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    SubjectRe: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable
    On Mon, Aug 26, 2019 at 09:40:35AM +0200, Michal Hocko wrote:
    > On Thu 22-08-19 18:29:34, Kirill A. Shutemov wrote:
    > > On Thu, Aug 22, 2019 at 02:56:56PM +0200, Vlastimil Babka wrote:
    > > > On 8/22/19 10:04 AM, Michal Hocko wrote:
    > > > > On Thu 22-08-19 01:55:25, Yang Shi wrote:
    > > > >> Available memory is one of the most important metrics for memory
    > > > >> pressure.
    > > > >
    > > > > I would disagree with this statement. It is a rough estimate that tells
    > > > > how much memory you can allocate before going into a more expensive
    > > > > reclaim (mostly swapping). Allocating that amount still might result in
    > > > > direct reclaim induced stalls. I do realize that this is simple metric
    > > > > that is attractive to use and works in many cases though.
    > > > >
    > > > >> Currently, the deferred split THPs are not accounted into
    > > > >> available memory, but they are reclaimable actually, like reclaimable
    > > > >> slabs.
    > > > >>
    > > > >> And, they seems very common with the common workloads when THP is
    > > > >> enabled. A simple run with MariaDB test of mmtest with THP enabled as
    > > > >> always shows it could generate over fifteen thousand deferred split THPs
    > > > >> (accumulated around 30G in one hour run, 75% of 40G memory for my VM).
    > > > >> It looks worth accounting in MemAvailable.
    > > > >
    > > > > OK, this makes sense. But your above numbers are really worrying.
    > > > > Accumulating such a large amount of pages that are likely not going to
    > > > > be used is really bad. They are essentially blocking any higher order
    > > > > allocations and also push the system towards more memory pressure.
    > > > >
    > > > > IIUC deferred splitting is mostly a workaround for nasty locking issues
    > > > > during splitting, right? This is not really an optimization to cache
    > > > > THPs for reuse or something like that. What is the reason this is not
    > > > > done from a worker context? At least THPs which would be freed
    > > > > completely sound like a good candidate for kworker tear down, no?
    > > >
    > > > Agreed that it's a good question. For Kirill :) Maybe with kworker approach we
    > > > also wouldn't need the cgroup awareness?
    > >
    > > I don't remember a particular locking issue, but I cannot say there's
    > > none :P
    > >
    > > It's artifact from decoupling PMD split from compound page split: the same
    > > page can be mapped multiple times with combination of PMDs and PTEs. Split
    > > of one PMD doesn't need to trigger split of all PMDs and underlying
    > > compound page.
    > >
    > > Other consideration is the fact that page split can fail and we need to
    > > have fallback for this case.
    > >
    > > Also in most cases THP split would be just waste of time if we would do
    > > them at the spot. If you don't have memory pressure it's better to wait
    > > until process termination: less pages on LRU is still beneficial.
    >
    > This might be true but the reality shows that a lot of THPs might be
    > waiting for the memory pressure that is essentially freeable on the
    > spot. So I am not really convinced that "less pages on LRUs" is really a
    > plausible justification. Can we free at least those THPs which are
    > unmapped completely without any pte mappings?

    Unmapped completely pages will be freed with current code. Deferred split
    only applies to partly mapped THPs: at least on 4k of the THP is still
    mapped somewhere.

    > > Main source of partly mapped THPs comes from exit path. When PMD mapping
    > > of THP got split across multiple VMAs (for instance due to mprotect()),
    > > in exit path we unmap PTEs belonging to one VMA just before unmapping the
    > > rest of the page. It would be total waste of time to split the page in
    > > this scenario.
    > >
    > > The whole deferred split thing still looks as a reasonable compromise
    > > to me.
    >
    > Even when it leads to all other problems mentioned in this and memcg
    > deferred reclaim series?

    Yes.

    You would still need deferred split even if you *try* to split the page on
    the spot. split_huge_page() can fail (due to pin on the page) and you will
    need to have a way to try again later.

    You'll not win anything in complexity by trying split_huge_page()
    immediately. I would ague you'll create much more complexity.

    > > We may have some kind of watermark and try to keep the number of deferred
    > > split THP under it. But it comes with own set of problems: what if all
    > > these pages are pinned for really long time and effectively not available
    > > for split.
    >
    > Again, why cannot we simply push the freeing where there are no other
    > mappings? This should be pretty common case, right?

    Partly mapped THP is not common case at all.

    To get to this point you will need to create a mapping, fault in THP and
    then unmap part of it. It requires very active memory management on
    application side. This kind of applications usually knows if THP is a fit
    for them.

    > I am still not sure that waiting for the memory reclaim is a general
    > win.

    It wins CPU cycles by not doing the work that is likely unneeded.
    split_huge_page() is not particularly lightweight operation from locking
    and atomic ops POV.

    > Do you have any examples of workloads that measurably benefit from
    > this lazy approach without any other downsides? In other words how
    > exactly do we measure cost/benefit model of this heuristic?

    Example? Sure.

    Compiling mm/memory.c in my setup generates 8 deferred split. 4 of them
    triggered from exit path. The rest 4 comes from MADV_DONTNEED. It doesn't
    make sense to convert any of them to in-place split: for short-lived
    process any split if waste of time without any benefit.

    --
    Kirill A. Shutemov

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