netdev - Re: [PATCH net-next 0/3] v3 GRE: TCP segmentation offload

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Date:	Fri, 15 Feb 2013 17:41:28 -0800
From:	Pravin Shelar <pshelar@...ira.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	edumazet@...gle.com, jesse@...ira.com, bhutchings@...arflare.com,
	mirqus@...il.com
Subject: Re: [PATCH net-next 0/3] v3 GRE: TCP segmentation offload

On Fri, Feb 15, 2013 at 4:52 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2013-02-15 at 15:18 -0500, David Miller wrote:
>
>> All applied, incorporating the suggestions/fixes from Eric.  Specifically,
>> using skb_reset_mac_len() in patch #2 and computing pkt_len before ip_local_out()
>> in patch #3.
>
> Thanks David
>
> There is this "tx-nocache-copy" issue :
>
> We currently enable the nocache copy for all devices but loopback.
>
> But its a loss of performance with tunnel devices
>
> Actually, it seems a loss even for regular ethernet devices :(
>
>
>
> # ethtool -K gre1 tx-nocache-copy on
> # perf stat netperf -H 7.7.8.84
> MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.8.84 () port 0 AF_INET
> Recv   Send    Send
> Socket Socket  Message  Elapsed
> Size   Size    Size     Time     Throughput
> bytes  bytes   bytes    secs.    10^6bits/sec
>
>  87380  16384  16384    10.00    4252.42
>
>  Performance counter stats for 'netperf -H 7.7.8.84':
>
>        9967.965824 task-clock                #    0.996 CPUs utilized
>                 54 context-switches          #    0.005 K/sec
>                  3 CPU-migrations            #    0.000 K/sec
>                261 page-faults               #    0.026 K/sec
>     27,964,187,393 cycles                    #    2.805 GHz
>     20,902,040,632 stalled-cycles-frontend   #   74.75% frontend cycles idle
>     13,524,565,776 stalled-cycles-backend    #   48.36% backend  cycles idle
>     15,929,463,578 instructions              #    0.57  insns per cycle
>                                              #    1.31  stalled cycles per insn
>      2,065,830,063 branches                  #  207.247 M/sec
>         35,891,035 branch-misses             #    1.74% of all branches
>
>       10.003882959 seconds time elapsed
>
>
> Now we use regular memory copy :
>
> # ethtool -K gre1 tx-nocache-copy off
> # perf stat netperf -H 7.7.8.84
> MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.8.84 () port 0 AF_INET
> Recv   Send    Send
> Socket Socket  Message  Elapsed
> Size   Size    Size     Time     Throughput
> bytes  bytes   bytes    secs.    10^6bits/sec
>
>  87380  16384  16384    10.00    7706.50
>
>  Performance counter stats for 'netperf -H 7.7.8.84':
>
>        5708.284991 task-clock                #    0.571 CPUs utilized
>              5,138 context-switches          #    0.900 K/sec
>                 24 CPU-migrations            #    0.004 K/sec
>                260 page-faults               #    0.046 K/sec
>     15,990,404,388 cycles                    #    2.801 GHz
>     10,903,764,099 stalled-cycles-frontend   #   68.19% frontend cycles idle
>      6,089,332,139 stalled-cycles-backend    #   38.08% backend  cycles idle
>     10,680,845,426 instructions              #    0.67  insns per cycle
>                                              #    1.02  stalled cycles per insn
>      1,401,663,288 branches                  #  245.549 M/sec
>         15,380,428 branch-misses             #    1.10% of all branches
>
>       10.004025020 seconds time elapsed
>
>

I am not seeing such big difference with these setting, are you
running this test on special hardware or in VM?

Thanks,
Pravin.
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