Estou tentando executar algum código R e está travando por causa da memória. O erro que recebo é:

Error in sendMaster(try(lapply(X = S, FUN = FUN, ...), silent = TRUE)) : 
  long vectors not supported yet: memory.c:3100

A função que cria os problemas é a seguinte:

StationUserX <- function(userNDX){
  lat1 = deg2rad(geolocation$latitude[userNDX])
  long1 = deg2rad(geolocation$longitude[userNDX])
  session_user_id = as.character(geolocation$session_user_id[userNDX])
  #Find closest station
  Distance2Stations <- unlist(lapply(stationNDXs, Distance2StationX, lat1, long1))
  # Return index for closest station and distance to closest station
  stations_userX = data.frame(session_user_id = session_user_id, 
                              station = ghcndstations$ID[stationNDXs], 
                              Distance2Station = Distance2Stations)    
  stations_userX = stations_userX[with(stations_userX, order(Distance2Station)), ]
  stations_userX = stations_userX[1:100,] #only the 100 closest stations...
  row.names(stations_userX)<-NULL
  return(stations_userX)
}

Eu executo essa função usando o mclapply 50k vezes. O StationUserX chama o Distance2StationX 90k vezes.

Existe uma maneira óbvia de otimizar a função StationUserX?