presto-main/src/main/java/io/prestosql/memory/QueryContext.java/QueryContext/freeSpill
class QueryContext:    public synchronized void freeSpill(long bytes)
    {
        checkArgument(spillUsed - bytes >= 0, "tried to free more memory than is reserved");
        spillUsed -= bytes;
        spillSpaceTracker.free(bytes);
    }