Re: [RFC 0/6] mm: improve page allocator scalability via splitting zones

["Huang, Ying" <ying.huang@intel.com>]

Hi, Dave,

Dave Hansen <dave.hansen@intel.com> writes:

> On 5/10/23 23:56, Huang Ying wrote:
>> To improve the scalability of the page allocation, in this series, we
>> will create one zone instance for each about 256 GB memory of a zone
>> type generally.  That is, one large zone type will be split into
>> multiple zone instances. 
>
> A few anecdotes for why I think _some_ people will like this:
>
> Some Intel hardware has a "RAM" caching mechanism.  It either caches
> DRAM in High-Bandwidth Memory or Persistent Memory in DRAM.  This cache
> is direct-mapped and can have lots of collisions.  One way to prevent
> collisions is to chop up the physical memory into cache-sized zones and
> let users choose to allocate from one zone.  That fixes the conflicts.
>
> Some other Intel hardware a ways to chop a NUMA node representing a
> single socket into slices.  Usually one slice gets a memory controller
> and its closest cores.  Intel calls these approaches Cluster on Die or
> Sub-NUMA Clustering and users can select it from the BIOS.
>
> In both of these cases, users have reported scalability improvements.
> We've gone as far as to suggest the socket-splitting options to folks
> today who are hitting zone scalability issues on that hardware.
>
> That said, those _same_ users sometimes come back and say something
> along the lines of: "So... we've got this app that allocates a big hunk
> of memory.  It's going slower than before."  They're filling up one of
> the chopped-up zones, hitting _some_ kind of undesirable reclaim
> behavior and they want their humpty-dumpty zones put back together again
> ... without hurting scalability.  Some people will never be happy. :)

Thanks a lot for your valuable input!

> Anyway, _if_ you do this, you might also consider being able to
> dynamically adjust a CPU's zonelists somehow.  That would relieve
> pressure on one zone for those uneven allocations.  That wasn't an
> option in the two cases above because users had ulterior motives for
> sticking inside a single zone.  But, in your case, the zones really do
> have equivalent performance.

Yes.  For the requirements you mentioned above, we need a mechanism to
adjust a CPU's zonelists dynamically.  I will not implement that in this
series.  But I think that it's doable based on the multiple zone
instances per zone type implementation in this series.

Best Regards,
Huang, Ying