On Tue, Oct 18, 2016 at 12:26 PM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote: > 18 окт. 2016 г. 18:29 пользователь "Dan Streetman" <ddstreet@xxxxxxxx> > написал: > > >> >> On Tue, Oct 18, 2016 at 10:51 AM, Vitaly Wool <vitalywool@xxxxxxxxx> >> wrote: >> > 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? >> >> Why not just compact the page in z3fold_unmap()? > > That would give a huge performance penalty (checked). my core concern with the shrinker is, it has no context about which pages need compacting and which don't, while the alloc/free/unmap functions do. If the alloc/free/unmap fast paths are impacted too much by compacting directly, then yeah setting a flag for deferred action would be better than the shrinker just scanning all pages. However, in that the case, then a shrinker still seems unnecessary - all the pages that need compacting are pre-marked, there's no need to scan any more. Isn't a simple workqueue to deferred-compact pages better? > >> >> 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. >> >> But that comparison is without taking the spinlock in map/unmap right? > > Right, but from the recent measurements it looks like per-page locks don't > slow things down that much. > > ~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