On Tue, Nov 12, 2024 at 8:30 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > On Thu, Nov 7, 2024 at 2:10 AM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > From: Barry Song <v-songbaohua@xxxxxxxx> > > > > When large folios are compressed at a larger granularity, we observe > > a notable reduction in CPU usage and a significant improvement in > > compression ratios. > > > > mTHP's ability to be swapped out without splitting and swapped back in > > as a whole allows compression and decompression at larger granularities. > > > > This patchset enhances zsmalloc and zram by adding support for dividing > > large folios into multi-page blocks, typically configured with a > > 2-order granularity. Without this patchset, a large folio is always > > divided into `nr_pages` 4KiB blocks. > > > > The granularity can be set using the `ZSMALLOC_MULTI_PAGES_ORDER` > > setting, where the default of 2 allows all anonymous THP to benefit. > > > > Examples include: > > * A 16KiB large folio will be compressed and stored as a single 16KiB > > block. > > * A 64KiB large folio will be compressed and stored as four 16KiB > > blocks. > > > > For example, swapping out and swapping in 100MiB of typical anonymous > > data 100 times (with 16KB mTHP enabled) using zstd yields the following > > results: > > > > w/o patches w/ patches > > swap-out time(ms) 68711 49908 > > swap-in time(ms) 30687 20685 > > compression ratio 20.49% 16.9% > > The data looks very promising :) My understanding is it also results > in memory saving as well right? Since zstd operates better on bigger > inputs. > > Is there any end-to-end benchmarking? My intuition is that this patch > series overall will improve the situations, assuming we don't fallback > to individual zero order page swapin too often, but it'd be nice if > there is some data backing this intuition (especially with the > upstream setup, i.e without any private patches). If the fallback > scenario happens frequently, the patch series can make a page fault > more expensive (since we have to decompress the entire chunk, and > discard everything but the single page being loaded in), so it might > make a difference. > > Not super qualified to comment on zram changes otherwise - just a > casual observer to see if we can adopt this for zswap. zswap has the > added complexity of not supporting THP zswap in (until Usama's patch > series lands), and the presence of mixed backing states (due to zswap > writeback), increasing the likelihood of fallback :) Correct. As I mentioned to Usama[1], this could be a problem, and we are collecting data. The simplest approach to work around the issue is to fall back to four small folios instead of just one, which would prevent the need for three extra decompressions. [1] https://lore.kernel.org/linux-mm/CAGsJ_4yuZLOE0_yMOZj=KkRTyTotHw4g5g-t91W=MvS5zA4rYw@xxxxxxxxxxxxxx/ Thanks Barry
