netdev - Re: 200 millisecond timeouts in TCP

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Message-ID: <20100604150202.GA8268@localhost.localdomain>
Date:	Sat, 5 Jun 2010 00:02:02 +0900
From:	Satoru SATOH <satoru.satoh@...il.com>
To:	Ryousei Takano <ryousei@...il.com>
Cc:	Ivan Novick <novickivan@...il.com>, netdev@...r.kernel.org
Subject: Re: 200 millisecond timeouts in TCP

It's possible to tune the RTO min value (rto_min) per route since
2.6.23+.  (see also the manual of iproute2, ip(8) )

- satoru

On Fri, Jun 04, 2010 at 03:58:57PM +0900, Ryousei Takano wrote:
> On Fri, Jun 4, 2010 at 7:37 AM, Ivan Novick <novickivan@...il.com> wrote:
> > Hello,
> >
> > Using tcpdump and systemtap I am seeing that sometimes retransmission
> > of data is sent after waiting 200 milliseconds.  However sometimes
> > retransmissions happen quicker.
> >
> > Is there a specifc event that causes these 200 milisec delays to kick
> > in?  Are those events identifiable in netstat -s output?
> >
> > Also do you know if the timeout numbers for TCP are configurable parameters?
> >
> The minimum RTO value is fixed to 200 ms.  It is useful to make the min/max
> RTO values tunable.  For example, reducing the minimum RTO value is effective
> for TCP incast problem [1].  Of course, it may occur spurious retransmissions.
> 
> [1] Vijay Vasudevan, et al, Safe and Effective Fine-grained TCP Retransmissions
> for Datacenter Communication, SIGCOMM2009
> 
> In Solaris, there are two tunable parameters: tcp_rexmit_interval_min/max.
> 
> Do you have plan to introduce sysctl parameters like these to the Linux.
> 
> Thanks,
> Ryousei
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