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    SubjectRe: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
    On Mon, 16 Jul 2018 05:41:15 -0700 Matthew Wilcox <willy@infradead.org> wrote:

    > On Mon, Jul 16, 2018 at 11:09:01AM +0200, Michal Hocko wrote:
    > > On Fri 13-07-18 10:36:14, Dave Chinner wrote:
    > > [...]
    > > > By limiting the number of negative dentries in this case, internal
    > > > slab fragmentation is reduced such that reclaim cost never gets out
    > > > of control. While it appears to "fix" the symptoms, it doesn't
    > > > address the underlying problem. It is a partial solution at best but
    > > > at worst it's another opaque knob that nobody knows how or when to
    > > > tune.
    > >
    > > Would it help to put all the negative dentries into its own slab cache?
    >
    > Maybe the dcache should be more sensitive to its own needs. In __d_alloc,
    > it could check whether there are a high proportion of negative dentries
    > and start recycling some existing negative dentries.

    Well, yes.

    The proposed patchset adds all this background reclaiming. Problem is
    a) that background reclaiming sometimes can't keep up so a synchronous
    direct-reclaim was added on top and b) reclaiming dentries in the
    background will cause non-dentry-allocating tasks to suffer because of
    activity from the dentry-allocating tasks, which is inappropriate.

    I expect a better design is something like

    __d_alloc()
    {
    ...
    while (too many dentries)
    call the dcache shrinker
    ...
    }

    and that's it. This way we have a hard upper limit and only the tasks
    which are creating dentries suffer the cost.


    Regarding the slab page fragmentation issue: I'm wondering if the whole
    idea of balancing the slab scan rates against the page scan rates isn't
    really working out. Maybe shrink_slab() should be sitting there
    hammering the caches until they have freed up a particular number of
    pages. Quite a big change, conceptually and implementationally.

    Aside: about a billion years ago we were having issues with processes
    getting stuck in direct reclaim because other processes were coming in
    and stealing away the pages which the direct-reclaimer had just freed.
    One possible solution to that was to make direct-reclaiming tasks
    release the freed pages into a list on the task_struct. So those pages
    were invisible to other allocating tasks and were available to the
    direct-reclaimer when it returned from the reclaim effort. I forget
    what happened to this.

    It's quite a small code change and would provide a mechanism for
    implementing the hammer-cache-until-youve-freed-enough design above.



    Aside 2: if we *do* do something like the above __d_alloc() pseudo code
    then perhaps it could be cast in terms of pages, not dentries. ie,

    __d_alloc()
    {
    ...
    while (too many pages in dentry_cache)
    call the dcache shrinker
    ...
    }

    and, apart from the external name thing (grr), that should address
    these fragmentation issues, no? I assume it's easy to ask slab how
    many pages are presently in use for a particular cache.

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