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Subject[RFC 0/7] Slab object migration for xarray V2
V1->V2:
- Works now on top of 4.20-rc7
- A couple of bug fixes


This is a patchset on top of Matthew Wilcox Xarray code and implements
slab object migration for xarray nodes. The migration is integrated into
the defragmetation and shrinking logic of the slab allocator.

Defragmentation will ensure that all xarray slab pages have
less objects available than specified by the slab defrag ratio.

Slab shrinking will create a slab cache with optimal object
density. Only one slab page will have available objects per node.

To test apply this patchset and run a workload that uses lots of radix tree objects


Then go to

/sys/kernel/slab/radix_tree_node

Inspect the number of total objects that the slab can handle

cat total_objects

qmdr:/sys/kernel/slab/radix_tree_node# cat objects
868 N0=448 N1=168 N2=56 N3=196

And the number of slab pages used for those

cat slabs

qmdr:/sys/kernel/slab/radix_tree_node# cat slabs
31 N0=16 N1=6 N2=2 N3=7


Perform a cache shrink operation

echo 1 >shrink


Now see how the slab has changed:

qmdr:/sys/kernel/slab/radix_tree_node# cat slabs
30 N0=15 N1=6 N2=2 N3=7
qmdr:/sys/kernel/slab/radix_tree_node# cat objects
713 N0=349 N1=141 N2=52 N3=171


This is just a barebones approach using a special mode
of the slab migration patchset that does not require refcounts.



If this is acceptable then additional functionality can be added:

1. Migration of objects to a specific node. Not sure how to implement
that. Using another sysfs field?

2. Dispersion of objects across all nodes (MPOL_INTERLEAVE)

3. Subsystems can request to move an object to a specific node.
How to design such functionality best?

4. Tying into the page migration and page defragmentation logic so
that so far unmovable pages that are in the way of creating a
contiguous block of memory will become movable.
This would mean checking for slab pages in the migration logic
and calling slab to see if it can move the page by migrating
all objects.

This is only possible for xarray for now but it would be worthwhile
to extend this to dentries and inodes.

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