On Fri, Feb 2, 2024 at 4:57 AM Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> wrote: > > On 2024/2/2 02:12, Johannes Weiner wrote: > > On Thu, Feb 01, 2024 at 03:49:05PM +0000, Chengming Zhou wrote: > >> The !zswap_exclusive_loads_enabled mode will leave compressed copy in > >> the zswap tree and lru list after the folio swapin. > >> > >> There are some disadvantages in this mode: > >> 1. It's a waste of memory since there are two copies of data, one is > >> folio, the other one is compressed data in zswap. And it's unlikely > >> the compressed data is useful in the near future. > >> > >> 2. If that folio is dirtied, the compressed data must be not useful, > >> but we don't know and don't invalidate the trashy memory in zswap. > >> > >> 3. It's not reclaimable from zswap shrinker since zswap_writeback_entry() > >> will always return -EEXIST and terminate the shrinking process. > >> > >> On the other hand, the only downside of zswap_exclusive_loads_enabled > >> is a little more cpu usage/latency when compression, and the same if > >> the folio is removed from swapcache or dirtied. > >> > >> Not sure if we should accept the above disadvantages in the case of > >> !zswap_exclusive_loads_enabled, so send this out for disscusion. > >> > >> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> > > > > This is interesting. > > > > First, I will say that I never liked this config option, because it's > > nearly impossible for a user to answer this question. Much better to > > just pick a reasonable default. > > Agree. > > > > > What should the default be? > > > > Caching "swapout work" is helpful when the system is thrashing. Then > > recently swapped in pages might get swapped out again very soon. It > > certainly makes sense with conventional swap, because keeping a clean > > copy on the disk saves IO work and doesn't cost any additional memory. > > > > But with zswap, it's different. It saves some compression work on a > > thrashing page. But the act of keeping compressed memory contributes > > to a higher rate of thrashing. And that can cause IO in other places > > like zswap writeback and file memory. > > > > It would be useful to have an A/B test to confirm that not caching is > > better. Can you run your test with and without keeping the cache, and > > in addition to the timings also compare the deltas for pgscan_anon, > > pgscan_file, workingset_refault_anon, workingset_refault_file? > > I just A/B test kernel building in tmpfs directory, memory.max=2GB. > (zswap writeback enabled and shrinker_enabled, one 50GB swapfile) > > From the below results, exclusive mode has fewer scan and refault. > > zswap-invalidate-entry zswap-invalidate-entry-exclusive > real 63.80 63.01 > user 1063.83 1061.32 > sys 290.31 266.15 > zswap-invalidate-entry zswap-invalidate-entry-exclusive This is one of those cases where something might make sense conceptually, but does not pan out in practice. Removing non-invalidate seems to simplify the code a bit, and that's one less thing to worry about for users, so I like this :) Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx> > workingset_refault_anon 2383084.40 1976397.40 > workingset_refault_file 44134.00 45689.40 > workingset_activate_anon 837878.00 728441.20 > workingset_activate_file 4710.00 4085.20 > workingset_restore_anon 732622.60 639428.40 > workingset_restore_file 1007.00 926.80 > workingset_nodereclaim 0.00 0.00 > pgscan 14343003.40 12409570.20 > pgscan_kswapd 0.00 0.00 > pgscan_direct 14343003.40 12409570.20 > pgscan_khugepaged 0.00 0.00
