Re: [PATCH v2 0/2] mm/shrinker: make per-memcg shrinker units cheaper

[Konstantin Khorenko]

On 10/14/26 22:50, Andrew Morton wrote:
> On Wed, 14 Oct 2026 16:54:36 +0200 Konstantin Khorenko <khorenko@xxxxxxxxxxxxx> wrote:
> 
>> Every memory cgroup allocates, per NUMA node, one shrinker_info_unit
>> per 64 memcg-aware shrinker ids up to shrinker_nr_max, whether or not it
>> ever charges an object to those shrinkers. Every superblock registers
>> such a shrinker, so the cost is
>>
>>   memory cgroups x NUMA nodes x shrinker_nr_max / 64 x 1 KiB
>>
>> because the unit is 520 bytes and comes from kmalloc-1k.
> 
> So we could almost halve this just by using a dedicated kmem_cache?

Yeap, a dedicated cache gets approximately the same cost per shrinker id.
On a 48-CPU box SLUB puts 31 units in an order-2 slab, about 528 bytes each:

                                      ids/unit  bytes/unit  bytes/id
    today, kmalloc-1k                    64        1024        16
    patch 1, kmalloc-512                 63         512       ~8.1
    dedicated cache                      64        ~528       ~8.3
    dedicated cache, HWCACHE_ALIGN       64         576        9

So the two are ~equal on memory, and the choice is about the code.

I went with 63 ids per unit because the unit then simply fills the
kmalloc slot it already comes from.

The price is an odd SHRINKER_UNIT_BITS and roundup() instead of round_up(),
plus the static_assert.

If you prefer the cache, I'm happy to redo patch 1 that way.
Patch 2 does not care about the unit size.

--
Best regards,
Konstantin