On Sun, Oct 27, 2024 at 3:45 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > On Sat, Oct 26, 2024 at 6:20 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > From: Barry Song <v-songbaohua@xxxxxxxx> > > > > When the proportion of folios from the zero map is small, missing their > > accounting may not significantly impact profiling. However, it’s easy > > to construct a scenario where this becomes an issue—for example, > > allocating 1 GB of memory, writing zeros from userspace, followed by > > MADV_PAGEOUT, and then swapping it back in. In this case, the swap-out > > and swap-in counts seem to vanish into a black hole, potentially > > causing semantic ambiguity. > > I agree. It also makes developing around this area more challenging. > I'm working on the swap abstraction, and sometimes I can't tell if I > screwed up somewhere, or if a proportion of these allocated entries go > towards this optimization... > > Thanks for taking a stab at fixing this, Barry! > > > > > We have two ways to address this: > > > > 1. Add a separate counter specifically for the zero map. > > 2. Continue using the current accounting, treating the zero map like > > a normal backend. (This aligns with the current behavior of zRAM > > when supporting same-page fills at the device level.) > > Hmm, my understanding of the pswpout/pswpin counters is that they only > apply to IO done directly to the backend device, no? That's why we > have a separate set of counters for zswap, and do not count them > towards pswp(in|out). > > For users who have swap files on physical disks, the performance > difference between reading directly from the swapfile and going > through these optimizations could be really large. I think it makes > sense to have a separate set of counters for zero-mapped pages > (ideally, both at the host level and at the cgroup level?) agree it is better to have a separate counter for zeromap. then it raises a question: what is the proper name for it :-) zeromap_swpin, zeromap_swpout seems too long? and zswpin and zswpout have been used by zswap Thanks barry
