kern/120057: [patch] Allow proper settings of ALTQ_HFSC. The check i wrong since even with the values forbidden from this check you get a concave curve.

[Ermal Luçi]

The following reply was made to PR kern/120057; it has been noted by GNATS.

From: "=?ISO-8859-1?Q?Ermal_Lu=E7i?=" <ermal.luci at gmail.com>
To: "Max Laier" <max at love2party.net>
Cc: bug-followup at freebsd.org, eri at freebsd.org
Subject: Re: kern/120057: [patch] Allow proper settings of ALTQ_HFSC. The check i wrong since even with the values forbidden from this check you get a concave curve.
Date: Tue, 29 Jan 2008 22:11:32 +0100

 Following up, since i noticed that the mail was truncated.
 
 Also not that the link you gave me, has the note:
 <snip>
 In order to decouple delay and bandwidth allocation, HFSC is designed
 based on the service curve service model. In HFSC, only two-piece
 linear service curves are used for simplicity. A two-piece linear
 service curve is characterized by three parameters:
 
    * m1, the slope of the first segment
    * m2, the slope of the second segment
    * d, the x-projection of the intersection point of the two segments
 
 The following figure illustrates the two types of two-piece linear
 service curves used in HFSC. For a convex curve (when m1 is less than
 m2), m1 is always zero.
 </snip>
 
 But beware, that m1 here is in slope terms while m1 parameter of
 service curves is not a slope! It is bytes per tick.
 To check if a curve is concave, in the paper there is a proper formula
 but needs some info that is not available at configuration time. But
 as i said you cannot really configure a convex service curve.