Re: what is the purpose of SLAB and SLUB (was: Re: [PATCH v3] mm/slab: Improve performance of gathering slabinfo) stats

[Michal Hocko]

On Wed 24-08-16 10:15:02, Joonsoo Kim wrote:
> On Tue, Aug 23, 2016 at 05:38:08PM +0200, Michal Hocko wrote:
> > On Tue 23-08-16 11:13:03, Joonsoo Kim wrote:
> > > On Thu, Aug 18, 2016 at 01:52:19PM +0200, Michal Hocko wrote:
> > [...]
> > > > I am not opposing the patch (to be honest it is quite neat) but this
> > > > is buggering me for quite some time. Sorry for hijacking this email
> > > > thread but I couldn't resist. Why are we trying to optimize SLAB and
> > > > slowly converge it to SLUB feature-wise. I always thought that SLAB
> > > > should remain stable and time challenged solution which works reasonably
> > > > well for many/most workloads, while SLUB is an optimized implementation
> > > > which experiment with slightly different concepts that might boost the
> > > > performance considerably but might also surprise from time to time. If
> > > > this is not the case then why do we have both of them in the kernel. It
> > > > is a lot of code and some features need tweaking both while only one
> > > > gets testing coverage. So this is mainly a question for maintainers. Why
> > > > do we maintain both and what is the purpose of them.
> > > 
> > > I don't know full history about it since I joined kernel communitiy
> > > recently(?). Christoph would be a better candidate for this topic.
> > > Anyway,
> > > 
> > > SLAB if SLUB beats SLAB completely. But, there are fundamental
> > > differences in implementation detail so they cannot beat each other
> > > for all the workloads. It is similar with filesystem case that various
> > > filesystems exist for it's own workload.
> > 
> > Do we have any documentation/study about which particular workloads
> > benefit from which allocator? It seems that most users will use whatever
> > the default or what their distribution uses. E.g. SLES kernel use SLAB
> > because this is what we used to have for ages and there was no strong
> > reason to change that default. From such a perspective having a stable
> > allocator with minimum changes - just bug fixes - makes a lot of sense.
> 
> It doesn't make sense to me. Even if someone uses SLAB due to
> conventional reason, they would want to use shiny new feature and get
> performance improvement.
> 
> And, it is not only reason to use SLAB. There would be many different
> reasons to use SLAB.

Could you be more specific please? Are there any inherent problems that
would make one allocator unsuitable for specific workloads?

> > I remember Mel doing some benchmarks when "why opensuse kernels do not
> > use the default SLUB allocator" came the last time and he didn't see any
> > large winner there
> > https://lists.opensuse.org/opensuse-kernel/2015-08/msg00098.html
> > This set of workloads is of course not comprehensive to rule one or
> > other but I am wondering whether there are still any pathological
> > workloads where we really want to keep SLAB or add new features to it.
> 
> AFAIK, some network benchmark still shows regression in SLUB.
> 
> http://lkml.kernel.org/r/20150907113026.5bb28ca3@xxxxxxxxxx

That suggests that this is not an inherent problem of SLUB though.

-- 
Michal Hocko
SUSE Labs