netdev - Interpreting perf stat on netperf and netserver

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Date:	Wed, 18 Jan 2012 12:33:39 +0100
From:	Jean-Michel Hautbois <jhautbois@...il.com>
To:	netdev@...r.kernel.org
Subject: Interpreting perf stat on netperf and netserver

Hi all,

I am currently using netperf/netserver in order to characterize a
benet emulex network device on a machine with 2 Xeon5670.
I am using the latest linux kernel from git (3.2.0+).
I am facing several issues, and I am trying to understand the
following perf stat launched on netserver :

 Performance counter stats for process id '5043':

      15452.992135 task-clock                #    0.450 CPUs utilized
            189678 context-switches          #    0.012 M/sec
                 5 CPU-migrations            #    0.000 M/sec
               275 page-faults               #    0.000 M/sec
       48490467936 cycles                    #    3.138 GHz
       33005879963 stalled-cycles-frontend   #   68.07% frontend cycles idle
       16325855769 stalled-cycles-backend    #   33.67% backend  cycles idle
       27340520316 instructions              #    0.56  insns per cycle
                                             #    1.21  stalled cycles per insn
        4745604818 branches                  #  307.099 M/sec
          67513124 branch-misses             #    1.42% of all branches

      34.303567279 seconds time elapsed

I am trying to understand the "stalled-cycles-frontend" and
"stalled-cycles-backend" lines.
It seems that frontend is high, and in red :) but I can't say why...

The be2net driver seems to have difficulties woth IRQ affinity also,
because it always uses CPU0 even if the affinity is 0-23 !
netperf result is quite good, and perf top shows :
   PerfTop:     689 irqs/sec  kernel:99.6% us: 0.3% guest kernel: 0.0%
guest us: 0.0% exact:  0.0% [1000Hz cycles],  (all, 24 CPUs)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    29.28%  [kernel]          [k] csum_partial
     8.46%  [kernel]          [k] copy_user_generic_string
     3.70%  [be2net]          [k] be_poll_rx
     3.08%  [be2net]          [k] event_handle
     2.37%  [kernel]          [k] irq_entries_start
     2.21%  [be2net]          [k] be_rx_compl_get
     1.65%  [be2net]          [k] be_post_rx_frags
     1.64%  [kernel]          [k] __napi_complete
     1.50%  [kernel]          [k] ip_defrag
     1.35%  [kernel]          [k] put_page
     1.34%  [kernel]          [k] get_page_from_freelist
     1.29%  [kernel]          [k] __netif_receive_skb
     1.16%  [kernel]          [k] __alloc_pages_nodemask
     1.14%  [kernel]          [k] debug_smp_processor_id
     1.08%  [kernel]          [k] add_preempt_count
     1.06%  [kernel]          [k] sub_preempt_count
     1.03%  [be2net]          [k] get_rx_page_info
     1.01%  [kernel]          [k] alloc_pages_current

Checksum calculation seems quite complex :).
Regards,
JM
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