I guess it is enough to show some examples:

[root@doppio linux-2.6-tip]# rm -f perf.data*
[root@doppio linux-2.6-tip]# ls -la perf.data*
ls: cannot access perf.data*: No such file or directory
[root@doppio linux-2.6-tip]# perf record -f find / > /dev/null
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.062 MB perf.data (~2699 samples) ]
[root@doppio linux-2.6-tip]# ls -la perf.data*
-rw------- 1 root root 74440 2009-12-14 20:03 perf.data
[root@doppio linux-2.6-tip]# perf record -f find / > /dev/null
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.062 MB perf.data (~2692 samples) ]
[root@doppio linux-2.6-tip]# ls -la perf.data*
-rw------- 1 root root 74280 2009-12-14 20:03 perf.data
-rw------- 1 root root 74440 2009-12-14 20:03 perf.data.old
[root@doppio linux-2.6-tip]# perf diff | head -5
   1        -34994580     /lib64/libc-2.10.1.so   _IO_vfprintf_internal
   2        -15307806         [kernel.kallsyms]   __kmalloc
   3    +1   +3665941     /lib64/libc-2.10.1.so   __GI_memmove
   4    +4  +23508995     /lib64/libc-2.10.1.so   _int_malloc
   5    +7  +38538813         [kernel.kallsyms]   __d_lookup
[root@doppio linux-2.6-tip]# perf diff -p | head -5
   1        +1.00%     /lib64/libc-2.10.1.so   _IO_vfprintf_internal
   2                       [kernel.kallsyms]   __kmalloc
   3    +1             /lib64/libc-2.10.1.so   __GI_memmove
   4    +4             /lib64/libc-2.10.1.so   _int_malloc
   5    +7  -1.00%         [kernel.kallsyms]   __d_lookup
[root@doppio linux-2.6-tip]# perf diff -v | head -5
   1        361449551 326454971 -34994580     /lib64/libc-2.10.1.so   _IO_vfprintf_internal
   2        151009241 135701435 -15307806         [kernel.kallsyms]   __kmalloc
   3    +1  101805328 105471269  +3665941     /lib64/libc-2.10.1.so   __GI_memmove
   4    +4   78041440 101550435 +23508995     /lib64/libc-2.10.1.so   _int_malloc
   5    +7   59536172  98074985 +38538813         [kernel.kallsyms]   __d_lookup
[root@doppio linux-2.6-tip]# perf diff -vp | head -5
   1        9.00% 8.00% +1.00%     /lib64/libc-2.10.1.so   _IO_vfprintf_internal
   2        3.00% 3.00%                [kernel.kallsyms]   __kmalloc
   3    +1  2.00% 2.00%            /lib64/libc-2.10.1.so   __GI_memmove
   4    +4  2.00% 2.00%            /lib64/libc-2.10.1.so   _int_malloc
   5    +7  1.00% 2.00% -1.00%         [kernel.kallsyms]   __d_lookup
[root@doppio linux-2.6-tip]#

This should be enough for diffs where the system is non
volatile, i.e. when one doesn't updates binaries.

For volatile environments, stay tuned for the next perf tool
feature: a buildid cache populated by 'perf record', managed by
'perf buildid-cache' a-la ccache, and used by all the report
tools.
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
LKML-Reference: <1260828571-3613-3-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>