Module: sip-router Branch: master Commit: d8fe0def83736c204dd1eace89e8ca1823704fc3 URL: http://git.sip-router.org/cgi-bin/gitweb.cgi/sip-router/?a=commit;h=d8fe0def...

Author: Andrei Pelinescu-Onciul andrei@iptel.org Committer: Andrei Pelinescu-Onciul andrei@iptel.org Date: Fri Mar 9 19:12:59 2012 +0100

dns: srv load balancing distribution fix

The load distribution when doing SRV based load balancing was not completely fair. The first listed server had always an extra (sum_of_weights-weight)/(sum_of_weights*(sum_of_weights+1) chance of being selected. For example for 2 servers both with weight 50, the first one will have an extra (100-50)/100*101 change of being selected => extra 0.495% which gives a 50.495% - 49.505% distribution instead of 50% - 50%. For large weight values this doesn't make a big difference (e.g. for 500-500 => 50.049% instead of 50%) but for small values it becomes very noticeable (e.g. for 1-1 => 66% - 33% distribution instead of 50%-50%).

The chance of selecting a 0-weight record was also changed. It was 1/(sum_of_weights+1) and now is 1/(sum_of_weights*1000).

Reported-by: Michal Matyska michal.matyska iptel org

---

dns_cache.c | 18 ++++++++++++++---- 1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/dns_cache.c b/dns_cache.c index da8f97d..1a7f1c5 100644 --- a/dns_cache.c +++ b/dns_cache.c @@ -100,6 +100,8 @@ #define DNS_HE_MAX_ADDR 10 /* maxium addresses returne in a hostent struct */ #define MAX_CNAME_CHAIN 10 #define SPACE_FORMAT " " /* format of view output */ +#define DNS_SRV_ZERO_W_CHANCE 1000 /* one in a 1000*weight_sum chance for + selecting a 0-weight record */

int dns_cache_init=1; /* if 0, the DNS cache is not initialized at startup */ static gen_lock_t* dns_hash_lock=0; @@ -2286,6 +2288,7 @@ inline static struct dns_rr* dns_srv_get_nxt_rr(struct dns_hash_entry* e, unsigned rand_w; int found; int saved_idx; + int zero_weight; /* number of records with 0 weight */ int i, idx; struct r_sums_entry{ unsigned r_sum; @@ -2307,6 +2310,7 @@ retry: prio=((struct srv_rdata*)start_grp->rdata)->priority; sum=0; saved_idx=-1; + zero_weight = 0; found=0; for (idx=0;rr && (prio==((struct srv_rdata*)rr->rdata)->priority) && (idx < MAX_SRV_GRP_IDX); idx++, rr=rr->next){ @@ -2328,6 +2332,7 @@ retry: if ((saved_idx==-1) || (((struct srv_rdata*)rr->rdata)->weight==0)){ saved_idx=idx; } + zero_weight += (((struct srv_rdata*)rr->rdata)->weight == 0); sum+=((struct srv_rdata*)rr->rdata)->weight; r_sums[idx].r_sum=sum; r_sums[idx].rr=rr; @@ -2338,10 +2343,15 @@ retry: n+=idx; /* next group start idx, last rr */ srv_reset_tried(tried); goto retry; - }else if ((found==1) || ((rand_w=dns_srv_random(sum))==0)){ - /* 1. if only one found, avoid a useless random() call or - * 2. if rand_w==0, immediately select a 0 weight record if present, - * or else the first record found + }else if ((found==1) || (sum==0) || + (((rand_w=(dns_srv_random(sum-1)+1))==1) && zero_weight && + (dns_srv_random(DNS_SRV_ZERO_W_CHANCE)==0))){ + /* 1. if only one found, avoid a useless random() call + and select it (saved_idx will point to it). + * 2. if the sum of weights is 0 (all have 0 weight) or + * 3. rand_w==1 and there are records with 0 weight and + * random(probab. of selecting a 0-weight) + * immediately select a 0 weight record. * (this takes care of the 0-weight at the beginning requirement) */ i=saved_idx; /* saved idx contains either first 0 weight or first valid record */