Re: [RFC][PATCH 07/10] zsmalloc: introduce auto-compact support

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (06/04/15 15:27), Minchan Kim wrote:
[..]
> 
> The problem is migration/freeing old zspage/allocating new zspage is
> not a cheap, either.
> If the system has no problem with small fragmented space, there is
> no point to keep such overheads.
>
> So, ideal is we should trigger compaction once we realized system
> is trouble but I don't have any good idea to detect it.
> That's why i wanted to rely on the decision from user via
> compact_threshold_ratio.

that'll be extremly hard to understand knob.

well, we can do something like
-- don't let the number of "CLASS_ALMOST_EMPTY" to become N times greater
than "CLASS_ALMOST_FULL".

or

-- don't let the number of pages in ZS_ALMOST_EMPTY pages to contribute 70%
of class memory usage. that is 70% of all pages allocated for this class belong
to ZS_ALMOST_EMPTY zspages, thus potentially we can compact it.

> > 
> > > It's simple design of mm/compaction.c to prevent pointless overhead
> > > but historically it made pains several times and required more
> > > complicated logics but it's still painful.
> > > 
> > > Other thing I found recently is that it's not always win zsmalloc
> > > for zram is not fragmented. The fragmented space could be used
> > > for storing upcoming compressed objects although it is wasted space
> > > at the moment but if we don't have any hole(ie, fragment space)
> > > via frequent compaction, zsmalloc should allocate a new zspage
> > > which could be allocated on movable pageblock by fallback of
> > > nonmovable pageblock request on highly memory pressure system
> > > so it accelerates fragment problem of the system memory.
> > 
> > yes, but compaction almost always leave classes fragmented. I think
> > it's a corner case, when the number of unused allocated objects was
> > exactly the same as the number of objects that we migrated and the
> > number of migrated objects was exactly N*maxobj_per_zspage, so we
> > left the class w/o any unused objects (OBJ_ALLOCATED == OBJ_USED).
> > classes have 'holes' after compaction.
> > 
> > 
> > > So, I want to pass the policy to userspace.
> > > If we found it's really trobule on userspace, then, we need more
> > > thinking.
> > 
> > well, it can be under config "aggressive compaction" or "automatic
> > compaction" option.
> > 
> 
> If you really want to do it automatically without any feedback
> form the userspace, we should find better algorithm.

ok. I'll drop auto-compaction part for now and will resend
general/minor zsmalloc tweaks today.

	-ss

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