调整txqueuelen 有什么作用

depthblue_xsc

系统默认的100
ifconfig ethN txqueuelen 1000
会对系统有什么影响？

xjarmstrong

首先确定一点，这个参数对于ppp拨号上网或其他低带宽的网络作用较大，对于局域网作用不大，局域网中你可以将其设置成0，这样就会减少延时，对像telnet这样对交互式响应要求较高的应用是很有作用的。下面主要是分析ppp拨号中该参数对网络性能的影响：

txqueuelen如果过大，会影响交互式应用（延时会加长），比如telnet，在网络环境不佳时，延时会很大。
对于像3G这样的网络，如果应用层有大量的数据上传，txqueuelen过大，网络协议栈对数据的限制就会变小，这样可能会对驱动层造成一定的压力，严重的话，可能会导致驱动卡死。

txqueuelen如果过小，对交互式应用的响应效果会比较好，因为这个值较小时，对于大量数据的丢包率较高，这样会触发TCP加强对数据的重传（这里的理解可能不对），提高实时响应。但丢包率太大会影响我们数据传输的效率。

所以建议根据不用的应用需求与网络环境设置合理的txqueuelen。

如果使用ppp上网，上传的数据量不高并且需要较高的实时响应，建议将此参数调低一点。当然以现在的3G网络带宽，我们不需要设的太低，ppp中默认的3，我觉得也是电话线时代的经验值了，3G完全可以设置到50以上。

如果每秒上传的数据较多，建议将该参数设置大一点，比如100左右，但不能太高，毕竟我们的3g带宽有限，性能本来就不行，设置太高了不仅无效，反而会对对驱动造成压力。

下面是一封来自linux内核邮件，解释了txqueuelen与ppp性能的关系，可供参考：

Poor PPP performance and txqueuelen
From: Arthur Kelly (arthur@sevenkings.net)
Date: Thu Jan 06 2000 - 18:21:38 EST

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>From the archives:


>> If not, wouldn't it be better to keep txquueuelen high and
>> just grin and bear the latency for interactive use?
>
> No you want it smaller, this makes packet loss feedback happen much
> more quickly, and thus TCP's heuristics respond to the situation faster.
>
> David S. Miller


That does not work well here. And it seems to contradict the ethernet
default of 100 for txqueuelen.


I have a 2.2.14aa1 box routing between ethernet and dialup PPP
interfaces. With the default PPP txqueuelen of 10, or if I set the
PPP txqueuelen to a smaller value such as 3, I get dropped packets and
poor FTP performance thru the Linux router.


But if I set Linux PPP txqueuelen to 30 or more, there are no dropped
packets and FTP performance thru the Linux router is perfect.


In ppp.c, the default value for receive queue length is 32. In my
local ppp.c, I've also changed the default transmit queue length to
32, and it works much better now.




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