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    SubjectRe: ext3 allocate-with-reservation latencies

    * Mingming Cao <cmm@us.ibm.com> wrote:

    > It seems we are holding the rsv_block while searching the bitmap for a
    > free bit. In alloc_new_reservation(), we first find a available to
    > create a reservation window, then we check the bitmap to see if it
    > contains any free block. If not, we will search for next available
    > window, so on and on. During the whole process we are holding the
    > global rsv_lock. We could, and probably should, avoid that. Just
    > unlock the rsv_lock before the bitmap search and re-grab it after it.
    > We need to make sure that the available space that are still available
    > after we re- grab the lock.
    >
    > Another option is to hold that available window before we release the
    > rsv_lock, and if there is no free bit inside that window, we will
    > remove it from the tree in the next round of searching for next
    > available window.
    >
    > I prefer the second option, and plan to code it up soon. Any comments?

    doesnt the first option also allow searches to be in parallel? This
    particular one took over 1 msec, so it seems there's a fair room for
    parallellizing multiple writers and thus improving scalability. (or is
    this codepath serialized globally anyway?)

    Ingo
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