Re: ext3 allocate-with-reservation latencies

[Ingo Molnar <mingo@elte.hu>]

* 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