netdev - Re: [PATCH net-next] tcp: RFC6298 supersedes RFC2988bis

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Message-ID: <CAPshTCjNyRe_pHUX+kASgJshupPGwg7RP3sL2WFPuXX5zkUMgw@mail.gmail.com>
Date:	Sun, 15 Apr 2012 19:21:05 -0700
From:	Jerry Chu <hkchu@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH net-next] tcp: RFC6298 supersedes RFC2988bis

n Sun, Apr 15, 2012 at 1:01 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Sun, 2012-04-15 at 11:33 -0700, Jerry Chu wrote:
>> [send again - it looks like my previous comment was lost...]
>>
>> On Thu, Apr 12, 2012 at 10:48 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > Updates some comments to track RFC6298
>> >
>> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> > Cc: H.K. Jerry Chu <hkchu@...gle.com>
>> > Cc: Tom Herbert <therbert@...gle.com>
>> > ---
>> > BTW, one side effect of the TCP_TIMEOUT_INIT change (3 -> 1) is
>> > () latency is 200% worse:
>>
>> Or even worse - 300% (3/1)?
>
> well, 3 instead of 1 is a 200% increase ;)
>
>>
>> >
>> > It fires every 200ms and scans 40% of hash table each time, listener
>> > socket held.
>>
>> If this becomes a real issue we could decrease TCP_SYNQ_INTERVAL,
>> essentially making the keepalive timer fire more often, but each time with
>> less work to do?
>
> Hmm... 200ms is already aggressive for power saving

Not sure how much power saving can one attain when syn_table is non-empty
anyway.

>
>>
>> Also why is
>> budget = 2 * (lopt->nr_table_entries / (timeout / interval));
>> rather than
>> budget = (lopt->nr_table_entries / (timeout / interval)) + 1;
>> ?
>
> Thats because if we do that, retransmits could be delayed by 100%,
> instead of 50% with this solution.

Oh, that's right - the delay can be upto the time to scan the whole table
so it's a balance between how much delay vs the processing overhead
in the current data structure...

Thanks,

Jerry

>
> (right now it takes 2.5 rounds to scan whole table, so a one sec 'timer'
> can be fired after 1.6 second)
>
>
>
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