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

Huang, Ying
2023-05-12T03:09:44Z

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