netdev - Re: [PATCH 6/6] [TCP]: Reorganize struct tcp_sock to save 16 bytes on 64-bit arch

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Message-ID: <20080131201720.GE11940@ghostprotocols.net>
Date:	Thu, 31 Jan 2008 18:17:20 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	dccp@...r.kernel.org
Subject: Re: [PATCH 6/6] [TCP]: Reorganize struct tcp_sock to save 16 bytes
	on 64-bit arch

Em Thu, Jan 31, 2008 at 08:57:53PM +0100, Eric Dumazet escreveu:
> Arnaldo Carvalho de Melo a écrit :
>> /home/acme/git/net-2.6/net/ipv6/tcp_ipv6.c:
>>   struct tcp_sock  |  -16
>>   struct tcp6_sock |  -16
>>  2 structs changed
>>
>> Now it is at:
>>
>> /* size: 1552, cachelines: 25 */
>> /* paddings: 2, sum paddings: 8 */
>> /* last cacheline: 16 bytes */
>>
>> As soon as we stop using skb_queue_list we'll get it down to 24 cachelines.
>>
>> Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
>> ---
>>  include/linux/tcp.h |    6 ++++--
>>  1 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
>> index 08027f1..f48644d 100644
>> --- a/include/linux/tcp.h
>> +++ b/include/linux/tcp.h
>> @@ -304,7 +304,6 @@ struct tcp_sock {
>>  	u32	rtt_seq;	/* sequence number to update rttvar	*/
>>   	u32	packets_out;	/* Packets which are "in flight"	*/
>> -	u32	retrans_out;	/* Retransmitted packets out		*/
>>  /*
>>   *      Options received (usually on last packet, some only on SYN packets).
>>   */
>> @@ -332,6 +331,8 @@ struct tcp_sock {
>>   	struct tcp_sack_block recv_sack_cache[4];
>>  +	u32	retrans_out;	/* Retransmitted packets out		*/
>> +
>
> Hum... retrans_out should sit close to packets_out (or lost_out/sacked_out 
> ???), please.
>
> 'struct tcp_sock' is very large on 64 bits, so I would prefer to make sure 
> most paths dont need to touch all 24 cache lines (or 25 cache lines).

That is perfectly fine, I'll replace my patch with another, that states
this beyond doubt.

- Arnaldo
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