netdev - Re: [PATCH 0/3] net: RCU lookups for UDP, DCCP and TCP protocol

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Message-ID: <87k5b7czli.fsf@basil.nowhere.org>
Date:	Thu, 13 Nov 2008 18:20:09 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Corey Minyard <minyard@....org>,
	Stephen Hemminger <shemminger@...tta.com>,
	benny+usenet@...rsen.dk,
	Linux Netdev List <netdev@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Evgeniy Polyakov <zbr@...emap.net>,
	Christian Bell <christian@...i.com>
Subject: Re: [PATCH 0/3] net: RCU lookups for UDP, DCCP and TCP protocol

Eric Dumazet <dada1@...mosbay.com> writes:


> 1) Introduce hlist_nulls variant of hlist
>
>   hlist uses NULL value to finish a chain.
>   hlist_nulls variant use the low order bit set to 1 to signal an end marker.
>   This allows to store many different end markers, so that some RCU lockless
>   algos (used in TCP/UDP stack for example) can save some memory barriers in
>   fast paths.

Do you have any numbers that demonstrate the read memory barriers being
a performance problem? At least on x86 they should be very cheap
because they're normally nops.

-Andi

-- 
ak@...ux.intel.com
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