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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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