linux-kernel - Re: [PATCH] Percpu tag allocator

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Date:	Thu, 13 Jun 2013 12:15:07 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kent Overstreet <koverstreet@...gle.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Jens Axboe <axboe@...nel.dk>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Jeff Layton <jlayton@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: [PATCH] Percpu tag allocator

Hello, Andrew.

On Thu, Jun 13, 2013 at 12:04:39PM -0700, Andrew Morton wrote:
> > The thing is that id[r|a] guarantee that the lowest available slot is
> > allocated
> 
> That isn't the case for ida_get_new_above() - the caller gets to
> control the starting index.

Hmmm?  get_new_above() is the same, it must allocate the first
available ID above the given low bound - used to exclude unused or
reserved IDs.

> The worst outcome here is that idr.c remains unimproved and we merge a
> new allocator which does basically the same thing.

The lowest number guarantee makes them different.  Maybe tag
allocation can be layered on top as a caching layer, I don't know, but
at any rate we need at least two different operation modes.

> The best outcome is that idr.c gets improved and we don't have to merge
> duplicative code.
>
> So please, let's put aside the shiny new thing for now and work out how
> we can use the existing tag allocator for these applications.  If we
> make a genuine effort to do this and decide that it's fundamentally
> hopeless then this is the time to start looking at new implementations.
> 
> (I can think of at least two ways of making ida_get_new_above() an
> order of magnitude faster for this application and I'm sure you guys
> can as well.)

Oh, I'm sure the current id[r|a] can be improved upon a lot but I'm
very skeptical one can reach the level of scalability necessary for,
say, pci-e attached extremely high-iops devices while still keeping
the lowest number allocation, which can't be achieved without strong
synchronization on each alloc/free.

Maybe we can layer things so that we have percpu layer on top of
id[r|a] and, say, mapping id to point is still done by idr, or the
percpu tag allocator uses ida for tag chunk allocations, but it's
still gonna be something extra on top.

Thanks.

-- 
tejun
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