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procfs: add num_to_str() to speed up /proc/stat

Browse source 
== stat_check.py
num = 0
with open("/proc/stat") as f:
        while num < 1000 :
                data = f.read()
                f.seek(0, 0)
                num = num + 1
==

perf shows

    20.39%  stat_check.py  [kernel.kallsyms]    [k] format_decode
    13.41%  stat_check.py  [kernel.kallsyms]    [k] number
    12.61%  stat_check.py  [kernel.kallsyms]    [k] vsnprintf
    10.85%  stat_check.py  [kernel.kallsyms]    [k] memcpy
     4.85%  stat_check.py  [kernel.kallsyms]    [k] radix_tree_lookup
     4.43%  stat_check.py  [kernel.kallsyms]    [k] seq_printf

This patch removes most of calls to vsnprintf() by adding num_to_str()
and seq_print_decimal_ull(), which prints decimal numbers without rich
functions provided by printf().

On my 8cpu box.
== Before patch ==
[root@bluextal test]# time ./stat_check.py

real    0m0.150s
user    0m0.026s
sys     0m0.121s

== After patch ==
[root@bluextal test]# time ./stat_check.py

real    0m0.055s
user    0m0.022s
sys     0m0.030s

[[email protected]: remove incorrect comment, use less statck in num_to_str(), move comment from .h to .c, simplify seq_put_decimal_ull()]
[[email protected]: avoid breaking the ABI in /proc/stat]
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrea Righi <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Glauber Costa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Russell King <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
KAMEZAWA Hiroyuki 2012-03-23 15:02:54 -07:00 • committed by Linus Torvalds
commit 1ac101a5d6
5 changed files with 84 additions and 29 deletions
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20
lib/vsprintf.c
View file

@ -212,6 +212,26 @@ char *put_dec(char *buf, unsigned long long num)
}
}
/*
* Convert passed number to decimal string.
* Returns the length of string. On buffer overflow, returns 0.
*
* If speed is not important, use snprintf(). It's easy to read the code.
*/
int num_to_str(char *buf, int size, unsigned long long num)
{
char tmp[21]; /* Enough for 2^64 in decimal */
int idx, len;
len = put_dec(tmp, num) - tmp;
if (len > size)
return 0;
for (idx = 0; idx < len; ++idx)
buf[idx] = tmp[len - idx - 1];
return len;
}
#define ZEROPAD 1 /* pad with zero */
#define SIGN 2 /* unsigned/signed long */
#define PLUS 4 /* show plus */