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authorDenis Vlasenko <vda.linux@googlemail.com>2007-07-15 23:41:56 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-07-16 09:05:52 -0700
commit4277eedd7908a0ca8b66fad46ee76b0ad96e6ef2 (patch)
tree88780b40c23883af5e9958a7f397f23ff5619ff7 /lib
parentb39a734097d5095d63eb9c709a6aaf965633bb01 (diff)
downloadlinux-4277eedd7908a0ca8b66fad46ee76b0ad96e6ef2.tar.gz
vsprintf.c: optimizing, part 2: base 10 conversion speedup, v2
Optimize integer-to-string conversion in vsprintf.c for base 10. This is by far the most used conversion, and in some use cases it impacts performance. For example, top reads /proc/$PID/stat for every process, and with 4000 processes decimal conversion alone takes noticeable time. Using code from http://www.cs.uiowa.edu/~jones/bcd/decimal.html (with permission from the author, Douglas W. Jones) binary-to-decimal-string conversion is done in groups of five digits at once, using only additions/subtractions/shifts (with -O2; -Os throws in some multiply instructions). On i386 arch gcc 4.1.2 -O2 generates ~500 bytes of code. This patch is run tested. Userspace benchmark/test is also attached. I tested it on PIII and AMD64 and new code is generally ~2.5 times faster. On AMD64: # ./vsprintf_verify-O2 Original decimal conv: .......... 151 ns per iteration Patched decimal conv: .......... 62 ns per iteration Testing correctness 12895992590592 ok... [Ctrl-C] # ./vsprintf_verify-O2 Original decimal conv: .......... 151 ns per iteration Patched decimal conv: .......... 62 ns per iteration Testing correctness 26025406464 ok... [Ctrl-C] More realistic test: top from busybox project was modified to report how many us it took to scan /proc (this does not account any processing done after that, like sorting process list), and then I test it with 4000 processes: #!/bin/sh i=4000 while test $i != 0; do sleep 30 & let i-- done busybox top -b -n3 >/dev/null on unpatched kernel: top: 4120 processes took 102864 microseconds to scan top: 4120 processes took 91757 microseconds to scan top: 4120 processes took 92517 microseconds to scan top: 4120 processes took 92581 microseconds to scan on patched kernel: top: 4120 processes took 75460 microseconds to scan top: 4120 processes took 66451 microseconds to scan top: 4120 processes took 67267 microseconds to scan top: 4120 processes took 67618 microseconds to scan The speedup comes from much faster generation of /proc/PID/stat by sprintf() calls inside the kernel. Signed-off-by: Douglas W Jones <jones@cs.uiowa.edu> Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib')
-rw-r--r--lib/vsprintf.c108
1 files changed, 105 insertions, 3 deletions
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index e94b4bd25bc..6b6734df6d2 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -135,6 +135,103 @@ static int skip_atoi(const char **s)
return i;
}
+/* Decimal conversion is by far the most typical, and is used
+ * for /proc and /sys data. This directly impacts e.g. top performance
+ * with many processes running. We optimize it for speed
+ * using code from
+ * http://www.cs.uiowa.edu/~jones/bcd/decimal.html
+ * (with permission from the author, Douglas W. Jones). */
+
+/* Formats correctly any integer in [0,99999].
+ * Outputs from one to five digits depending on input.
+ * On i386 gcc 4.1.2 -O2: ~250 bytes of code. */
+static char* put_dec_trunc(char *buf, unsigned q)
+{
+ unsigned d3, d2, d1, d0;
+ d1 = (q>>4) & 0xf;
+ d2 = (q>>8) & 0xf;
+ d3 = (q>>12);
+
+ d0 = 6*(d3 + d2 + d1) + (q & 0xf);
+ q = (d0 * 0xcd) >> 11;
+ d0 = d0 - 10*q;
+ *buf++ = d0 + '0'; /* least significant digit */
+ d1 = q + 9*d3 + 5*d2 + d1;
+ if (d1 != 0) {
+ q = (d1 * 0xcd) >> 11;
+ d1 = d1 - 10*q;
+ *buf++ = d1 + '0'; /* next digit */
+
+ d2 = q + 2*d2;
+ if ((d2 != 0) || (d3 != 0)) {
+ q = (d2 * 0xd) >> 7;
+ d2 = d2 - 10*q;
+ *buf++ = d2 + '0'; /* next digit */
+
+ d3 = q + 4*d3;
+ if (d3 != 0) {
+ q = (d3 * 0xcd) >> 11;
+ d3 = d3 - 10*q;
+ *buf++ = d3 + '0'; /* next digit */
+ if (q != 0)
+ *buf++ = q + '0'; /* most sign. digit */
+ }
+ }
+ }
+ return buf;
+}
+/* Same with if's removed. Always emits five digits */
+static char* put_dec_full(char *buf, unsigned q)
+{
+ /* BTW, if q is in [0,9999], 8-bit ints will be enough, */
+ /* but anyway, gcc produces better code with full-sized ints */
+ unsigned d3, d2, d1, d0;
+ d1 = (q>>4) & 0xf;
+ d2 = (q>>8) & 0xf;
+ d3 = (q>>12);
+
+ /* Possible ways to approx. divide by 10 */
+ /* gcc -O2 replaces multiply with shifts and adds */
+ // (x * 0xcd) >> 11: 11001101 - shorter code than * 0x67 (on i386)
+ // (x * 0x67) >> 10: 1100111
+ // (x * 0x34) >> 9: 110100 - same
+ // (x * 0x1a) >> 8: 11010 - same
+ // (x * 0x0d) >> 7: 1101 - same, shortest code (on i386)
+
+ d0 = 6*(d3 + d2 + d1) + (q & 0xf);
+ q = (d0 * 0xcd) >> 11;
+ d0 = d0 - 10*q;
+ *buf++ = d0 + '0';
+ d1 = q + 9*d3 + 5*d2 + d1;
+ q = (d1 * 0xcd) >> 11;
+ d1 = d1 - 10*q;
+ *buf++ = d1 + '0';
+
+ d2 = q + 2*d2;
+ q = (d2 * 0xd) >> 7;
+ d2 = d2 - 10*q;
+ *buf++ = d2 + '0';
+
+ d3 = q + 4*d3;
+ q = (d3 * 0xcd) >> 11; /* - shorter code */
+ /* q = (d3 * 0x67) >> 10; - would also work */
+ d3 = d3 - 10*q;
+ *buf++ = d3 + '0';
+ *buf++ = q + '0';
+ return buf;
+}
+/* No inlining helps gcc to use registers better */
+static noinline char* put_dec(char *buf, unsigned long long num)
+{
+ while (1) {
+ unsigned rem;
+ if (num < 100000)
+ return put_dec_trunc(buf, num);
+ rem = do_div(num, 100000);
+ buf = put_dec_full(buf, rem);
+ }
+}
+
#define ZEROPAD 1 /* pad with zero */
#define SIGN 2 /* unsigned/signed long */
#define PLUS 4 /* show plus */
@@ -182,6 +279,11 @@ static char *number(char *buf, char *end, unsigned long long num, int base, int
i = 0;
if (num == 0)
tmp[i++] = '0';
+ /* Generic code, for any base:
+ else do {
+ tmp[i++] = digits[do_div(num,base)];
+ } while (num != 0);
+ */
else if (base != 10) { /* 8 or 16 */
int mask = base - 1;
int shift = 3;
@@ -190,9 +292,9 @@ static char *number(char *buf, char *end, unsigned long long num, int base, int
tmp[i++] = digits[((unsigned char)num) & mask];
num >>= shift;
} while (num);
- } else do { /* generic code, works for any base */
- tmp[i++] = digits[do_div(num,10 /*base*/)];
- } while (num);
+ } else { /* base 10 */
+ i = put_dec(tmp, num) - tmp;
+ }
/* printing 100 using %2d gives "100", not "00" */
if (i > precision)