Re: slab: introduce the flag SLAB_MINIMIZE_WASTE

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 16 Apr 2018, Mike Snitzer wrote:

> On Mon, Apr 16 2018 at  8:38am -0400,
> Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> > On 04/13/2018 05:10 PM, Mike Snitzer wrote:
> > > On Fri, Apr 13 2018 at  5:22am -0400,
> > > Vlastimil Babka <vbabka@suse.cz> wrote:
> > >>
> > >> Would this perhaps be a good LSF/MM discussion topic? Mikulas, are you
> > >> attending, or anyone else that can vouch for your usecase?
> > > 
> > > Any further discussion on SLAB_MINIMIZE_WASTE should continue on list.
> > > 
> > > Mikulas won't be at LSF/MM.  But I included Mikulas' dm-bufio changes
> > > that no longer depend on this proposed SLAB_MINIMIZE_WASTE (as part of
> > > the 4.17 merge window).
> > 
> > Can you or Mikulas briefly summarize how the dependency is avoided, and
> > whether if (something like) SLAB_MINIMIZE_WASTE were implemented, the
> > dm-bufio code would happily switch to it, or not?
> 
> git log eeb67a0ba04df^..45354f1eb67224669a1 -- drivers/md/dm-bufio.c
> 
> But the most signficant commit relative to SLAB_MINIMIZE_WASTE is: 
> 359dbf19ab524652a2208a2a2cddccec2eede2ad ("dm bufio: use slab cache for 
> dm_buffer structure allocations")
> 
> So no, I don't see why dm-bufio would need to switch to
> SLAB_MINIMIZE_WASTE if it were introduced in the future.

Currently, the slab cache rounds up the size of the slab to the next power 
of two (if the size is large). And that wastes memory if that memory were 
to be used for deduplication tables.

Generally, the performance of the deduplication solution depends on how 
much data can you put to memory. If you round 640KB buffer to 1MB (this is 
what the slab and slub subsystem currently do), you waste a lot of memory. 
Deduplication indices with 640KB blocks are already used in the wild, so 
it can't be easily changed.

> Mike

Mikulas