Re: [RFC 2/2] mm/slub: prefer NUMA locality over slight memory saving on NUMA machines

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Aug 3, 2023 at 11:54 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 7/23/23 21:09, Hyeonggon Yoo wrote:
> > By default, SLUB sets remote_node_defrag_ratio to 1000, which makes it
> > (in most cases) take slabs from remote nodes first before trying allocating
> > new folios on the local node from buddy.
> >
> > Documentation/ABI/testing/sysfs-kernel-slab says:
> >> The file remote_node_defrag_ratio specifies the percentage of
> >> times SLUB will attempt to refill the cpu slab with a partial
> >> slab from a remote node as opposed to allocating a new slab on
> >> the local node.  This reduces the amount of wasted memory over
> >> the entire system but can be expensive.
> >
> > Although this made sense when it was introduced, the portion of
> > per node partial lists in the overall SLUB memory usage has been decreased
> > since the introduction of per cpu partial lists. Therefore, it's worth
> > reevaluating its overhead on performance and memory usage.
> >
> > [
> >       XXX: Add performance data. I tried to measure its impact on
> >       hackbench with a 2 socket NUMA  machine. but it seems hackbench is
> >       too synthetic to benefit from this, because the skbuff_head_cache's
> >       size fits into the last level cache.
> >
> >       Probably more realistic workloads like netperf would benefit
> >       from this?
> > ]
> >
> > Set remote_node_defrag_ratio to zero by default, and the new behavior is:
> >       1) try refilling per CPU partial list from the local node
> >       2) try allocating new slabs from the local node without reclamation
> >       3) try refilling per CPU partial list from remote nodes
> >       4) try allocating new slabs from the local node or remote nodes
> >
> > If user specified remote_node_defrag_ratio, it probabilistically tries
> > 3) first and then try 2) and 4) in order, to avoid unexpected behavioral
> > change from user's perspective.
>
> It makes sense to me, but as you note it would be great to demonstrate
> benefits, because it adds complexity, especially in the already complex
> ___slab_alloc(). Networking has been indeed historically a workload very
> sensitive to slab performance, so seems a good candidate.

Thank you for looking at it!

Yeah, it was a PoC for what I thought "oh, it might be useful"
and definitely I will try to measure it.

> We could also postpone this until we have tried the percpu arrays
> improvements discussed at LSF/MM.

Possibly, but can you please share your plans/opinions on it?
I think one possible way is simply to allow the cpu freelist to be
mixed by objects from different slabs
if we want to minimize changes, Or introduce a per cpu array similar
to what SLAB does now.

And one thing I'm having difficulty understanding is - what is the
mind behind/or impact of managing objects
on a slab basis, other than avoiding array queues in 2007?