[tip:x86/asm] x86/defconfig: Turn on CONFIG_CC_OPTIMIZE_FOR_SIZE= y in the 64-bit defconfig

[tip-bot for Ma Ling <ling.ml@alipay.com>]

Commit-ID:  d94ffd677469ef729e9d6e968191872577a6119e
Gitweb:     http://git.kernel.org/tip/d94ffd677469ef729e9d6e968191872577a6119e
Author:     Ma Ling <ling.ml@alipay.com>
AuthorDate: Fri, 25 Jan 2013 09:11:01 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 26 Jan 2013 13:09:15 +0100

x86/defconfig: Turn on CONFIG_CC_OPTIMIZE_FOR_SIZE=y in the 64-bit defconfig

Currently we use O2 as compiler option for better performance,
although it will enlarge code size, in modern CPUs larger
instructon and unified cache, sophisticated instruction prefetch
weaken instruction cache miss, meanwhile flags such as
 -falign-functions, -falign-jumps, -falign-loops, -falign-labels
are very helpful to improve CPU front-end throughput because CPU
fetch instruction by 16 aligned–bytes code block per cycle.

In order to save power and get higher performance, Sandy Bridge
starts to introduce decoded-cache, instructions will be kept in
it after decode stage. When CPU refetches the instruction,
decoded cache could provide 32 aligned-bytes instruction block,
instead of 16 bytes from I-cache, fewer branch miss penalty
resulted from shorter pipeline. It requires hot code should be
put into decoded cache as possible we can. Sandy Bridge, Ivy
Bridge, and Haswell all implemented this feature, Os-Optimize
for size should be better than O2 on them.

Based on above reasons, we compiled linux kernel 3.6.9 with O2
and Os respectively. The results show Os improve performance
netperf 4.8%, 2.7% for volano as below:

O2 + netperf
Performance counter stats for 'netperf' (3 runs):

       5416.157986 task-clock                #    0.541 CPUs utilized            ( +-  0.19% )
           348,249 context-switches          #    0.064 M/sec                    ( +-  0.17% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +-  0.00% )
               353 page-faults               #    0.000 M/sec                    ( +-  0.16% )
    13,166,254,384 cycles                    #    2.431 GHz                      ( +-  0.18% )
     8,827,499,807 stalled-cycles-frontend   #   67.05% frontend cycles idle     ( +-  0.29% )
     5,951,234,060 stalled-cycles-backend    #   45.20% backend  cycles idle     ( +-  0.44% )
     8,122,481,914 instructions              #    0.62  insns per cycle
                                             #    1.09  stalled cycles per insn  ( +-  0.17% )
     1,415,864,138 branches                  #  261.415 M/sec                    ( +-  0.17% )
        16,975,308 branch-misses             #    1.20% of all branches          ( +-  0.61% )

      10.007215371 seconds time elapsed                                          ( +-  0.03% )

Os + netperf

Performance counter stats for 'netperf' (3 runs):

       5395.386704 task-clock                #    0.539 CPUs utilized            ( +-  0.14% )
           345,880 context-switches          #    0.064 M/sec                    ( +-  0.25% )
                 0 CPU-migrations            #    0.000 M/sec                    ( +-  0.00% )
               354 page-faults               #    0.000 M/sec                    ( +-  0.00% )
    13,142,706,297 cycles                    #    2.436 GHz                      ( +-  0.23% )
     8,379,382,641 stalled-cycles-frontend   #   63.76% frontend cycles idle     ( +-  0.50% )
     5,513,722,219 stalled-cycles-backend    #   41.95% backend  cycles idle     ( +-  0.71% )
     8,554,202,795 instructions              #    0.65  insns per cycle
                                             #    0.98  stalled cycles per insn  ( +-  0.25% )
     1,530,020,505 branches                  #  283.579 M/sec                    ( +-  0.25% )
        17,710,406 branch-misses             #    1.16% of all branches          ( +-  1.00% )

      10.004859867 seconds time elapsed

During the same time (10.004859867 seconds) IPC from Os is 0.65,
O2 is 0.62, Os improved performance 4.8%.

O2 + volano
Performance counter stats for './loopclient.sh openjdk' (3 runs):

     210627.115313 task-clock                #    0.781 CPUs utilized            ( +-  0.92% )
        13,812,610 context-switches          #    0.066 M/sec                    ( +-  0.17% )
         2,352,755 CPU-migrations            #    0.011 M/sec                    ( +-  0.84% )
           208,333 page-faults               #    0.001 M/sec                    ( +-  1.58% )
   525,627,073,405 cycles                    #    2.496 GHz                      ( +-  0.96% )
   428,177,571,365 stalled-cycles-frontend   #   81.46% frontend cycles idle     ( +-  1.09% )
   370,885,224,739 stalled-cycles-backend    #   70.56% backend  cycles idle     ( +-  1.18% )
   187,662,577,544 instructions              #    0.36  insns per cycle
                                             #    2.28  stalled cycles per insn  ( +-  0.31% )
    35,684,976,425 branches                  #  169.423 M/sec                    ( +-  0.45% )
     1,062,086,942 branch-misses             #    2.98% of all branches          ( +-  0.08% )

     269.764578435 seconds time elapsed

Os + volano
Performance counter stats for './loopclient.sh openjdk' (3 runs):

     209545.786941 task-clock                #    0.778 CPUs utilized            ( +-  0.66% )
        13,864,142 context-switches          #    0.066 M/sec                    ( +-  0.29% )
         2,326,826 CPU-migrations            #    0.011 M/sec                    ( +-  0.83% )
           205,575 page-faults               #    0.001 M/sec                    ( +-  2.63% )
   523,366,588,452 cycles                    #    2.498 GHz                      ( +-  0.75% )
   419,200,472,430 stalled-cycles-frontend   #   80.10% frontend cycles idle     ( +-  0.86% )
   362,044,374,737 stalled-cycles-backend    #   69.18% backend  cycles idle     ( +-  0.96% )
   193,274,857,837 instructions              #    0.37  insns per cycle
                                             #    2.17  stalled cycles per insn  ( +-  0.51% )
    37,657,832,686 branches                  #  179.712 M/sec                    ( +-  0.42% )
     1,061,005,300 branch-misses             #    2.82% of all branches          ( +-  0.86% )

     269.410275674 seconds time elapsed                                          ( +-  0.06% )

During the same  time (269.410275674 seconds) IPC from Os is
0.37, O2 is 0.36, Os improved performance 2.7%.

Signed-off-by: Ma Ling <ling.ml@alipay.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1359123061-6139-1-git-send-email-ling.ma@alipay.com
[ So, this is a bit symbolic as most people don't use the defconfig,
  but the measurements are useful nevertheless so let's commit this
  if there are no objections. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/configs/x86_64_defconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/configs/x86_64_defconfig b/arch/x86/configs/x86_64_defconfig
index 671524d..2fcde13 100644
--- a/arch/x86/configs/x86_64_defconfig
+++ b/arch/x86/configs/x86_64_defconfig
@@ -18,6 +18,7 @@ CONFIG_CGROUP_CPUACCT=y
 CONFIG_RESOURCE_COUNTERS=y
 CONFIG_CGROUP_SCHED=y
 CONFIG_BLK_DEV_INITRD=y
+CONFIG_CC_OPTIMIZE_FOR_SIZE=y
 # CONFIG_COMPAT_BRK is not set
 CONFIG_PROFILING=y
 CONFIG_KPROBES=y