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    SubjectRe: [patch 31/35] fs: icache per-zone inode LRU
    On Tue, Oct 19, 2010 at 02:42:47PM +1100, wrote:
    > Per-zone LRUs and shrinkers for inode cache.

    Regardless of whether this is the right way to scale or not, I don't
    like the fact that this moves the cache LRUs into the memory
    management structures, and expands the use of MM specific structures
    throughout the code. It ties the cache implementation to the current
    VM implementation. That, IMO, goes against all the principle of
    modularisation at the source code level, and it means we have to tie
    all shrinker implemenations to the current internal implementation
    of the VM. I don't think that is wise thing to do because of the
    dependencies and impedance mismatches it introduces.

    As an example: XFS inodes to be reclaimed are simply tagged in a
    radix tree so the shrinker can reclaim inodes in optimal IO order
    rather strict LRU order. It simply does not match a zone-based
    shrinker implementation in any way, shape or form, nor does it's
    inherent parallelism match that of the way shrinkers are called.

    Any change in shrinker infrastructure needs to be able to handle
    these sorts of impedance mismatches between the VM and the cache
    subsystem. The current API doesn't handle this very well, either,
    so it's something that we need to fix so that scalability is easy
    for everyone.

    Anyway, my main point is that tying the LRU and shrinker scaling to
    the implementation of the VM is a one-off solution that doesn't work
    for generic infrastructure. Other subsystems need the same
    large-machine scaling treatment, and there's no way we should be
    tying them all into the struct zone. It needs further abstraction.


    Dave Chinner

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