Trouble shooting low rate MCS9 in 802.11ac

Vu Hai NGUYEN vh.nguyen at
Mon Aug 18 02:45:21 PDT 2014

>> I used ethtool to disable Checksum offloading on my Ethernet interface and increase rate from
>> 400 Mbps to 500 Mbps, now the thread "csum_partial_copy_nocheck" has disappeared and there
>> is no more particular thread which takes most of the processor resource. But my processor is
>> always exhausted.
>> I notice some warning when starting perf top
>> [ath10k_core] with build id f31b98301ddea35e320bacaef048e4adbae00a6d not found, continuing without symbols
>> [ath10k_pci] with build id a1303b5587ceef676d03d5a68fa5c7d717861c5d not found, continuing without symbols
>> [mac80211] with build id f73251a940dedf51ac599141e5edefe05da2cb6f not found, continuing without symbols
>> [cfg80211] with build id 94dd0809b98902252ce78cc3bc4ae385dd5e8fdf not found, continuing without symbols
>> This may cause some unknown thread like the 2nd thread $a below of [mac80211]?
>> PerfTop:      51 irqs/sec  kernel:98.0%  exact:  0.0% [1000Hz cycles],  (all, 1 CPU)
>> -------------------------------------------------------------------------------
>>      8.79%  [mac80211]     [k] ieee80211_tx_status
>>      5.02%  [mac80211]     [k] $a
>>      5.02%  [kernel]       [k] __memzero
>>      4.60%  [kernel]       [k] memcpy
>>      4.60%  [kernel]       [k] mv_eth_poll
>>      4.18%  [kernel]       [k] dev_queue_xmit
>>      4.18%  [kernel]       [k] br_parse_ip_options
>>      3.77%  [kernel]       [k] dev_hard_start_xmit
>>      3.77%  [kernel]       [k] nf_hook_slow
>>      3.35%  [kernel]       [k] local_bh_enable
>>      3.35%  [kernel]       [k] nf_iterate
>>      3.35%  [kernel]       [k] br_nf_forward_ip
>>      2.93%  [kernel]       [k] kfree
>>      2.93%  [kernel]       [k] br_nf_pre_routing
>>      2.93%  [kernel]       [k] mv_eth_tx
>>      2.93%  [kernel]       [k] br_handle_frame_finish
>>      2.93%  [ath10k_pci]   [k] ath10k_ce_send_nolock
>>      2.93%  [ath10k_core]  [k] ath10k_txrx_tx_unref
>>      2.51%  [kernel]       [k] eth_type_trans
>>      2.51%  [ath10k_core]  [k] ath10k_htt_rx_amsdu_pop

>This accounts for 76.58% of CPU usage. You've most likely trimmed down
>the remaining entries accounting for 24% before posting.

You can find in the attached file the report of perf using in my AP and STA. Thank you in advance for the review or any comment

>By the looks of it you might be able to strip down netfilter
>(iptables) rules to none on all chains and tables if you just want to
>max out the performance at the cost of having no firewall whatsoever.

I'd cleaned all rules of iptables and removed all modules of iptables from kernel by command modprobe -r
But there's still some process of netfilter process running. I disabled netfilter on bridge interface by writing 0 to these files:

and the performance increases from 500 to 550 Mbps (iperf in tcp reports between 530 ~ 570Mbps).
There is no more process like  "br_nf_pre_routing" and "br_nf_forward_ip".
Do you think I can do anything more to strip down netfilter?

>You also might want to change qdisc (`tc` command) on all interfaces
>involved to pfifo in case you're using codel to further reduce CPU
>load at the cost of buffer bloating the system.

I installed tc from package iproute2 and did : tc qdisc but it doesn't improve the performance
One more question: Could the antennas create any effect on the MCS in use?
We are using this antenna from PCTEL:
And in the specification, I saw that it supports the standard 802.11n, so may be this is the reason
that the best rate for me is MCS7?

Route de Mayres - B.P. 9
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