Re: [PATCH v3 10/11] mm: vmalloc: Set nr_nodes based on CPUs in a system

[Dave Chinner <david@fromorbit.com>]

On Mon, Jan 15, 2024 at 08:09:29PM +0100, Uladzislau Rezki wrote:
> > On Tue, Jan 02, 2024 at 07:46:32PM +0100, Uladzislau Rezki (Sony) wrote:
> > > A number of nodes which are used in the alloc/free paths is
> > > set based on num_possible_cpus() in a system. Please note a
> > > high limit threshold though is fixed and corresponds to 128
> > > nodes.
> > 
> > Large CPU count machines are NUMA machines. ALl of the allocation
> > and reclaim is NUMA node based i.e. a pgdat per NUMA node.
> > 
> > Shrinkers are also able to be run in a NUMA aware mode so that
> > per-node structures can be reclaimed similar to how per-node LRU
> > lists are scanned for reclaim.
> > 
> > Hence I'm left to wonder if it would be better to have a vmalloc
> > area per pgdat (or sub-node cluster) rather than just base the
> > number on CPU count and then have an arbitrary maximum number when
> > we get to 128 CPU cores. We can have 128 CPU cores in a
> > single socket these days, so not being able to scale the vmalloc
> > areas beyond a single socket seems like a bit of a limitation.
> > 
> >
> > Hence I'm left to wonder if it would be better to have a vmalloc
> > area per pgdat (or sub-node cluster) rather than just base the
> >
> > Scaling out the vmalloc areas in a NUMA aware fashion allows the
> > shrinker to be run in numa aware mode, which gets rid of the need
> > for the global shrinker to loop over every single vmap area in every
> > shrinker invocation. Only the vm areas on the node that has a memory
> > shortage need to be scanned and reclaimed, it doesn't need reclaim
> > everything globally when a single node runs out of memory.
> > 
> > Yes, this may not give quite as good microbenchmark scalability
> > results, but being able to locate each vm area in node local memory
> > and have operation on them largely isolated to node-local tasks and
> > vmalloc area reclaim will work much better on large multi-socket
> > NUMA machines.
> > 
> Currently i fix the max nodes number to 128. This is because i do not
> have an access to such big NUMA systems whereas i do have an access to
> around ~128 ones. That is why i have decided to stop on that number as
> of now.

I suspect you are confusing number of CPUs with number of NUMA nodes.

A NUMA system with 128 nodes is a large NUMA system that will have
thousands of CPU cores, whilst above you talk about basing the
count on CPU cores and that a single socket can have 128 cores?

> We can easily set nr_nodes to num_possible_cpus() and let it scale for
> anyone. But before doing this, i would like to give it a try as a first
> step because i have not tested it well on really big NUMA systems.

I don't think you need to have large NUMA systems to test it. We
have the "fakenuma" feature for a reason.  Essentially, once you
have enough CPU cores that catastrophic lock contention can be
generated in a fast path (can take as few as 4-5 CPU cores), then
you can effectively test NUMA scalability with fakenuma by creating
nodes with >=8 CPUs each.

This is how I've done testing of numa aware algorithms (like
shrinkers!) for the past decade - I haven't had direct access to a
big NUMA machine since 2008, yet it's relatively trivial to test
NUMA based scalability algorithms without them these days.

-Dave.
-- 
Dave Chinner
david@fromorbit.com