It The Fq_codel settings prevent excess buffering on the push of data out over the cable modem itself / it will prevent unnecessary premature reduction of the  tcp sliding window size further preventing a cascading backlog that ends up further reducing the sliding window and slowing down the overall outbound transfer rate. 

A buffering problem in the cable modem only happens if you feed it data to quickly. 






Sent from my iPhone

On Aug 20, 2012, at 10:33 PM, dpreed@reed.com wrote:

> I'm confused.  Fq_codel does not (to my knowledge) fix bufferbloat *in your cable modem*  or *in the CMTS head-end*.
>  
> How could it?   In order for that to be fixed, you need to manage the buffer in the cable modem itself.
>  
> -----Original Message-----
> From: "Sebastian Moeller" <moeller0@gmx.de>
> Sent: Monday, August 20, 2012 2:24pm
> To: "Dave Taht" <dave.taht@gmail.com>
> Cc: cerowrt-devel@lists.bufferbloat.net
> Subject: Re: [Cerowrt-devel] cerowrt 3.3.8-17 is released
> 
> Hi Dave,
> 
> so I went to play around with this a bit more. I turned to UDP flooding my cable modem through the router and this surely allows me to create enough load on the wndr3700v2 to cause the allocation errors and as a "bonus" also to drive the router to reboot (driven by the watchdog timer?). Here is the script I used over 5G wireless (from http://blog.ioshints.info/2008/03/udp-flood-in-perl.html)
> 
> #!/usr/bin/perl
> ##############
> 
> # udp flood.
> ##############
> 
> use Socket;
> use strict;
> 
> if ($#ARGV != 3) {
> print "flood.pl <ip> <port> <size> <time>\n\n";
> print " port=0: use random ports\n";
> print " size=0: use random size between 64 and 1024\n";
> print " time=0: continuous flood\n";
> exit(1);
> }
> 
> my ($ip,$port,$size,$time) = @ARGV;
> 
> my ($iaddr,$endtime,$psize,$pport);
> 
> $iaddr = inet_aton("$ip") or die "Cannot resolve hostname $ip\n";
> $endtime = time() + ($time ? $time : 1000000);
> 
> socket(flood, PF_INET, SOCK_DGRAM, 17);
> 
> 
> print "Flooding $ip " . ($port ? $port : "random") . " port with " . 
> ($size ? "$size-byte" : "random size") . " packets" . 
> ($time ? " for $time seconds" : "") . "\n";
> print "Break with Ctrl-C\n" unless $time;
> 
> for (;time() <= $endtime;) {
> $psize = $size ? $size : int(rand(1024-64)+64) ;
> $pport = $port ? $port : int(rand(65500))+1;
> 
> send(flood, pack("a$psize","flood"), 0, pack_sockaddr_in($pport, $iaddr));}
> 
> called as either
> udp_flood.pl 192.168.100.1 0 1024 240
> or 
> udp_flood.pl 192.168.100.1 32000 1024 240
> 
> The first version with randomized port number spreads the load nicely over many fq_codel bins/flows and seems slightly more likely to cause allocation errors and reboots than the 2nd invocation which restricts itself to port 32000 and presumably just one flow.
> I wonder how to make cerowrt survive this kind of stress test… 
> 
> best
> Sebastian
> 
> 
> On Aug 15, 2012, at 9:08 PM, Dave Taht wrote:
> 
> > re: ath: skbuff alloc of size 1926 failed
> > 
> > as for the ath skbuff problem, I've seen that a lot. I had put hard
> > packet limits (~600) on fq_codel in -11 and prior that were too low
> > and it mostly went away, but I hit tail drop behavior everywhere,
> > instead of codel behavior. What I have now (typically 1200) may well
> > be too high, but not as overly high as the default (10k packets).
> > There may be another means of increasing the size of that slab pool or
> > making it less onerous.
> > 
> > I would like it if codel "kicked in" earlier than it currently does.
> > The code in ns2 is currently using half the period that the linux code
> > is. This would control things better, or so I hope (planning on trying
> > this as I get time)
> > 
> > I am also considering means of artificially upscaling the drop
> > scheduler when we get close to queue limits.
> > 
> > See some discussions on the codel list for these issues. (sims are
> > easier to deal with than cerowrt, too!)
> > 
> > as for bind, it should be automagically restarted from xinetd, no need
> > to fiddle with anything. However, since you are already under massive
> > memory pressure, it may well fail to start up that way, too. At the
> > moment, I've largely given up on bind on anything but a more core home
> > gw, and am running dnsmasq on everything (3700v2, picostations,
> > nanostations) but the 3800s. (and the ones I run it on, aren't being
> > used for wifi right now).
> > 
> > Lastly: Swap space won't help you on exhausting kernel limits.
> > 
> > I'm glad you can reproduce the ath: slab problem - I can get it too at
> > high rates using netperf over wifi. I will try a 3700v2 with and
> > without bind to see if it's still there in 3.3.8-17. In the meantime
> > if anyone knows how to get more allocations in that (2048? 4096?) slab
> > by default, perhaps that will help?
> > 
> > 
> > 
> > On Wed, Aug 15, 2012 at 10:23 AM, Sebastian Moeller <moeller0@gmx.de> wrote:
> >> Hi Dave,
> >> 
> >> great work, as always I upgraded my production router to the latest and greatest (since I only have one router…). And it works quite well for normal usage…
> >> Netalyzr reports around 2800ms seconds of uplink buffering, yet saturating the uplink does not affect ping times to a remote target noticeably, basically the same as for all codellized ceo versions I tested so far...
> >> 
> >> Some notes and a question:
> >> I noticed that even given plenty of swap space (1GB on a usb stick), using http://broadband.mpi-sws.org/residential/ to exercise UDP stress (on the uplink I assume) I can easily produce (I run the test from a macosx via 5GHz wireless over 1.5 yards):
> >> Aug 15 01:16:29 nacktmulle kern.err kernel: [175395.132812] ath: skbuff alloc of size 1926 failed
> >> (and plenty of those…).
> >> What then happens is that the OOM killer will aim for bind (reasonable since it is the largest single process) and kill it. When I try to restart bind by:
> >> root@nacktmulle:~# /etc/rc.d/S47namedprep start
> >> root@nacktmulle:~# /etc/rc.d/S48named restart
> >> Stopping isc-bind
> >> /etc/chroot/named//var/run/named/named.pid not found, trying brute force
> >> killall: named: no process killed
> >> Kicking isc-bind in xinetd
> >> rndc: connect failed: 127.0.0.1#953: connection refused
> >> And bind does not start again and the router becomes less than useful. Now I assume I am doing something wrong, but what, if you have any idea how to solve this short of a reboot of the router (my current method) I would be happy to learn
> >> 
> >> 
> >> 
> >> best regards
> >> sebastian
> >> 
> >> On Aug 12, 2012, at 11:08 PM, Dave Taht wrote:
> >> 
> >>> I'm too tired to write up a full set of release notes, but I've been
> >>> testing it all day,
> >>> and it looks better than -10 and certainly better than -11, but I won't know
> >>> until some more folk sit down and test it, so here it is.
> >>> 
> >>> http://huchra.bufferbloat.net/~cero1/3.3/3.3.8-17/
> >>> 
> >>> fresh merge with openwrt, fix to a bind CVE, fixes for 6in4 and quagga
> >>> routing problems,
> >>> and a few tweaks to fq_codel setup that might make voip better.
> >>> 
> >>> Go forth and break things!
> >>> 
> >>> In other news:
> >>> 
> >>> Van Jacobson gave a great talk about bufferbloat, BQL, codel, and fq_codel
> >>> at last week's ietf meeting. Well worth watching. At the end he outlines
> >>> the deployment problems in particular.
> >>> 
> >>> http://recordings.conf.meetecho.com/Recordings/watch.jsp?recording=IETF84_TSVAREA&chapter=part_3
> >>> 
> >>> Far more interesting than this email!
> >>> 
> >>> 
> >>> --
> >>> Dave Täht
> >>> http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
> >>> with fq_codel!"
> >>> _______________________________________________
> >>> Cerowrt-devel mailing list
> >>> Cerowrt-devel@lists.bufferbloat.net
> >>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
> >> 
> > 
> > 
> > 
> > -- 
> > Dave Täht
> > http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
> > with fq_codel!"
> 
> _______________________________________________
> Cerowrt-devel mailing list
> Cerowrt-devel@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cerowrt-devel
> _______________________________________________
> Cerowrt-devel mailing list
> Cerowrt-devel@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cerowrt-devel