examples.txt

		------------------------------
		****** perf by examples ******
		------------------------------

[ From an e-mail by Ingo Molnar, https://lore.kernel.org/lkml/[email protected] ]


First, discovery/enumeration of available counters can be done via
'perf list':

titan:~> perf list
  [...]
  kmem:kmalloc                             [Tracepoint event]
  kmem:kmem_cache_alloc                    [Tracepoint event]
  kmem:kmalloc_node                        [Tracepoint event]
  kmem:kmem_cache_alloc_node               [Tracepoint event]
  kmem:kfree                               [Tracepoint event]
  kmem:kmem_cache_free                     [Tracepoint event]
  kmem:mm_page_free                        [Tracepoint event]
  kmem:mm_page_free_batched                [Tracepoint event]
  kmem:mm_page_alloc                       [Tracepoint event]
  kmem:mm_page_alloc_zone_locked           [Tracepoint event]
  kmem:mm_page_pcpu_drain                  [Tracepoint event]
  kmem:mm_page_alloc_extfrag               [Tracepoint event]

Then any (or all) of the above event sources can be activated and
measured. For example the page alloc/free properties of a 'hackbench
run' are:

 titan:~> perf stat -e kmem:mm_page_pcpu_drain -e kmem:mm_page_alloc
 -e kmem:mm_page_free_batched -e kmem:mm_page_free ./hackbench 10
 Time: 0.575

 Performance counter stats for './hackbench 10':

          13857  kmem:mm_page_pcpu_drain
          27576  kmem:mm_page_alloc
           6025  kmem:mm_page_free_batched
          20934  kmem:mm_page_free

    0.613972165  seconds time elapsed

You can observe the statistical properties as well, by using the
'repeat the workload N times' feature of perf stat:

 titan:~> perf stat --repeat 5 -e kmem:mm_page_pcpu_drain -e
   kmem:mm_page_alloc -e kmem:mm_page_free_batched -e
   kmem:mm_page_free ./hackbench 10
 Time: 0.627
 Time: 0.644
 Time: 0.564
 Time: 0.559
 Time: 0.626

 Performance counter stats for './hackbench 10' (5 runs):

          12920  kmem:mm_page_pcpu_drain    ( +-   3.359% )
          25035  kmem:mm_page_alloc         ( +-   3.783% )
           6104  kmem:mm_page_free_batched  ( +-   0.934% )
          18376  kmem:mm_page_free	    ( +-   4.941% )

    0.643954516  seconds time elapsed   ( +-   2.363% )

Furthermore, these tracepoints can be used to sample the workload as
well. For example the page allocations done by a 'git gc' can be
captured the following way:

 titan:~/git> perf record -e kmem:mm_page_alloc -c 1 ./git gc
 Counting objects: 1148, done.
 Delta compression using up to 2 threads.
 Compressing objects: 100% (450/450), done.
 Writing objects: 100% (1148/1148), done.
 Total 1148 (delta 690), reused 1148 (delta 690)
 [ perf record: Captured and wrote 0.267 MB perf.data (~11679 samples) ]

To check which functions generated page allocations:

 titan:~/git> perf report
 # Samples: 10646
 #
 # Overhead          Command               Shared Object
 # ........  ...............  ..........................
 #
    23.57%       git-repack  /lib64/libc-2.5.so
    21.81%              git  /lib64/libc-2.5.so
    14.59%              git  ./git
    11.79%       git-repack  ./git
     7.12%              git  /lib64/ld-2.5.so
     3.16%       git-repack  /lib64/libpthread-2.5.so
     2.09%       git-repack  /bin/bash
     1.97%               rm  /lib64/libc-2.5.so
     1.39%               mv  /lib64/ld-2.5.so
     1.37%               mv  /lib64/libc-2.5.so
     1.12%       git-repack  /lib64/ld-2.5.so
     0.95%               rm  /lib64/ld-2.5.so
     0.90%  git-update-serv  /lib64/libc-2.5.so
     0.73%  git-update-serv  /lib64/ld-2.5.so
     0.68%             perf  /lib64/libpthread-2.5.so
     0.64%       git-repack  /usr/lib64/libz.so.1.2.3

Or to see it on a more finegrained level:

titan:~/git> perf report --sort comm,dso,symbol
# Samples: 10646
#
# Overhead          Command               Shared Object  Symbol
# ........  ...............  ..........................  ......
#
     9.35%       git-repack  ./git                       [.] insert_obj_hash
     9.12%              git  ./git                       [.] insert_obj_hash
     7.31%              git  /lib64/libc-2.5.so          [.] memcpy
     6.34%       git-repack  /lib64/libc-2.5.so          [.] _int_malloc
     6.24%       git-repack  /lib64/libc-2.5.so          [.] memcpy
     5.82%       git-repack  /lib64/libc-2.5.so          [.] __GI___fork
     5.47%              git  /lib64/libc-2.5.so          [.] _int_malloc
     2.99%              git  /lib64/libc-2.5.so          [.] memset

Furthermore, call-graph sampling can be done too, of page
allocations - to see precisely what kind of page allocations there
are:

 titan:~/git> perf record -g -e kmem:mm_page_alloc -c 1 ./git gc
 Counting objects: 1148, done.
 Delta compression using up to 2 threads.
 Compressing objects: 100% (450/450), done.
 Writing objects: 100% (1148/1148), done.
 Total 1148 (delta 690), reused 1148 (delta 690)
 [ perf record: Captured and wrote 0.963 MB perf.data (~42069 samples) ]

 titan:~/git> perf report -g
 # Samples: 10686
 #
 # Overhead          Command               Shared Object
 # ........  ...............  ..........................
 #
    23.25%       git-repack  /lib64/libc-2.5.so
                |
                |--50.00%-- _int_free
                |
                |--37.50%-- __GI___fork
                |          make_child
                |
                |--12.50%-- ptmalloc_unlock_all2
                |          make_child
                |
                 --6.25%-- __GI_strcpy
    21.61%              git  /lib64/libc-2.5.so
                |
                |--30.00%-- __GI_read
                |          |
                |           --83.33%-- git_config_from_file
                |                     git_config
                |                     |
   [...]

Or you can observe the whole system's page allocations for 10
seconds:

titan:~/git> perf stat -a -e kmem:mm_page_pcpu_drain -e
kmem:mm_page_alloc -e kmem:mm_page_free_batched -e
kmem:mm_page_free sleep 10

 Performance counter stats for 'sleep 10':

         171585  kmem:mm_page_pcpu_drain
         322114  kmem:mm_page_alloc
          73623  kmem:mm_page_free_batched
         254115  kmem:mm_page_free

   10.000591410  seconds time elapsed

Or observe how fluctuating the page allocations are, via statistical
analysis done over ten 1-second intervals:

 titan:~/git> perf stat --repeat 10 -a -e kmem:mm_page_pcpu_drain -e
   kmem:mm_page_alloc -e kmem:mm_page_free_batched -e
   kmem:mm_page_free sleep 1

 Performance counter stats for 'sleep 1' (10 runs):

          17254  kmem:mm_page_pcpu_drain    ( +-   3.709% )
          34394  kmem:mm_page_alloc         ( +-   4.617% )
           7509  kmem:mm_page_free_batched  ( +-   4.820% )
          25653  kmem:mm_page_free	    ( +-   3.672% )

    1.058135029  seconds time elapsed   ( +-   3.089% )

Or you can annotate the recorded 'git gc' run on a per symbol basis
and check which instructions/source-code generated page allocations:

 titan:~/git> perf annotate __GI___fork
 ------------------------------------------------
  Percent |      Source code & Disassembly of libc-2.5.so
 ------------------------------------------------
          :
          :
          :      Disassembly of section .plt:
          :      Disassembly of section .text:
          :
          :      00000031a2e95560 <__fork>:
 [...]
     0.00 :        31a2e95602:   b8 38 00 00 00          mov    $0x38,%eax
     0.00 :        31a2e95607:   0f 05                   syscall
    83.42 :        31a2e95609:   48 3d 00 f0 ff ff       cmp    $0xfffffffffffff000,%rax
     0.00 :        31a2e9560f:   0f 87 4d 01 00 00       ja     31a2e95762 <__fork+0x202>
     0.00 :        31a2e95615:   85 c0                   test   %eax,%eax

( this shows that 83.42% of __GI___fork's page allocations come from
  the 0x38 system call it performs. )

etc. etc. - a lot more is possible. I could list a dozen of
other different usecases straight away - neither of which is
possible via /proc/vmstat.

/proc/vmstat is not in the same league really, in terms of
expressive power of system analysis and performance
analysis.

All that the above results needed were those new tracepoints
in include/tracing/events/kmem.h.

	Ingo