I've created a OpenWRT ticket[1] on this issue, as it seems that someone[2] closed Felix'es OpenWRT email account (bad choice! emails bouncing). Sounds like OpenWRT and the LEDE https://www.lede-project.org/ project is in some kind of conflict.
OpenWRT ticket [1] https://dev.openwrt.org/ticket/22349 [2] http://thread.gmane.org/gmane.comp.embedded.openwrt.devel/40298/focus=40335 On Fri, 6 May 2016 11:42:43 +0200 Jesper Dangaard Brouer <bro...@redhat.com> wrote: > Hi Felix, > > This is an important fix for OpenWRT, please read! > > OpenWRT changed the default fq_codel sch->limit from 10240 to 1024, > without also adjusting q->flows_cnt. Eric explains below that you must > also adjust the buckets (q->flows_cnt) for this not to break. (Just > adjust it to 128) > > Problematic OpenWRT commit in question: > http://git.openwrt.org/?p=openwrt.git;a=patch;h=12cd6578084e > 12cd6578084e ("kernel: revert fq_codel quantum override to prevent it from > causing too much cpu load with higher speed (#21326)") > > > I also highly recommend you cherry-pick this very recent commit: > net-next: 9d18562a2278 ("fq_codel: add batch ability to fq_codel_drop()") > https://git.kernel.org/davem/net-next/c/9d18562a227 > > This should fix very high CPU usage in-case fq_codel goes into drop mode. > The problem is that drop mode was considered rare, and implementation > wise it was chosen to be more expensive (to save cycles on normal mode). > Unfortunately is it easy to trigger with an UDP flood. Drop mode is > especially expensive for smaller devices, as it scans a 4K big array, > thus 64 cache misses for small devices! > > The fix is to allow drop-mode to bulk-drop more packets when entering > drop-mode (default 64 bulk drop). That way we don't suddenly > experience a significantly higher processing cost per packet, but > instead can amortize this. > > To Eric, should we recommend OpenWRT to adjust default (max) 64 bulk > drop, given we also recommend bucket size to be 128 ? (thus the amount > of memory to scan is less, but their CPU is also much smaller). > > --Jesper > > > On Thu, 05 May 2016 12:23:27 -0700 Eric Dumazet <eric.duma...@gmail.com> > wrote: > > > On Thu, 2016-05-05 at 19:25 +0300, Roman Yeryomin wrote: > > > On 5 May 2016 at 19:12, Eric Dumazet <eric.duma...@gmail.com> wrote: > > > > On Thu, 2016-05-05 at 17:53 +0300, Roman Yeryomin wrote: > > > > > > > >> > > > >> qdisc fq_codel 0: dev eth0 root refcnt 2 limit 1024p flows 1024 > > > >> quantum 1514 target 5.0ms interval 100.0ms ecn > > > >> Sent 12306 bytes 128 pkt (dropped 0, overlimits 0 requeues 0) > > > >> backlog 0b 0p requeues 0 > > > >> maxpacket 0 drop_overlimit 0 new_flow_count 0 ecn_mark 0 > > > >> new_flows_len 0 old_flows_len 0 > > > > > > > > > > > > Limit of 1024 packets and 1024 flows is not wise I think. > > > > > > > > (If all buckets are in use, each bucket has a virtual queue of 1 packet, > > > > which is almost the same than having no queue at all) > > > > > > > > I suggest to have at least 8 packets per bucket, to let Codel have a > > > > chance to trigger. > > > > > > > > So you could either reduce number of buckets to 128 (if memory is > > > > tight), or increase limit to 8192. > > > > > > Will try, but what I've posted is default, I didn't change/configure > > > that. > > > > fq_codel has a default of 10240 packets and 1024 buckets. > > > > http://lxr.free-electrons.com/source/net/sched/sch_fq_codel.c#L413 > > > > If someone changed that in the linux variant you use, he probably should > > explain the rationale. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
