netdev - Re: 200 millisecond timeouts in TCP

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Message-ID: <AANLkTilsYS8P2VqATmasXjjdhjPMMZznIHBByXc1i8cU@mail.gmail.com>
Date:	Fri, 4 Jun 2010 15:58:57 +0900
From:	Ryousei Takano <ryousei@...il.com>
To:	Ivan Novick <novickivan@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: 200 millisecond timeouts in TCP

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

> I am testing on RHEL5 with this kernel: 2.6.18-164.15.1.el5.
>
> Cheers,
> Ivan Novick
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