• Alexey Dobriyan's avatar
    proc: much faster /proc/vmstat · 68ba0326
    Alexey Dobriyan authored
    Every current KDE system has process named ksysguardd polling files
    below once in several seconds:
    
    	$ strace -e trace=open -p $(pidof ksysguardd)
    	Process 1812 attached
    	open("/etc/mtab", O_RDONLY|O_CLOEXEC)   = 8
    	open("/etc/mtab", O_RDONLY|O_CLOEXEC)   = 8
    	open("/proc/net/dev", O_RDONLY)         = 8
    	open("/proc/net/wireless", O_RDONLY)    = -1 ENOENT (No such file or directory)
    	open("/proc/stat", O_RDONLY)            = 8
    	open("/proc/vmstat", O_RDONLY)          = 8
    
    Hell knows what it is doing but speed up reading /proc/vmstat by 33%!
    
    Benchmark is open+read+close 1.000.000 times.
    
    			BEFORE
    $ perf stat -r 10 taskset -c 3 ./proc-vmstat
    
     Performance counter stats for 'taskset -c 3 ./proc-vmstat' (10 runs):
    
          13146.768464      task-clock (msec)         #    0.960 CPUs utilized            ( +-  0.60% )
                    15      context-switches          #    0.001 K/sec                    ( +-  1.41% )
                     1      cpu-migrations            #    0.000 K/sec                    ( +- 11.11% )
                   104      page-faults               #    0.008 K/sec                    ( +-  0.57% )
        45,489,799,349      cycles                    #    3.460 GHz                      ( +-  0.03% )
         9,970,175,743      stalled-cycles-frontend   #   21.92% frontend cycles idle     ( +-  0.10% )
         2,800,298,015      stalled-cycles-backend    #   6.16% backend cycles idle       ( +-  0.32% )
        79,241,190,850      instructions              #    1.74  insn per cycle
                                                      #    0.13  stalled cycles per insn  ( +-  0.00% )
        17,616,096,146      branches                  # 1339.956 M/sec                    ( +-  0.00% )
           176,106,232      branch-misses             #    1.00% of all branches          ( +-  0.18% )
    
          13.691078109 seconds time elapsed                                          ( +-  0.03% )
          ^^^^^^^^^^^^
    
    			AFTER
    $ perf stat -r 10 taskset -c 3 ./proc-vmstat
    
     Performance counter stats for 'taskset -c 3 ./proc-vmstat' (10 runs):
    
           8688.353749      task-clock (msec)         #    0.950 CPUs utilized            ( +-  1.25% )
                    10      context-switches          #    0.001 K/sec                    ( +-  2.13% )
                     1      cpu-migrations            #    0.000 K/sec
                   104      page-faults               #    0.012 K/sec                    ( +-  0.56% )
        30,384,010,730      cycles                    #    3.497 GHz                      ( +-  0.07% )
        12,296,259,407      stalled-cycles-frontend   #   40.47% frontend cycles idle     ( +-  0.13% )
         3,370,668,651      stalled-cycles-backend    #  11.09% backend cycles idle       ( +-  0.69% )
        28,969,052,879      instructions              #    0.95  insn per cycle
                                                      #    0.42  stalled cycles per insn  ( +-  0.01% )
         6,308,245,891      branches                  #  726.058 M/sec                    ( +-  0.00% )
           214,685,502      branch-misses             #    3.40% of all branches          ( +-  0.26% )
    
           9.146081052 seconds time elapsed                                          ( +-  0.07% )
           ^^^^^^^^^^^
    
    vsnprintf() is slow because:
    
    1. format_decode() is busy looking for format specifier: 2 branches
       per character (not in this case, but in others)
    
    2. approximately million branches while parsing format mini language
       and everywhere
    
    3.  just look at what string() does /proc/vmstat is good case because
       most of its content are strings
    
    Link: http://lkml.kernel.org/r/20160806125455.GA1187@p183.telecom.bySigned-off-by: default avatarAlexey Dobriyan <adobriyan@gmail.com>
    Cc: Joe Perches <joe@perches.com>
    Cc: Andi Kleen <andi@firstfloor.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    68ba0326
vmstat.c 47.4 KB