On Mon, Jul 29, 2024 at 8:39 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > Seek is a fixed coefficient for the scan rate. > > We want to slow writeback when recent zswapouts dominate the zswap > pool (expanding or thrashing), and speed it up when recent entries > make up a small share of the pool (stagnating). > > This is what the second chance accomplishes. Wow, this is something that I did not even consider. Thanks for pointing this out, Johannes. shrinker->seeks tuning allows you to adjust writeback pacing, as a ratio of the pool size. When the pool is static (no/few zswpin or zswpout), then these two are similar (on average). But with concurrent activities (pages coming in and out), the dynamics can potentially be different. Second chance allows you to have different dynamics depending on recent pool activities. The recent zswpouts will be protected by virtue of the reference bits (and given another chance, which will be taken if it's used again soon), and the pages concurrently zswapped in obviously will be too, whereas the stale objects who have already been touched by the shrinker once in the past will be evicted immediately. IOW, all of the above activities (zswpin, zswpout, reclaim pressure) can harmonize seamlessly to adjust the effective rate of writeback. Without any additional heuristics (old or new), increasing seek (i.e decreasing the writeback rate) by itself only has a static effect, and definitely does not accomplish the aformentioned dynamic writeback rate adjustment. Now, we can (and did try to) mimic the above behavior somewhat with the protection size scheme: only return the unprotected size, carefully increase it on zswpout and swpin (so that zswpouts are not considered), carefully prevent shrinker from reclaiming into protected area, etc.. But it's incredibly brittle - with all these hacks, it becomes even more complicated and unintuitive than the second chance algorithm. If it's performing well, then sure, but it's not. Might as well do the simpler thing? :) Besides, the problem with the haphazard aging (i.e protection decaying) remains - at which point do we decay, and how much do we decay? Compare this with the new second chance scheme, which gives you a natural aging mechanism, and can elegantly adjust itself with reclaim/memory pressure.
