On Tue, Oct 18, 2016 at 4:27 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote: > On Mon, Oct 17, 2016 at 10:45 PM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote: >> Hi Dan, >> >> On Tue, Oct 18, 2016 at 4:06 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote: >>> On Sat, Oct 15, 2016 at 8:05 AM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote: >>>> This patch implements shrinker for z3fold. This shrinker >>>> implementation does not free up any pages directly but it allows >>>> for a denser placement of compressed objects which results in >>>> less actual pages consumed and higher compression ratio therefore. >>>> >>>> This update removes z3fold page compaction from the freeing path >>>> since we can rely on shrinker to do the job. Also, a new flag >>>> UNDER_COMPACTION is introduced to protect against two threads >>>> trying to compact the same page. >>> >>> i'm completely unconvinced that this should be a shrinker. The >>> alloc/free paths are much, much better suited to compacting a page >>> than a shrinker that must scan through all the unbuddied pages. Why >>> not just improve compaction for the alloc/free paths? >> >> Basically the main reason is performance, I want to avoid compaction on hot >> paths as much as possible. This patchset brings both performance and >> compression ratio gain, I'm not sure how to achieve that with improving >> compaction on alloc/free paths. > > It seems like a tradeoff of slight improvement in hot paths, for > significant decrease in performance by adding a shrinker, which will > do a lot of unnecessary scanning. The alloc/free/unmap functions are > working directly with the page at exactly the point where compaction > is needed - when adding or removing a bud from the page. I can see that sometimes there are substantial amounts of pages that are non-compactable synchronously due to the MIDDLE_CHUNK_MAPPED bit set. Picking up those seems to be a good job for a shrinker, and those end up in the beginning of respective unbuddied lists, so the shrinker is set to find them. I can slightly optimize that by introducing a COMPACT_DEFERRED flag or something like that to make shrinker find those pages faster, would that make sense to you? > Sorry if I missed it in earlier emails, but have you done any > performance measurements comparing with/without the shrinker? The > compression ratio gains may be possible with only the > z3fold_compact_page() improvements, and performance may be stable (or > better) with only a per-z3fold-page lock, instead of adding the > shrinker...? I'm running some tests with per-page locks now, but according to the previous measurements the shrinker version always wins on multi-core platforms. > If a shrinker really is needed, it seems like it would be better > suited to coalescing separate z3fold pages via migration, like > zsmalloc does (although that's a significant amount of work). I really don't want to go that way to keep z3fold applicable to an MMU-less system. ~vitaly -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>