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OPTIM: backend: skip LB when we know the backend is full

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For some algos (roundrobin, static-rr, leastconn, first) we know that
if there is any request queued in the backend, it's because a previous
attempt failed at finding a suitable server after trying all of them.
This alone is sufficient to decide that the next request will skip the
LB algo and directly reach the backend's queue. Doing this alone avoids
an O(N) lookup when load-balancing on a saturated farm of N servers,
which starts to be very expensive for hundreds of servers, especially
under the lbprm lock. This change alone has increased the request rate
from 110k to 148k RPS for 200 saturated servers on 8 threads, and
fwlc_reposition_srv() doesn't show up anymore in perf top. See github
issue #880 for more context.

It could have been the same for random, except that random is performed
using a consistent hash and it only considers a small set of servers (2
by default), so it may result in queueing at the backend despite having
some free slots on unknown servers. It's no big deal though since random()
only performs two attempts by default.

For hashing algorithms this is pointless since we don't queue at the
backend, except when there's no hash key found, which is the least of
our concerns here.
This commit is contained in:
Willy Tarreau 2020-09-29 17:07:21 +02:00
parent b88ae18021
commit 82cd5c13a5
1 changed files with 12 additions and 1 deletions
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13
src/backend.c
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@ -640,14 +640,25 @@ int assign_server(struct stream *s)
}
}
}
if (s->be->lbprm.algo & BE_LB_KIND) {
if (s->be->lbprm.algo & BE_LB_KIND) {
/* we must check if we have at least one server available */
if (!s->be->lbprm.tot_weight) {
err = SRV_STATUS_NOSRV;
goto out;
}
/* if there's some queue on the backend, with certain algos we
* know it's because all servers are full.
*/
if (s->be->nbpend &&
(((s->be->lbprm.algo & BE_LB_KIND) == BE_LB_KIND_CB) || // conn-based: leastconn & first
((s->be->lbprm.algo & (BE_LB_KIND|BE_LB_NEED|BE_LB_PARM)) == BE_LB_ALGO_RR) || // roundrobin
((s->be->lbprm.algo & (BE_LB_KIND|BE_LB_NEED|BE_LB_PARM)) == BE_LB_ALGO_SRR))) { // static-rr
err = SRV_STATUS_FULL;
goto out;
}
/* First check whether we need to fetch some data or simply call
* the LB lookup function. Only the hashing functions will need
* some input data in fact, and will support multiple algorithms.