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

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Message-ID: <20081123223300.GA7094@linux.vnet.ibm.com>
Date:	Sun, 23 Nov 2008 14:33:00 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <dada1@...mosbay.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

On Sun, Nov 23, 2008 at 09:18:17PM +0100, Eric Dumazet wrote:
> Paul E. McKenney a écrit :
>> On Sun, Nov 23, 2008 at 07:42:14PM +0100, Eric Dumazet wrote:
>>> 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 :)
>> 200MB per CPU, right?
>> But yes, now that you mention it, 800GB of memory dedicated to TCP
>> connections sounds almost as ridiculous as did 640K of memory in the
>> late 1970s.  ;-)
>
> ;)
>
>> Nevertheless, I don't have an overwhelming objection to the current
>> code.  Easy enough to change should it become a problem, right?
>
> Sure. By that time, cpus might be 128 bits or 256 bits anyway :)

Or even 640K bits.  ;-)

							Thanx, Paul
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