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Date:	Wed, 05 Sep 2007 15:58:18 +0200
From:	Jesper Dangaard Brouer <jdb@...x.dk>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Jesper Dangaard Brouer <hawk@...u.dk>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: Re: [PATCH 2/2]: [NET_SCHED]: Making rate table lookups
	more	flexible.

On Tue, 2007-09-04 at 18:25 +0200, Patrick McHardy wrote:
> Jesper Dangaard Brouer wrote:
> > On Sun, 2007-09-02 at 23:16 +0200, Patrick McHardy wrote:
> > 
> >>Jesper Dangaard Brouer wrote:
> >>
> >>>On Sun, 2 Sep 2007, Patrick McHardy wrote:
> >>>
> >>>Lets focus on the general case, where the functionality actually is 
> >>>needed right away.
> >>>
> >>>In the general case:
> >>>
> >>>- The rate table needs to be aligned (cell_align=-1).
> >>>  (currently, we miscalculates up to 7 bytes on every lookup)
> >>
> >>We will always do that, thats a consequence of storing the
> >>transmission times for multiples of 8b.
> > 
> > 
> > The issue is that we use the lower boundary for calculating the transmit
> > cost. Thus, a 15 bytes packet only have a transmit cost of 8 bytes.
> 
> I believe this is something that should be fixed anyway,
> its better to overestimate than underestimate to stay
> in control of the queue. 

Well, I have attached a patch that uses the upper boundry instead.
The patch uses the cell_align feature.

The patch is very simple it self, but figure out what happens the rtab
array requires a little illustration:

Illustrating the rate table array:
 Legend description
  rtab[x]   : Array index x of rtab[x]
  xmit_sz   : Transmit size contained in rtab[x] (normal transmit time)
  maps[a-b] : Packet sizes from a to b, will map into rtab[x]

Current/old rate table mapping (cell_log:3):
 rtab[0]:=xmit_sz:0  maps[0-7]
 rtab[1]:=xmit_sz:8  maps[8-15]
 rtab[2]:=xmit_sz:16 maps[16-23]
 rtab[3]:=xmit_sz:24 maps[24-31]
 rtab[4]:=xmit_sz:32 maps[32-39]
 rtab[5]:=xmit_sz:40 maps[40-47]
 rtab[6]:=xmit_sz:48 maps[48-55]

New rate table mapping, with kernel cell_align support.
 rtab[0]:=xmit_sz:8  maps[0-8]
 rtab[1]:=xmit_sz:16 maps[9-16]
 rtab[2]:=xmit_sz:24 maps[17-24]
 rtab[3]:=xmit_sz:32 maps[25-32]
 rtab[4]:=xmit_sz:40 maps[33-40]
 rtab[5]:=xmit_sz:48 maps[41-48]
 rtab[6]:=xmit_sz:56 maps[49-56]

New TC util on a kernel WITHOUT support for cell_align
 rtab[0]:=xmit_sz:8 maps[0-7]
 rtab[1]:=xmit_sz:16 maps[8-15]
 rtab[2]:=xmit_sz:24 maps[16-23]
 rtab[3]:=xmit_sz:32 maps[24-31]
 rtab[4]:=xmit_sz:40 maps[32-39]
 rtab[5]:=xmit_sz:48 maps[40-47]
 rtab[6]:=xmit_sz:56 maps[48-55]

Notice that without the kernel cell_align feature, we are only off by
one byte.  That should be acceptable, when somebody uses a new TC util
on a old kernel. 

> We could additionally make the
> rate tables more finegrained (optionally).

That is actually already possible with the approach used to handle
overflow of the rate table ("TSO" large packet support).  By setting
cell_log=0, and letting the overflow code handle the rest, we get a very
fingrained lookup.


> >>>- The existing tc overhead calc can be made more accurate.
> >>>  (by adding overhead before doing the lookup, instead of the
> >>>   current solution where the rate table is modified with its
> >>>   limited resolution)
> >>
> >>Please demonstrate this with patches (one for the overhead
> >>calculation, one for the cell_align thing), then we can
> >>continue this discussion.
> > 
> > 
> > I have attached a patch for the overhead calculation.

Attached is a patch that uses "the cell_align thing".

> Thanks, I probably won't get to looking into this until
> after the netfilter workshop next week.

Okay, but I'll see you at the workshop, so I might bug you there ;-)

-- 
Med venlig hilsen / Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network developer
  Cand. Scient Datalog / MSc.
  Author of http://adsl-optimizer.dk

View attachment "upperbound_rate_table_aligned.patch" of type "text/x-patch" (2156 bytes)

View attachment "cleanup_tc_calc_rtable_git.patch" of type "text/x-patch" (6028 bytes)

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