Re: [RFC V2 SLEB 00/14] The Enhanced(hopefully) Slab Allocator

[Nick Piggin <npiggin@suse.de>]

On Thu, May 27, 2010 at 10:52:52AM -0500, Christoph Lameter wrote:
> On Fri, 28 May 2010, Nick Piggin wrote:
> 
> > > I am just amazed at the tosses and turns by you. Didnt you write SLQB on
> > > the basis of SLUB? And then it was abandoned? If you really believe ths
> >
> > Sure I hoped it would be able to conclusively beat SLAB, and I'd
> > thought it might be a good idea. I stopped pushing it because I
> > realized that incremental improvements to SLAB would likely be a
> > far better idea.
> 
> It looked to me as if there was a major conceptual issue with the linked
> lists used for objects that impacted performance

With SLQB's linked list? No. Single threaded cache hot performance was
the same (+/- a couple of cycles IIRC) as SLUB on your microbenchmark.
On Intel's OLTP workload it was as good as SLAB.

The linked lists were similar to SLOB/SLUB IIRC.


> plus unresolved issues
> with crashes on boot.

Was due to a hack using per_cpu definition for a node field (some
systems would have nodes not equal to a CPU number). I don't think
there were any problems left.


> I did not see you work on SLAB improvements. Seemed
> that other things had higher priority. The work on slab allocators in
> general is not well funded, not high priority and is a side issue. The
> time that I can spend on this is also limited.

I heard SLUB was just about to get there with new per-cpu accessors.
That didn't seem to help too much in real world. I would have liked more
time on SLAB but unfortunately have not until now.

It seems that it is *still* the best and most mature allocator we have
for most users, and the most widely deployed one. So AFAIKS it still
makes sense to incrementally improve it rather than take something
else.


> > > and want to get this done then please invest some time in SLAB to get it
> > > cleaned up. I have some doubt that you are aware of the difficulties that
> > > you will encounter.
> >
> > I am working on it. We'll see.
> 
> I think we agreee on one thing regardless of SLAB or SLUB as a base: It
> would be good to put the best approaches together to form a superior slab
> allocator. I just think its much easier to do give a mature and clean code
> base in SLUB. If we both work on this then this may coalesce at some
> point.

And I've listed my gripes with SLUB countless times, so I won't any more.

 
> The main gripes with SLAB
> 
> - Code base difficult to maintain. Has grown over almost 2 decades.
> - Alien caches need to be kept under control. Various hacky ways
>   are implemented to bypass that problem.
> - Locking issues because of long hold times of per node lock. SLUB
>   has locking on per page level. This is important for high number of
>   threads per node. Westmere has already 12. EX 24 and it way grow
>   from there.
> - Debugging features and recovery mechanisms.
> - Off or on page slab metadata causes space wastage, complex allocation
>   and locking and alignment issues. SLED replaces that metadata structure
>   with a bitfield in the page struct. This may also save access to
>   additional cacheline and maybe allow freeing of objects to a slab page
>   without taking locks.
> - Variable and function naming is confusing.
> - OS noise caused by periodic cache cleaning (which requires scans over
>   all caches of all slabs on every processor).


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