[PATCH v2 2/2] powerpc/mm: Add memory_block_size as a kernel parameter

[Michael Ellerman]

David Hildenbrand <david at redhat.com> writes:
> On 09.06.23 08:08, Aneesh Kumar K.V wrote:
>> Certain devices can possess non-standard memory capacities, not constrained
>> to multiples of 1GB. Provide a kernel parameter so that we can map the
>> device memory completely on memory hotplug.
>
> So, the unfortunate thing is that these devices would have worked out of 
> the box before the memory block size was increased from 256 MiB to 1 GiB 
> in these setups. Now, one has to fine-tune the memory block size. The 
> only other arch that I know, which supports setting the memory block 
> size, is x86 for special (large) UV systems -- and at least in the past 
> 128 MiB vs. 2 GiB memory blocks made a performance difference during 
> boot (maybe no longer today, who knows).
>
>
> Obviously, less tunable and getting stuff simply working out of the box 
> is preferable.
>
> Two questions:
>
> 1) Isn't there a way to improve auto-detection to fallback to 256 MiB in 
> these setups, to avoid specifying these parameters?
>
> 2) Is the 256 MiB -> 1 GiB memory block size switch really worth it? On 
> x86-64, experiments (with direct map fragmentation) showed that the 
> effective performance boost is pretty insignificant, so I wonder how big 
> the 1 GiB direct map performance improvement is.

The other issue is simply the number of sysfs entries.

With 64TB of memory and a 256MB block size you end up with ~250,000
directories in /sys/devices/system/memory.

cheers