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Date:	Tue, 03 Feb 2015 06:27:40 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

On Tue, 2015-02-03 at 12:50 +0100, Michal Kazior wrote:
> On 3 February 2015 at 02:18, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Mon, 2015-02-02 at 10:52 -0800, Eric Dumazet wrote:
> >
> >> It seems to break ACK clocking badly (linux stack has a somewhat buggy
> >> tcp_tso_should_defer(), which relies on ACK being received smoothly, as
> >> no timer is setup to split the TSO packet.)
> >
> > Following patch might help the TSO split defer logic.
> >
> > It would avoid setting the TSO defer 'pseudo timer' twice, if/when TCP
> > Small Queue logic prevented the xmit at the expiration of first 'timer'.
> >
> > This patch clears the tso_deferred variable only if we could really
> > send something.
> >
> > Please try it, thanks !
> [..patch..]
> 
> I've done a second round of tests. I've added the A-MSDU count
> parameter I've mentioned in my other email into the mix.
> 
>  net - net/master (includes stretch ack patches)
>  net-tso - net/master + your TSO defer patch
>  net-gro - net/master + my ath10k GRO patch
>  net-gro-tso - net/master + duh
> 
> Here's the best of amsdu count 1 and 3:
> 
>  ; for (i in */output.txt) { echo $i; for (j in (1 3)) { cat $i | awk
> 'x && /Mbits/ {y=$0}; x && y && !/Mbits/ {print y; x=0; y=""}; /set
> amsdu cnt to '$j'/{x=1}' | awk '{ if (x < $(NF-1)) {x=$(NF-1)} }
> END{print "A-MSDU limit='$j', " x " Mbits/sec"}' } }
>  net-gro-tso/output.txt
>  A-MSDU limit=1, 436 Mbits/sec
>  A-MSDU limit=3, 284 Mbits/sec
>  net-gro/output.txt
>  A-MSDU limit=1, 444 Mbits/sec
>  A-MSDU limit=3, 283 Mbits/sec
>  net-tso/output.txt
>  A-MSDU limit=1, 376 Mbits/sec
>  A-MSDU limit=3, 251 Mbits/sec
>  net/output.txt
>  A-MSDU limit=1, 387 Mbits/sec
>  A-MSDU limit=3, 260 Mbits/sec
> 
> IOW:
>  - stretch acks / TSO defer don't seem to help much (when compared to
> throughput results from yesterday)
>  - GRO helps
>  - disabling A-MSDU on sender helps
>  - net/master+GRO still doesn't reach the performance from before the
> regression (~600mbps w/ GRO)
> 
> You can grab logs and dumps here: http://www.filedropper.com/test2tar
> 

Thanks for these traces.

There is absolutely a problem at the sender, as we can see a big 2ms
delay between reception of ACK and send of following packets.
TCP stack should generate them immediately.
Are you using some kind of netem qdisc ?

These 2ms delays, in a flow with a 5ms RTT are terrible.

06:54:57.408391 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 4294899240, win 11268, options [nop,nop,TS val 1053302 ecr 1052250], length 0
06:54:57.408418 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 4294910824, win 11268, options [nop,nop,TS val 1053303 ecr 1052251], length 0
06:54:57.408431 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 4294936888, win 11268, options [nop,nop,TS val 1053303 ecr 1052251], length 0
06:54:57.408453 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 4294962952, win 11268, options [nop,nop,TS val 1053303 ecr 1052251], length 0
06:54:57.408474 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 0, win 11268, options [nop,nop,TS val 1053303 ecr 1052251], length 0
<this 2ms delay is not generated by TCP stack.>
06:54:57.410243 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 82536:83984, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410271 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 83984:85432, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410289 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 85432:86880, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410310 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 86880:88328, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410326 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 88328:89776, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410339 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 89776:91224, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410353 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 91224:92672, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448
06:54:57.410370 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 92672:94120, ack 1, win 457, options [nop,nop,TS val 1052256 ecr 1053303], length 1448

...
06:54:57.411178 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 28960, win 11268, options [nop,nop,TS val 1053306 ecr 1052253], length 0
06:54:57.411190 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 52128, win 11268, options [nop,nop,TS val 1053306 ecr 1052254], length 0
06:54:57.411220 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 78192, win 11268, options [nop,nop,TS val 1053306 ecr 1052254], length 0
06:54:57.411243 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 82536, win 11268, options [nop,nop,TS val 1053306 ecr 1052254], length 0
< same 2ms unexplained gap here >
06:54:57.412912 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 165072:166520, ack 1, win 457, options [nop,nop,TS val 1052259 ecr 1053306], length 1448
06:54:57.412935 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 166520:167968, ack 1, win 457, options [nop,nop,TS val 1052259 ecr 1053306], length 1448
06:54:57.412948 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 167968:169416, ack 1, win 457, options [nop,nop,TS val 1052259 ecr 1053306], length 1448
...
06:54:57.413650 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 244712:246160, ack 1, win 457, options [nop,nop,TS val 1052260 ecr 1053306], length 1448
06:54:57.413662 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 246160:247608, ack 1, win 457, options [nop,nop,TS val 1052260 ecr 1053306], length 1448
06:54:57.413712 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 112944, win 11268, options [nop,nop,TS val 1053308 ecr 1052256], length 0
06:54:57.413730 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 130320, win 11268, options [nop,nop,TS val 1053308 ecr 1052257], length 0
06:54:57.413754 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 160728, win 11268, options [nop,nop,TS val 1053309 ecr 1052257], length 0
06:54:57.413779 IP 192.168.1.2.5001 > 192.168.1.3.51645: Flags [.], ack 165072, win 11268, options [nop,nop,TS val 1053309 ecr 1052257], length 0
< same 2ms delay >
06:54:57.415682 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 247608:249056, ack 1, win 457, options [nop,nop,TS val 1052262 ecr 1053309], length 1448
06:54:57.415709 IP 192.168.1.3.51645 > 192.168.1.2.5001: Flags [.], seq 249056:250504, ack 1, win 457, options [nop,nop,TS val 1052262 ecr 1053309], length 1448

Are packets TX completed after a timer or something ?

Some very heavy stuff might run from tasklet (or other softirq triggered) event.

BTW, traces tend to show that you 'receive' multiple ACK in the same burst,
its not clear if they are delayed at one side or the other.

GRO should delay only GRO candidates. ACK packets are not GRO candidates.

Have you tried to disable GSO on sender ?

(Or maybe wifi drivers should start to use skb->xmit_more as a signal to end aggregation)


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