this is the best approach. However, on a NUMA machine, the vm_pageout_lowmem() handlers have a system-wide effect, and so the non-invoking domains will be subject to the issue that I described earlier, i.e., their vmd_free_count's will be reduced by the system-wide effect of vm_pageout_lowmem(), and so it will appear to them that there has been a change in the rate of page allocation on their domains.

How hard will it be to make uma_reclaim() NUMA aware?

In D16606#353726, @alc wrote:

At the moment, I think that this is the best approach. However, on a NUMA machine, the vm_pageout_lowmem() handlers have a system-wide effect, and so the non-invoking domains will be subject to the issue that I described earlier, i.e., their vmd_free_count's will be reduced by the system-wide effect of vm_pageout_lowmem(), and so it will appear to them that there has been a change in the rate of page allocation on their domains.

How hard will it be to make uma_reclaim() NUMA aware?

UMA zones now have per-domain full bucket caches, but those caches are for allocations to a specific domain, rather than from a specific domain. Thus a given bucket will generally contain items from multiple domains, and if we were to free only a subset of the items we'd be caching partially full buckets. Then, to compute the domain of the slab for a given item we may require the keg lock, which is awkward since we currently need to drop the keg lock before releasing any item to the keg. I think these problems could be solved, but it's not as straightforward as it could be.

BTW, I'm finishing up a patch that Jeff and I discussed to compute a WSS estimate for each (per-domain) bucket cache. When the page daemon invokes the lowmem handlers, it'll only reclaim cached items in excess of the estimate, rather than freeing the entire cache. (If there is a severe free page shortage, we will still free the entire cache.) I hope to put the patch up for review today or tomorrow.