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    SubjectRe: IO scheduler benchmarking
    Andrea Arcangeli <andrea@suse.de> wrote:
    >
    > I don't
    > buy Andrew complaining about the write throttling when he still allows
    > several dozen mbytes of ram in flight and invisible to the VM,

    The 2.5 VM accounts for these pages (/proc/meminfo:Writeback) and throttling
    decisions are made upon the sum of dirty+writeback pages.

    The 2.5 VFS limits the amount of dirty+writeback memory, not just the amount
    of dirty memory.

    Throttling in both write() and the page allocator is fully decoupled from the
    queue size. An 8192-slot (4 gigabyte) queue on a 32M machine has been
    tested.

    The only tasks which block in get_request_wait() are the ones which we want
    to block there: heavy writers.

    Page reclaim will never block page allocators in get_request_wait(). That
    causes terrible latency if the writer is still active.

    Page reclaim will never block a page-allocating process on I/O against a
    particular disk block. Allocators are instead throttled against _any_ write
    I/O completion. (This is broken in several ways, but it works well enough to
    leave it alone I think).

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