linux-kernel - Re: [PATCH] idr: Document ida tree sections

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Message-ID: <20130813221928.GE28996@mtj.dyndns.org>
Date:	Tue, 13 Aug 2013 18:19:28 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] idr: Document ida tree sections

Hello,

On Tue, Aug 13, 2013 at 03:13:08PM -0700, Kent Overstreet wrote:
> If you're convinced this is a real issue though - how about

It is a real issue.  Large order allocation is fine for optimization
but shouldn't be depended upon.  It does fail easily without
compaction and compaction is heavy-ass operation which will blow up
any minute performance advantage you might get from avoiding proper
radix tree implementation.

> IDA_SECTION_SIZE conditional on CONFIG_COMPACTION, so we use order 2 or
> 3 allocations if CONFIG_COMPACTION=n?
> 
> Then the max size toplevel array of pointers to segments would be
> bigger, but that's only an issue when we're allocating up to near
> INT_MAX ids, so it's difficult to see how _that_ would be an issue on a
> small/embedded system... and we could even use vmalloc for that
> allocation when the size of that array is > IDA_SECTION_SIZE.

What about cyclic allocations then?  This is natrually a radix tree
problem.  I don't know why you're resisting radix tree so much here.

Thanks.

-- 
tejun
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