Re: [PATCH v5 3/3] z3fold: add shrinker

[Vitaly Wool]

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