On Mon, Jun 10, 2024 at 5:18 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote: > > Going back to the v1 implementation of the patchseries. The main reason > is that a correct version of v2 implementation requires another rmap > walk in shrink_folio_list to change the ptes from swap entry to zero pages to > work (i.e. more CPU used) [1], is more complex to implement compared to v1 > and is harder to verify correctness compared to v1, where everything is > handled by swap. > > --- > As shown in the patchseries that introduced the zswap same-filled > optimization [2], 10-20% of the pages stored in zswap are same-filled. > This is also observed across Meta's server fleet. > By using VM counters in swap_writepage (not included in this > patchseries) it was found that less than 1% of the same-filled > pages to be swapped out are non-zero pages. > > For conventional swap setup (without zswap), rather than reading/writing > these pages to flash resulting in increased I/O and flash wear, a bitmap > can be used to mark these pages as zero at write time, and the pages can > be filled at read time if the bit corresponding to the page is set. > > When using zswap with swap, this also means that a zswap_entry does not > need to be allocated for zero filled pages resulting in memory savings > which would offset the memory used for the bitmap. > > A similar attempt was made earlier in [3] where zswap would only track > zero-filled pages instead of same-filled. > This patchseries adds zero-filled pages optimization to swap > (hence it can be used even if zswap is disabled) and removes the > same-filled code from zswap (as only 1% of the same-filled pages are > non-zero), simplifying code. > > This patchseries is based on mm-unstable. Aside from saving swap/zswap space and simplifying the zswap code (thanks for that!), did you observe any performance benefits from not having to go into zswap code for zero-filled pages? In [3], I observed ~1.5% improvement in kernbench just by optimizing zswap's handling of zero-filled pages, and that benchmark only produced around 1.5% zero-filled pages. I imagine avoiding the zswap code entirely, and for workloads that have 10-20% zero-filled pages, the performance improvement should be more pronounced. When zswap is not being used and all swap activity translates to IO, I imagine the benefits will be much more significant. I am curious if you have any numbers with or without zswap :)
