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

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Message-ID: <20130613190831.GB19218@fieldses.org>
Date:	Thu, 13 Jun 2013 15:08:31 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>
Subject: Re: [PATCH] Percpu tag allocator

On Thu, Jun 13, 2013 at 11:53:18AM -0700, Tejun Heo wrote:
> The thing is that id[r|a] guarantee that the lowest available slot is
> allocated and this is important because it's used to name things which
> are visible to userland - things like block device minor number,
> device indicies and so on.  That alone pretty much ensures that
> alloc/free paths can't be very scalable which usually is fine for most
> id[r|a] use cases as long as lookup is fast.  I'm doubtful that it's a
> good idea to push per-cpu tag allocation into id[r|a].  The use cases
> are quite different.
> 
> In fact, maybe what we can do is adding some features on top of the
> tag allocator and moving id[r|a] users which don't require strict
> in-order allocation to it.  For example, NFS allocates an ID for each
> transaction it performs and uses it to index the associate command
> structure (Jeff, Bruce, please correct me if I'm getting it wrong).

It's not really per-transaction: it's more like an identifier for a
piece of lock state (an open, a lock, whatever), so a bit like a file
descriptor, but there's no requirement they be "small".

> The only requirement on IDs is that they shouldn't be recycled too
> fast.

Yep.--b.

> Currently, idr implements cyclic mode for it but it can easily
> be replaced with per-cpu tag allocator like this one and it'd be a lot
> more scalable.  There are a couple things to worry about tho - it
> probably should use the highbits as generation number as a tag is
> given out so that the actual ID doesn't get recycled quickly, and some
> form dynamic tag sizing would be nice too.
> 
> Thanks.
> 
> -- 
> tejun
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