in my case it automatically adjust SievePerRound:

Work received: height=2555695 diff=10.991005 latency=31ms
 * warning: no enough candidates (16071 available, must be more than 16384
increase sieves per round to 13
.
.
Work received: height=2555703 diff=10.99101 latency=46ms
 * warning: no enough candidates (13892 available, must be more than 14336
increase sieves per round to 12
.
.
Work received: height=2555704 diff=10.991011 latency=46ms
 * warning: no enough candidates (16325 available, must be more than 16384
increase sieves per round to 13
.
.
Work received: height=2555717 diff=10.991013 latency=31ms
 * warning: no enough candidates (15526 available, must be more than 16384
increase sieves per round to 14