Re: [PATCH 00/10] implement zsmalloc shrinking

[Minchan Kim <minchan@kernel.org>]

On Fri, Sep 12, 2014 at 01:05:11PM -0400, Dan Streetman wrote:
> On Fri, Sep 12, 2014 at 1:46 AM, Minchan Kim <minchan@kernel.org> wrote:
> > On Thu, Sep 11, 2014 at 04:53:51PM -0400, Dan Streetman wrote:
> >> Now that zswap can use zsmalloc as a storage pool via zpool, it will
> >> try to shrink its zsmalloc zs_pool once it reaches its max_pool_percent
> >> limit.  These patches implement zsmalloc shrinking.  The way the pool is
> >> shrunk is by finding a zspage and reclaiming it, by evicting each of its
> >> objects that is in use.
> >>
> >> Without these patches zswap, and any other future user of zpool/zsmalloc
> >> that attempts to shrink the zpool/zs_pool, will only get errors and will
> >> be unable to shrink its zpool/zs_pool.  With the ability to shrink, zswap
> >> can keep the most recent compressed pages in memory.
> >>
> >> Note that the design of zsmalloc makes it impossible to actually find the
> >> LRU zspage, so each class and fullness group is searched in a round-robin
> >> method to find the next zspage to reclaim.  Each fullness group orders its
> >> zspages in LRU order, so the oldest zspage is used for each fullness group.
> >>
> >
> > 1. Pz, Cc Mel who was strong against zswap with zsmalloc.
> > 2. I don't think LRU stuff should be in allocator layer. Exp, it's really
> >    hard to work well in zsmalloc design.
> 
> I didn't add any LRU - the existing fullness group LRU ordering is
> already there.  And yes, the zsmalloc design prevents any real LRU

I don't think It's not LRU for reclaiming but just simple linked list for
finding free slot.

> ordering, beyond per-fullness-group LRU ordering.

Yes.

> 
> > 3. If you want to add another writeback, make zswap writeback sane first.
> >    current implemenation(zswap store -> zbud reclaim -> zswap writeback,
> >    even) is really ugly.
> 
> why what's wrong with that?  how else can zbud/zsmalloc evict stored objects?

You can refer Mel's suggestion for zswap/zsmalloc and writeback problem.

http://www.spinics.net/lists/linux-mm/msg61601.html
http://lkml.iu.edu//hypermail/linux/kernel/1304.1/04334.html

I think LRU/writeback should be upper layer, not allocator itself.
Please, don't force every allocator to implement it for only zswap.

> 
> > 4. Don't make zsmalloc complicated without any data(benefit, regression)
> >    I will never ack if you don't give any number and real usecase.
> 
> ok, i'll run performance tests then, but let me know if you see any
> technical problems with any of the patches before then.
> 
> thanks!
> 
> >
> >> ---
> >>
> >> This patch set applies to linux-next.
> >>
> >> Dan Streetman (10):
> >>   zsmalloc: fix init_zspage free obj linking
> >>   zsmalloc: add fullness group list for ZS_FULL zspages
> >>   zsmalloc: always update lru ordering of each zspage
> >>   zsmalloc: move zspage obj freeing to separate function
> >>   zsmalloc: add atomic index to find zspage to reclaim
> >>   zsmalloc: add zs_ops to zs_pool
> >>   zsmalloc: add obj_handle_is_free()
> >>   zsmalloc: add reclaim_zspage()
> >>   zsmalloc: add zs_shrink()
> >>   zsmalloc: implement zs_zpool_shrink() with zs_shrink()
> >>
> >>  drivers/block/zram/zram_drv.c |   2 +-
> >>  include/linux/zsmalloc.h      |   7 +-
> >>  mm/zsmalloc.c                 | 314 +++++++++++++++++++++++++++++++++++++-----
> >>  3 files changed, 290 insertions(+), 33 deletions(-)
> >>
> >> --
> >> 1.8.3.1
> >>
> >> --
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> >
> > --
> > Kind regards,
> > Minchan Kim
> 
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-- 
Kind regards,
Minchan Kim