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Date: Fri, 30 Apr 2010 15:00:20 -0700
From: Maxim Sobolev <sobomax@FreeBSD.org>
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To: Julian Elischer <julian@elischer.org>
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Cc: freebsd-net@FreeBSD.org, "current@freebsd.org" <current@FreeBSD.org>
Subject: Re: Making IFQ_MAXLEN tunable
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Julian Elischer wrote:
> On 4/30/10 1:23 PM, Maxim Sobolev wrote:
>> Hi,
>>
>> Many network drivers in the FreeBSD kernel use the IFQ_MAXLEN value to
>> set length of the outgoing packets queue. The default value for that
>> parameter is only 50, which is pretty low especially for the cases when
>> the system handles lot of small packets and can cause ENOBUFS in
>> applications under the load. The following patch makes IFQ_MAXLEN a
>> tunable. I am also tempted to bump the default value for IFQ_MAXLEN
>> 10-fold, but would like to hear what do people think about it first.
>>
>> http://sobomax.sippysoft.com/IFQ_MAXLEN.diff
> 
> so just tunable? not a sysctl :-)

The sysctl would require much bigger rewrite. As long as I understand 
the value is now cached in many instances of the ifnet structure, and 
some drivers even use their own queue length instead of IFQ_MAXLEN. 
Therefore, even if I make this parameter a sysctl one would have to 
destroy interface and create it again in order for the change to have an 
effect. Therefore, keeping it tunable would be less confusing.

> patch could be a lot smaller if you defined IFQ_MAXLEN to be V_ifqmaxlen 
> (do different vimages want a different value?)

I am not quite sure about that. AFAIK vimage is more high-level thing, 
while this parameter controls queue length between kernel and hardware 
interface driver. vimage lies above that.

-Maxim