netdev - Re: [PATCH] net: Convert TCP/DCCP listening hash tables to use RCU

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Date:	Sun, 23 Nov 2008 19:42:14 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	David Miller <davem@...emloft.net>,
	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] net: Convert TCP/DCCP listening hash tables to use RCU

Paul E. McKenney a écrit :
> On Sun, Nov 23, 2008 at 10:33:28AM +0100, Eric Dumazet wrote:
>> Hi David
>>
>> Please find patch to convert TCP/DCCP listening hash tables
>> to RCU.
>>
>> A followup patch will cleanup all sk_node fields and macros
>> that are not used anymore.
>>
>> Thanks
>>
>> [PATCH] net: Convert TCP/DCCP listening hash tables to use RCU
>>
>> This is the last step to be able to perform full RCU lookups
>> in __inet_lookup() : After established/timewait tables, we
>> add RCU lookups to listening hash table.
>>
>> The only trick here is that a socket of a given type (TCP ipv4,
>> TCP ipv6, ...) can now flight between two different tables
>> (established and listening) during a RCU grace period, so we
>> must use different 'nulls' end-of-chain values for two tables.
>>
>> We define a large value :
>>
>> #define LISTENING_NULLS_BASE (1U << 29)
> 
> I do like this use of the full set up upper bits!  However, wouldn't it
> be a good idea to use a larger base value for 64-bit systems, perhaps
> using CONFIG_64BIT to choose?  500M entries might not seem like that
> many in a few years time...
> 

Well, this value is correct up to 2^29 slots, and a hash table of 2^32 bytes
(8 bytes per pointer)

A TCP socket uses about 1472 bytes on 64bit arches, so 2^29 sessions
would need 800 GB of ram, not counting dentries, inodes, ...

I really doubt a machine, even with 4096 cpus should/can handle so many
tcp sessions :)


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