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[PERF] use snprintf once in addReplyDouble (#11093) 

The previous implementation calls `snprintf` twice, the second time used to
'memcpy' the output of the first, which could be a very large string.
The new implementation reserves space for the protocol header ahead
of the formatted double, and then prepends the string length ahead of it.

Measured improvement of simple ZADD of some 25%.
This commit is contained in:
Ariel Shtul 2022-08-23 09:37:59 +03:00 committed by GitHub
parent 407b5c912f
commit 90223759a3
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 22 additions and 6 deletions
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28
src/networking.c
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@ -840,13 +840,29 @@ void addReplyDouble(client *c, double d) {
d > 0 ? 6 : 7);
}
} else {
char dbuf[MAX_LONG_DOUBLE_CHARS+3],
sbuf[MAX_LONG_DOUBLE_CHARS+32];
int dlen, slen;
char dbuf[MAX_LONG_DOUBLE_CHARS+32];
int dlen = 0;
if (c->resp == 2) {
dlen = snprintf(dbuf,sizeof(dbuf),"%.17g",d);
slen = snprintf(sbuf,sizeof(sbuf),"$%d\r\n%s\r\n",dlen,dbuf);
addReplyProto(c,sbuf,slen);
/* In order to prepend the string length before the formatted number,
* but still avoid an extra memcpy of the whole number, we reserve space
* for maximum header `$0000\r\n`, print double, add the resp header in
* front of it, and then send the buffer with the right `start` offset. */
int dlen = snprintf(dbuf+7,sizeof(dbuf) - 7,"%.17g",d);
int digits = digits10(dlen);
int start = 4 - digits;
dbuf[start] = '$';
/* Convert `dlen` to string, putting it's digits after '$' and before the
* formatted double string. */
for(int i = digits, val = dlen; val && i > 0 ; --i, val /= 10) {
dbuf[start + i] = "0123456789"[val % 10];
}
dbuf[5] = '\r';
dbuf[6] = '\n';
dbuf[dlen+7] = '\r';
dbuf[dlen+8] = '\n';
addReplyProto(c,dbuf+start,dlen+9-start);
} else {
dlen = snprintf(dbuf,sizeof(dbuf),",%.17g\r\n",d);
addReplyProto(c,dbuf,dlen);