apache spark - delay on driver - scala

Question

I am attaching image from spark UI, and i am asking what is causing the delay( represented by white space) based on the description of my code below
Description:
1) isEmpt: is a action triggered on a Dataset DS1. it takes fe milliseconds : 60ms.
2) The white space between "isEmpty" and " run at ThreadPool...".
3) "collect at graphUtil" : collection of Datasets created between 1) and 2)
The script is running on yarn cluster.
Between 1) and 2) i am declaring Datasets which uses sqlContext.implicits._, i am not collecting them here.so this is supposed to be work on Driver.Those Datasets contains Join/filter/....
Having that i am not collecting them between 1) and 2) what could be causing this delay.
Code between 1) and 2)
import sqlContext.implicits._
val intermediateInputFlowsIdsDS= intermediateInputFlowsDS
.map(x=>x.flow)
.toDF("flowid").distinct().as[Int].repartition($"flowid")
val df_exch_flow_interm_out=df_exch_flow.filter(df_exch_flow("flow_type")==="PRODUCT_FLOW"
&&df_exch_flow("is_input")==="0" )
val allproducersExchDS= intermediateInputFlowsIdsDS.join(df_exch_flow_interm_out,
intermediateInputFlowsIdsDS("flowid")===df_exch_flow_interm_out("f_flow") )
.repartition($"f_owner")
//proc{id,name,proctype}/inter{flowid}/df_exch{exch,proc,flow,direct,amount,provider,unit}/df_flow{id,name,type}/unit{id,src,factor,dest}
df_proc.join(allproducersExchDS,df_proc("Id")=== allproducersExchDS("f_owner"))
.map(row => {
/*(flowid,procid,value)*/
new FlowProducer( row.getInt(3),// flowid output of producer
row.getInt(0) ,// the process id of producer
row.getDouble(8),// value of the matrix A cell,
row.getDouble(16),//factor
row.getString(17),//destination unit
row.getString(2)//process type
)
}).repartition($"producer_flow")