concurrency reading problem ?

I have a task with a lot of lookups. They usually goes quite near in terms of keys.

The effect is I can not get more than about 5-6 cores working effectivly.

Other threads gets locked in the last line of code below (StoreDirectAbstract, lines 37+):

//TODO PERF indexPages are synchronized writes are protected by structural lock, but should it be read under locks?
protected val indexPages = if(isThreadSafe) LongArrayList().asSynchronized() else LongArrayList()

protected fun recidToOffset(recid2:Long):Long{
    var recid = recid2-1; //normalize recid so it starts from zero
    if(recid< StoreDirectJava.RECIDS_PER_ZERO_INDEX_PAGE){
        //zero index page
        return StoreDirectJava.HEAD_END + 16 + recid*8
    }
    //strip zero index page
    recid -= StoreDirectJava.RECIDS_PER_ZERO_INDEX_PAGE
    val pageNum = recid/ StoreDirectJava.RECIDS_PER_INDEX_PAGE
    return indexPages.get(pageNum.toInt()) + 16 + ((recid)% StoreDirectJava.RECIDS_PER_INDEX_PAGE)*8
}

seems that indexPages.get(pageNum.toInt()) causes the problem, as this call is synchronized.

below actual implementation of get (from eclipse collections):

public long get(int index)
{
    synchronized (this.getLock())
    {
        return this.getMutableLongList().get(index);
    }
}

Start in StoreDirectAbstract at recidToOffset and inspect the indexPages access, then review the shown Eclipse Collections get implementation to understand the synchronization. Reproduce the lookup workload and determine whether indexPages.get causes the contention; done should preserve thread safety while addressing the reported concurrency problem.