[OpenWrt-Devel] OpenWRT wrong adjustment of fq_codel defaults (Was: [Codel] fq_codel_drop vs a udp flood)

Roman Yeryomin leroi.lists at gmail.com
Sun May 15 18:34:36 EDT 2016 

On 6 May 2016 at 22:43, Dave Taht <dave.taht at gmail.com> wrote:
> On Fri, May 6, 2016 at 11:56 AM, Roman Yeryomin <leroi.lists at gmail.com> wrote:
>> On 6 May 2016 at 21:43, Roman Yeryomin <leroi.lists at gmail.com> wrote:
>>> On 6 May 2016 at 15:47, Jesper Dangaard Brouer <brouer at redhat.com> wrote:
>>>>
>>>> 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
>>>
>>> OK, so, after porting the patch to 4.1 openwrt kernel and playing a
>>> bit with fq_codel limits I was able to get 420Mbps UDP like this:
>>> tc qdisc replace dev wlan0 parent :1 fq_codel flows 16 limit 256
>>
>> Forgot to mention, I've reduced drop_batch_size down to 32
>
> 0) Not clear to me if that's the right line, there are 4 wifi queues,
> and the third one
> is the BE queue.

That was an example, sorry, should have stated that. I've applied same
settings to all 4 queues.

> That is too low a limit, also, for normal use. And:
> for the purpose of this particular UDP test, flows 16 is ok, but not
> ideal.

I played with different combinations, it doesn't make any
(significant) difference: 20-30Mbps, not more.
What numbers would you propose?

> 1) What's the tcp number (with a simultaneous ping) with this latest patchset?
> (I care about tcp performance a lot more than udp floods - surviving a
> udp flood yes, performance, no)

During the test (both TCP and UDP) it's roughly 5ms in average, not
running tests ~2ms. Actually I'm now wondering if target is working at
all, because I had same result with target 80ms..
So, yes, latency is good, but performance is poor.

> before/after?
>
> tc -s qdisc show dev wlan0 during/after results?

during the test:

qdisc mq 0: root
 Sent 1600496000 bytes 1057194 pkt (dropped 1421568, overlimits 0 requeues 17)
 backlog 1545794b 1021p requeues 17
qdisc fq_codel 8001: parent :1 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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
qdisc fq_codel 8002: parent :2 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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
qdisc fq_codel 8003: parent :3 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 1601271168 bytes 1057706 pkt (dropped 1422304, overlimits 0 requeues 17)
 backlog 1541252b 1018p requeues 17
  maxpacket 1514 drop_overlimit 1422304 new_flow_count 35 ecn_mark 0
  new_flows_len 0 old_flows_len 1
qdisc fq_codel 8004: parent :4 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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


after the test (60sec):

qdisc mq 0: root
 Sent 3084996052 bytes 2037744 pkt (dropped 2770176, overlimits 0 requeues 28)
 backlog 0b 0p requeues 28
qdisc fq_codel 8001: parent :1 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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
qdisc fq_codel 8002: parent :2 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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
qdisc fq_codel 8003: parent :3 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 3084996052 bytes 2037744 pkt (dropped 2770176, overlimits 0 requeues 28)
 backlog 0b 0p requeues 28
  maxpacket 1514 drop_overlimit 2770176 new_flow_count 64 ecn_mark 0
  new_flows_len 0 old_flows_len 1
qdisc fq_codel 8004: parent :4 limit 1024p flows 16 quantum 1514
target 80.0ms ce_threshold 32us interval 100.0ms ecn
 Sent 0 bytes 0 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


> IF you are doing builds for the archer c7v2, I can join in on this... (?)

I'm not but I have c7 somewhere, so I can do a build for it and also
test, so we are on the same page.

> I did do a test of the ath10k "before", fq_codel *never engaged*, and
> tcp induced latencies under load, e at 100mbit, cracked 600ms, while
> staying flat (20ms) at 100mbit. (not the same patches you are testing)
> on x86. I have got tcp 300Mbit out of an osx box, similar latency,
> have yet to get anything more on anything I currently have
> before/after patchsets.
>
> I'll go add flooding to the tests, I just finished a series comparing
> two different speed stations and life was good on that.
>
> "before" - fq_codel never engages, we see seconds of latency under load.
>
> root at apu2:~# tc -s qdisc show dev wlp4s0
> qdisc mq 0: root
>  Sent 8570563893 bytes 6326983 pkt (dropped 0, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
> qdisc fq_codel 0: parent :1 limit 10240p flows 1024 quantum 1514
> target 5.0ms interval 100.0ms ecn
>  Sent 2262 bytes 17 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
> qdisc fq_codel 0: parent :2 limit 10240p flows 1024 quantum 1514
> target 5.0ms interval 100.0ms ecn
>  Sent 220486569 bytes 152058 pkt (dropped 0, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
>   maxpacket 18168 drop_overlimit 0 new_flow_count 1 ecn_mark 0
>   new_flows_len 0 old_flows_len 1
> qdisc fq_codel 0: parent :3 limit 10240p flows 1024 quantum 1514
> target 5.0ms interval 100.0ms ecn
>  Sent 8340546509 bytes 6163431 pkt (dropped 0, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
>   maxpacket 68130 drop_overlimit 0 new_flow_count 120050 ecn_mark 0
>   new_flows_len 1 old_flows_len 3
> qdisc fq_codel 0: parent :4 limit 10240p flows 1024 quantum 1514
> target 5.0ms interval 100.0ms ecn
>  Sent 9528553 bytes 11477 pkt (dropped 0, overlimits 0 requeues 0)
>  backlog 0b 0p requeues 0
>   maxpacket 66 drop_overlimit 0 new_flow_count 1 ecn_mark 0
>   new_flows_len 1 old_flows_len 0
>   ```
>
>
>>> This is certainly better than 30Mbps but still more than two times
>>> less than before (900).
>
> The number that I still am not sure we got is that you were sending
> 900mbit udp and recieving 900mbit on the prior tests?

900 was sending, AP POV (wifi client is downloading)

>>> TCP also improved a little (550 to ~590).
>
> The limit is probably a bit low, also.  You might want to try target
> 20ms as well.

I've tried limit up to 1024 and target up to 80ms

>>>
>>> Felix, others, do you want to see the ported patch, maybe I did something wrong?
>>> Doesn't look like it will save ath10k from performance regression.
>
> what was tcp "before"? (I'm sorry, such a long thread)

750Mbps

>>>
>>>>
>>>> On Fri, 6 May 2016 11:42:43 +0200
>>>> Jesper Dangaard Brouer <brouer at 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.dumazet at 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.dumazet at 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
>
>
>
> --
> Dave Täht
> Let's go make home routers and wifi faster! With better software!
> http://blog.cerowrt.org
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