[Make-wifi-fast] [Codel] fq_codel_drop vs a udp flood
dave.taht at gmail.com
Mon May 2 22:26:45 EDT 2016
On Sun, May 1, 2016 at 11:20 AM, Jonathan Morton <chromatix99 at gmail.com> wrote:
>> On 1 May, 2016, at 20:59, Eric Dumazet <eric.dumazet at gmail.com> wrote:
>> fq_codel_drop() could drop _all_ packets of the fat flow, instead of a
>> single one.
> Unfortunately, that could have bad consequences if the “fat flow” happens to be a TCP in slow-start on a long-RTT path. Such a flow is responsive, but on an order-magnitude longer timescale than may have been configured as optimum.
> The real problem is that fq_codel_drop() performs the same (excessive) amount of work to cope with a single unresponsive flow as it would for a true DDoS. Optimising the search function is sufficient.
Don't think so.
I did some tests today, (not the fq_codel batch drop patch yet)
When hit with a 900mbit flood, cake shaping down to 250mbit, results
in nearly 100% cpu use in the ksoftirq1 thread on the apu2, and
150mbits of actual throughput (as measured by iperf3, which is now a
measurement I don't trust)
cake *does* hold the packet count down a lot better than fq_codel does.
fq_codel (pre eric's patch) basically goes to the configured limit and
In both cases I will eventually get an error like this (in my babel
routed environment) that suggests that we're also not delivering
packets from other flows (arp?) with either fq_codel or cake in these
iperf3 -c 172.26.64.200 -u -b900Mbit -t 600
[ 4] 47.00-48.00 sec 107 MBytes 895 Mbits/sec 13659
iperf3: error - unable to write to stream socket: No route to host
The results I get from iperf are a bit puzzling over the interval it
samples at - this is from a 100Mbit test (downshifting from 900mbit)
[ 15] 25.00-26.00 sec 152 KBytes 1.25 Mbits/sec 0.998 ms
[ 15] 26.00-27.00 sec 232 KBytes 1.90 Mbits/sec 1.207 ms
[ 15] 27.00-28.00 sec 72.0 KBytes 590 Kbits/sec 1.098 ms
[ 15] 28.00-29.00 sec 0.00 Bytes 0.00 bits/sec 1.098 ms 0/0 (-nan%)
[ 15] 29.00-30.00 sec 72.0 KBytes 590 Kbits/sec 1.044 ms
[ 15] 30.00-31.00 sec 64.0 KBytes 524 Kbits/sec 1.060 ms
[ 15] 31.00-32.00 sec 0.00 Bytes 0.00 bits/sec 1.060 ms 0/0 (-nan%)
^C[ 15] 32.00-32.66 sec 64.0 KBytes 797 Kbits/sec 1.050 ms
Not that I care all that much about how iperf is intepreting it's drop
rate (I guess pulling apart the actual caps is in order).
As for cake struggling to cope:
root at apu2:/home/d/git/tc-adv/tc# ./tc -s qdisc show dev enp2s0
qdisc cake 8018: root refcnt 9 bandwidth 100Mbit diffserv4 flows rtt 100.0ms raw
Sent 219736818 bytes 157121 pkt (dropped 989289, overlimits 1152272 requeues 0)
backlog 449646b 319p requeues 0
memory used: 2658432b of 5000000b
capacity estimate: 100Mbit
Bulk Best Effort Video Voice
thresh 100Mbit 93750Kbit 75Mbit 25Mbit
target 5.0ms 5.0ms 5.0ms 5.0ms
interval 100.0ms 100.0ms 100.0ms 100.0ms
pk_delay 0us 5.2ms 92us 48us
av_delay 0us 5.1ms 4us 2us
sp_delay 0us 5.0ms 4us 2us
pkts 0 1146649 31 49
bytes 0 1607004053 2258 8779
way_inds 0 0 0 0
way_miss 0 15 2 1
way_cols 0 0 0 0
drops 0 989289 0 0
marks 0 0 0 0
sp_flows 0 0 0 0
bk_flows 0 1 0 0
last_len 0 1514 66 138
max_len 0 1514 110 487
But I am very puzzled as to why flow isolation would fail in the face
of this overload.
> - Jonathan Morton
Let's go make home routers and wifi faster! With better software!
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