linux-kernel - Re: [Slightly off topic] A question about R/B trees.

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Message-ID: <48FCAA4D.3030802@gmail.com>
Date:	Mon, 20 Oct 2008 17:57:01 +0200
From:	Maxim Levitsky <maximlevitsky@...il.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Andi Kleen <andi@...stfloor.org>, Chris Snook <csnook@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [Slightly off topic] A question about R/B trees.

Pekka Enberg wrote:
> Hi Andi,
> 
> On Sat, Oct 18, 2008 at 10:53 AM, Andi Kleen <andi@...stfloor.org> wrote:
>> The problem with hash tables is that if they're big enough
>> or if the rest of the workload is memory intensive
>> each hit will be a cache miss. And you can do a lot of branch
>> mispredicts in the time of a single cache miss.
>>
>> In general trees can be much better for cache usage, although
>> it's generally better to use some tree that has nodes near
>> the cache line size. Binary trees like RB are too small for that.
> 
> Right, but even for binary trees, you can get some of the benefits by
> packing all the nodes in a slab cache of their own. That way many of
> the neighboring nodes will share the same cache line if you're
> allocating memory for the nodes in a top-down order. Of course, you
> lose the benefits if the tree is updated a lot because you're back to
> worst case allocation again.
> 
>                        Pekka

Thank you very much.

Best regards,
	Maxim Levitsky
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