On Thu, Mar 28, 2024 at 12:38 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Mon, Mar 25, 2024 at 11:50:15PM +0000, Yosry Ahmed wrote: > > After the rbtree to xarray conversion, and dropping zswap_entry.refcount > > and zswap_entry.value, the only members of zswap_entry utilized by > > zero-filled pages are zswap_entry.length (always 0) and > > zswap_entry.objcg. Store the objcg pointer directly in the xarray as a > > tagged pointer and avoid allocating a zswap_entry completely for > > zero-filled pages. > > > > This simplifies the code as we no longer need to special case > > zero-length cases. We are also able to further separate the zero-filled > > pages handling logic and completely isolate them within store/load > > helpers. Handling tagged xarray pointers is handled in these two > > helpers, as well as the newly introduced helper for freeing tree > > elements, zswap_tree_free_element(). > > > > There is also a small performance improvement observed over 50 runs of > > kernel build test (kernbench) comparing the mean build time on a skylake > > machine when building the kernel in a cgroup v1 container with a 3G > > limit. This is on top of the improvement from dropping support for > > non-zero same-filled pages: > > > > base patched % diff > > real 69.915 69.757 -0.229% > > user 2956.147 2955.244 -0.031% > > sys 2594.718 2575.747 -0.731% > > > > This probably comes from avoiding the zswap_entry allocation and > > cleanup/freeing for zero-filled pages. Note that the percentage of > > zero-filled pages during this test was only around 1.5% on average. > > Practical workloads could have a larger proportion of such pages (e.g. > > Johannes observed around 10% [1]), so the performance improvement should > > be larger. > > > > This change also saves a small amount of memory due to less allocated > > zswap_entry's. In the kernel build test above, we save around 2M of > > slab usage when we swap out 3G to zswap. > > > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@xxxxxxxxxxx/ > > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > --- > > mm/zswap.c | 137 ++++++++++++++++++++++++++++++----------------------- > > 1 file changed, 78 insertions(+), 59 deletions(-) > > Tbh, I think this makes the code worse overall. Instead of increasing > type safety, it actually reduces it, and places that previously dealt > with a struct zswap_entry now deal with a void *. > > If we go ahead with this series, I think it makes more sense to either > > a) do the explicit subtyping of struct zswap_entry I had proposed, or I suspect we won't get the small performance improvements (and memory saving) with this approach. Neither are too significant, but it'd be nice if we could keep them. > > b) at least keep the specialness handling of the xarray entry as local > as possible, so that instead of a proliferating API that operates > on void *, you have explicit filtering only where the tree is > accessed, and then work on struct zswap_entry as much as possible. I was trying to go for option (b) by isolating filtering and casting to the correct type in a few functions (zswap_tree_free_element(), zswap_store_zero_filled(), and zswap_load_zero_filled()). If we open-code filtering it will be repeated in a few places. What did you have in mind?
