[PATCH v5] z3fold: add shrinker

[Vitaly Wool <vitalywool@gmail.com>]

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18 окт. 2016 г. 18:29 пользователь "Dan Streetman" <ddstreet@ieee.org
<javascript:_e(%7B%7D,'cvml','ddstreet@ieee.org');>> написал:
>
> On Tue, Oct 18, 2016 at 10:51 AM, Vitaly Wool <vitalywool@gmail.com
<javascript:_e(%7B%7D,'cvml','vitalywool@gmail.com');>> wrote:
> > On Tue, Oct 18, 2016 at 4:27 PM, Dan Streetman <ddstreet@ieee.org
<javascript:_e(%7B%7D,'cvml','ddstreet@ieee.org');>> wrote:
> >> On Mon, Oct 17, 2016 at 10:45 PM, Vitaly Wool <vitalywool@gmail.com
<javascript:_e(%7B%7D,'cvml','vitalywool@gmail.com');>> wrote:
> >>> Hi Dan,
> >>>
> >>> On Tue, Oct 18, 2016 at 4:06 AM, Dan Streetman <ddstreet@ieee.org
<javascript:_e(%7B%7D,'cvml','ddstreet@ieee.org');>> wrote:
> >>>> On Sat, Oct 15, 2016 at 8:05 AM, Vitaly Wool <vitalywool@gmail.com
<javascript:_e(%7B%7D,'cvml','vitalywool@gmail.com');>> 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).

> >> 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

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