netdev - Re: [net-next PATCH v4 2/3] net: TCP thin linear timeouts

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Message-ID: <4B7D01CE.1030101@lastsummer.de>
Date:	Thu, 18 Feb 2010 10:01:02 +0100
From:	Franco Fichtner <franco@...tsummer.de>
To:	Andreas Petlund <apetlund@...ula.no>
CC:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Netdev <netdev@...r.kernel.org>, eric.dumazet@...il.com,
	hannemann@...s.rwth-aachen.de, LKML <linux-kernel@...r.kernel.org>,
	shemminger@...tta.com, william.allen.simpson@...il.com,
	damian@....rwth-aachen.de, ebiederm@...ssion.com,
	David Miller <davem@...emloft.net>
Subject: Re: [net-next PATCH v4 2/3] net: TCP thin linear timeouts

Hi,

Andreas Petlund wrote:
> On 02/18/2010 09:41 AM, Ilpo Järvinen wrote:
>   
>> On Wed, 17 Feb 2010, David Miller wrote:
>>
>>     
>>> From: Andreas Petlund <apetlund@...ula.no>
>>> Date: Tue, 16 Feb 2010 15:40:41 +0100
>>>
>>>       
>>>> @@ -341,6 +342,8 @@ struct tcp_sock {
>>>>  	u16	advmss;		/* Advertised MSS			*/
>>>>  	u8	frto_counter;	/* Number of new acks after RTO */
>>>>  	u8	nonagle;	/* Disable Nagle algorithm?             */
>>>> +	u8      thin_lto    : 1,/* Use linear timeouts for thin streams */
>>>> +		thin_undef  : 7;
>>>>  
>>>>         
>>> There is now a gap of 3 unused bytes here in this critical
>>> core TCP socket data structure.
>>>
>>> Please either find a way to avoid this hole, or document
>>> it with a comment.
>>>       
>> There would be multiple bits free for use in both frto_counter and nonagle 
>> byte.
>>
>>     
>
> I was playing aroud with this setup:
>
> =========
> u8	nonagle     : 4,/* Disable Nagle algorithm?             */
> 	thin_lto    : 1,/* Use linear timeouts for thin streams */
> 	thin_dupack : 1,/* Fast retransmit on first dupack      */
> 	thin_undef  : 2;
> =========
>
> Do you think that would do the trick?
>   

According to Ilpo, it would be ok to reduce both ftro_counter and
nonagle, so why not join all these into u16 and leave the remaining
free bits documented for other people. Like this:

    u16 frto_counter:x; /* Number of new acks after RTO */
    u16 nonagle:y; /* Disable Nagle algorithm? */
    u16 thin_lto:1; /* Use linear timeouts for thin streams */
    u16 unused:15-x-y;

Not sure about the y and x. Ilpo, can you comment on those values?


Thanks,
Franco
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