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Date:      Tue, 4 May 2004 15:55:32 -0400 
From:      Gerrit Nagelhout <gnagelhout@sandvine.com>
To:        freebsd-current@FreeBSD.org
Subject:   4.7 vs 5.2.1 SMP/UP bridging performance
Message-ID:  <FE045D4D9F7AED4CBFF1B3B813C85337021AB372@mail.sandvine.com>

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Hi,

For one of our applications in our testlab, we are running bridge(4) 
with several user land applications.  I have found that the bridging 
performance (64 byte packets, 2-port bridge) on 5.2.1 is 
significantly lower than that of RELENG_4, especially when running in
SMP.  The platform is a dual 2.8GHz xeon with a dual port em (100MHz 
PCI-X).  Invariants are disabled, and polling (with idle_polling 
enabled) is used.

Here are the various test results (packets per second, full duplex) 
[traffic generator] <=> [FreeBSD bridge] <=> [traffic generator]

	4.7 UP:     1.2Mpps
	4.7 SMP :   1.2Mpps
	5.2.1 UP:   850Kpps
	5.2.1 SMP:  500Kpps

I believe that for RELENG_4, the hardware is the bottleneck, which 
explains why there is no difference between UP and SMP.
In order to get these numbers for 5.2.1, I had to make a small change
to bridge.c (change ETHER_ADDR_EQ to BDG_MATCH in bridge_in to avoid 
calling bcmp).  This change boosted performance by about 20% 

I ran the kernel profiler for both UP and SMP (5.2.1), and included 
the results of the top functions below.  In the past, I have run the 
profiler against RELENG_4 also, and the main difference with that 
(explaining reduced UP performance) is more overhead due to bus_dma & 
mbuf handling.  When I compare the results of UP & SMP (5.2.1), all 
the functions using mutexes seem to get much more expensive, and 
critical_exit is taking more cycles.  A quick count of mutexes in the 
bridge code path showed that there were 10-20 locks & unlocks for 
each packet.  When as a quick test I added 10 more locks/unlocks to 
the code path, the SMP performance when down to 330Kpps.  This 
indicates that mutexes are much more expensive in SMP than in UP.

I would like to move to CURRENT for new hardware support, and the 
ability to properly use multi-threading in user-space, but can't do
this until the performance bottlenecks are solved.  I realize that 
5.x is still a work in progress and hasn't been tuned as well as 4.7 
yet, but are there any plans for optimizations in this area?  Does 
anyone have any suggestions on what else I can try?

Thanks,

Gerrit

(wheel)# sysctl net.link.ether.bridge
net.link.ether.bridge.version: $Revision: 1.72 $ $Date: 2003/10/31 18:32:08
$
net.link.ether.bridge.debug: 0
net.link.ether.bridge.ipf: 0
net.link.ether.bridge.ipfw: 0
net.link.ether.bridge.copy: 0
net.link.ether.bridge.ipfw_drop: 0
net.link.ether.bridge.ipfw_collisions: 0
net.link.ether.bridge.packets: 1299855421
net.link.ether.bridge.dropped: 0
net.link.ether.bridge.predict: 0
net.link.ether.bridge.enable: 1
net.link.ether.bridge.config: em0:1,em1:1

(wheel)# sysctl kern.polling
kern.polling.burst: 19
kern.polling.each_burst: 80
kern.polling.burst_max: 1000
kern.polling.idle_poll: 1
kern.polling.poll_in_trap: 0
kern.polling.user_frac: 5
kern.polling.reg_frac: 120
kern.polling.short_ticks: 0
kern.polling.lost_polls: 4297586
kern.polling.pending_polls: 0
kern.polling.residual_burst: 0
kern.polling.handlers: 3
kern.polling.enable: 1
kern.polling.phase: 0
kern.polling.suspect: 1030517
kern.polling.stalled: 40
kern.polling.idlepoll_sleeping: 0


Here are some of the interesting parts of the config file:
options         HZ=2500
options         NMBCLUSTERS=32768
#options        GDB_REMOTE_CHAT
#options        INVARIANTS
#options        INVARIANT_SUPPORT
#options        DIAGNOSTIC

options         DEVICE_POLLING



The following profiles show only the top functions (more than 0.2%):

UP:

granularity: each sample hit covers 16 byte(s) for 0.01% of 10.01 seconds 
	  
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 20.3       2.03     2.03                             ether_input [1]
 10.5       3.09     1.06                             mb_free [2]
  5.8       3.67     0.58
_bus_dmamap_load_buffer [3]
  5.6       4.23     0.56                             m_getcl [4]
  5.3       4.76     0.53                             em_encap [5]
  5.1       5.27     0.51                             m_free [6]
  5.1       5.78     0.51                             mb_alloc [7]
  4.9       6.27     0.49                             bdg_forward [8]
  4.9       6.76     0.49
em_process_receive_interrupts [9]
  4.1       7.17     0.41                             bridge_in [10]
  3.6       7.53     0.36                             generic_bcopy [11]
  3.6       7.89     0.36                             m_freem [12]
  2.6       8.14     0.26                             em_get_buf [13]
  2.2       8.37     0.22
em_clean_transmit_interrupts [14]
  2.2       8.59     0.22                             em_start_locked [15]
  2.0       8.79     0.20                             bus_dmamap_load_mbuf
[16]
  1.9       8.99     0.19                             bus_dmamap_load [17]
  1.3       9.11     0.13                             critical_exit [18]
  1.1       9.23     0.11                             em_start [19]
  1.0       9.32     0.10                             bus_dmamap_create [20]
  0.8       9.40     0.08                             em_receive_checksum
[21]
  0.6       9.46     0.06                             em_tx_cb [22]
  0.5       9.52     0.05                             __mcount [23]
  0.5       9.57     0.05
em_transmit_checksum_setup [24]
  0.5       9.62     0.05                             m_tag_delete_chain
[25]
  0.5       9.66     0.05                             m_adj [26]
  0.3       9.69     0.03                             mb_pop_cont [27]
  0.2       9.71     0.02                             bus_dmamap_destroy
[28]
  0.2       9.73     0.02                             mb_reclaim [29]
  0.2       9.75     0.02                             ether_ipfw_chk [30]
  0.2       9.77     0.02                             em_dmamap_cb [31]

SMP:

granularity: each sample hit covers 16 byte(s) for 0.00% of 20.14 seconds

  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 47.9       9.64     9.64                             cpu_idle_default [1]
  4.9      10.63     0.99                             critical_exit [2]
  4.6      11.56     0.93                             mb_free [3]
  4.3      12.41     0.86                             bridge_in [4]
  4.2      13.26     0.84                             bdg_forward [5]
  4.1      14.08     0.82                             mb_alloc [6]
  3.9      14.87     0.79
em_process_receive_interrupts [7]
  3.2      15.52     0.65                             em_start [8]
  3.1      16.15     0.63                             m_free [9]
  3.0      16.76     0.61
_bus_dmamap_load_buffer [10]
  2.5      17.27     0.51                             m_getcl [11]
  2.1      17.69     0.42                             em_start_locked [12]
  1.9      18.07     0.37                             ether_input [13]
  1.5      18.38     0.31                             em_encap [14]
  1.1      18.61     0.23                             bus_dmamap_load [15]
  1.0      18.82     0.21                             generic_bcopy [16]
  0.9      19.00     0.18                             bus_dmamap_load_mbuf
[17]
  0.8      19.16     0.17                             __mcount [18]
  0.6      19.29     0.13                             em_get_buf [19]
  0.6      19.41     0.12
em_clean_transmit_interrupts [20]
  0.5      19.52     0.11                             em_receive_checksum
[21]
  0.4      19.60     0.09                             m_gethdr_clrd [22]
  0.4      19.69     0.08                             bus_dmamap_create [23]
  0.3      19.75     0.06                             em_tx_cb [24]
  0.2      19.80     0.05                             m_freem [25]
  0.2      19.83     0.03                             m_adj [26]
  0.1      19.85     0.02                             m_tag_delete_chain
[27]
  0.1      19.87     0.02                             bus_dmamap_destroy
[28]
  0.1      19.89     0.02                             mb_pop_cont [29]
  0.1      19.91     0.02                             em_dmamap_cb [30]
  0.1      19.92     0.02
em_transmit_checksum_setup [31]
  0.1      19.94     0.01                             mb_alloc_wait [32]
  0.1      19.95     0.01                             em_poll [33]




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