Re: ext3 allocate-with-reservation latencies

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Wed, 2005-04-06 at 17:53, Mingming Cao wrote:

> > Possible, but not necessarily nice.  If you've got a nearly-full disk,
> > most bits will be already allocated.  As you scan the bitmaps, it may
> > take quite a while to find a free bit; do you really want to (a) lock
> > the whole block group with a temporary window just to do the scan, or
> > (b) keep allocating multiple smaller windows until you finally find a
> > free bit?  The former is bad for concurrency if you have multiple tasks
> > trying to allocate nearby on disk --- you'll force them into different
> > block groups.  The latter is high overhead.

> I am not quite understand what you mean about (a).  In this proposal, we
> will drop the lock before the scan. 

s/lock/reserve/.  

> And for (b), maybe I did not make myself clear: I am not proposing to
> keeping allocating multiple smaller windows until finally find a free
> bit. I mean, we book the window(just link the node into the tree) before
> we drop the lock, if there is no free bit inside that window, we will go
> back search for another window(call find_next_reserveable_window()),
> inside it, we will remove the temporary window we just created and find
> next window. SO we only have one temporary window at a time. 

And that's the problem.  Either we create small temporary windows, in
which case we may end up thrashing through vast numbers of them before
we find a bit that's available --- very expensive as the disk gets full
--- or we use large windows but get worse layout when there are parallel
allocators going on.

--Stephen

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