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    SubjectRe: [RFC][PATCH 6/8] mm: handle_speculative_fault()


    On Thu, 7 Jan 2010, Christoph Lameter wrote:
    > >
    > > depends on the workload; on a many-threads-java workload, you also get
    > > it for write quite a bit (lots of malloc/frees in userspace in addition
    > > to pagefaults).. at which point you do end up serializing on the
    > > zeroing.
    > >
    > > There's some real life real big workloads that show this pretty badly;
    > > so far the workaround is to have glibc batch up a lot of the free()s..
    > > but that's just pushing it a little further out.
    >
    > Again mmap_sem is a rwsem and only a read lock is held. Zeroing in
    > do_anonymous_page can occur concurrently on multiple processors in the
    > same address space. The pte lock is intentionally taken *after* zeroing to
    > allow concurrent zeroing to occur.

    You're missing what Arjan said - the jav workload does a lot of memory
    allocations too, causing mmap/munmap.

    So now some paths are indeed holding it for writing (or need to wait for
    it to become writable). And the fairness of rwsems quite possibly then
    impacts throughput a _lot_..

    (Side note: I wonder if we should wake up _all_ readers when we wake up
    any. Right now, we wake up all readers - but only until we hit a writer.
    Which is the _fair_ thing to do, but it does mean that we can end up in
    horrible patterns of alternating readers/writers, when it could be much
    better to just say "release the hounds" and let all pending readers go
    after a writer has had its turn).

    Linus


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