[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Tue, 09 Sep 2008 14:40:00 -0400
From: Chris Snook <csnook@...hat.com>
To: Andi Kleen <andi@...stfloor.org>
CC: Rick Jones <rick.jones2@...com>, Netdev <netdev@...r.kernel.org>
Subject: Re: RFC: Nagle latency tuning
Andi Kleen wrote:
> Chris Snook <csnook@...hat.com> writes:
>> I'd like to know where the 40 ms magic number comes from.
>
> From TCP_ATO_MIN
>
> #define TCP_ATO_MIN ((unsigned)(HZ/25))
>
>> That's the
>> one that really hurts, and if we could lower that without doing
>> horrible things elsewhere in the stack,
>
> You can lower it (with likely some bad side effects), but I don't think it
> would make these apps very happy in the end because they likely want
> no delay at all.
>
> -Andi
These apps have a love/hate relationship with TCP. They'll probably love SCTP 5
years from now, but it's not mature enough for them yet. They do want to
minimize all latencies, and many of the apps explicitly set TCP_NODELAY. The
goal here is to improve latencies on the supporting apps that aren't quite as
carefully optimized as the main message daemons themselves. If we can give them
a knob that bounds their worst-case latency to 2-3 times their average latency,
without risking network floods that won't show up in testing, they'll be much
happier.
-- Chris
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists