library(simmer)
library(simmer.plot)
set.seed(1234)

M/M/c/k systems

In Kendall’s notation, an M/M/c/k system has exponential arrivals (M/M/c/k), a \(c\) servers (M/M/c/k) with exponential service time (M/M/c/k) and an \(k-c\) positions in queue (M/M/c/k). For instance, a router may have several processors to handle packets, and the in/out queues are necessarily finite.

This is the simulation of an M/M/2/3 system (2 server, 1 position in queue). Note that the trajectory is identical to an M/M/1 system (one server, infinite queue).

lambda <- 3
mu <- 4

mm23.trajectory <- trajectory() %>%
  seize("server", amount=1) %>%
  timeout(function() rexp(1, mu)) %>%
  release("server", amount=1)

mm23.env <- simmer() %>%
  add_resource("server", capacity=2, queue_size=1) %>%
  add_generator("arrival", mm23.trajectory, function() rexp(1, lambda)) %>%
  run(until=2000)

There are rejections when the queue is full.

mm23.arrivals <- get_mon_arrivals(mm23.env)

rejection_rate <- mm23.arrivals %>%
  dplyr::summarise(rejection_rate = sum(!finished)/length(finished)) %>%
  dplyr::pull(rejection_rate)
rejection_rate
#> [1] 0.04253894

By solving the balance equations for this system, we obtain the following:

\[ \begin{aligned} p_0 &= \left[ \sum_{n=0}^{c-1}\frac{r^n}{n!} + \sum_{n=c}^{k}\frac{r^n}{c!c^{n-c}} \right]^{-1}\\ N &= \sum_{n=0}^{\infty}nP_n = p_0\left(\sum_{n=0}^{c-n}\frac{nr^n}{n!} + \sum_{n=c}^{k}\frac{nr^n}{c!c^{n-c}}\right) \end{aligned} \]

where \(r=\lambda/\mu\). Finally, we can see how the simulation quickly converges to the theoretical average number of customers in the system \(N\):

# Theoretical value
rho <- lambda/mu
div <- 1 / c(1, 1, factorial(2) * 2^(2:3-2))
mm23.N <- sum(0:3 * rho^(0:3) * div) / sum(rho^(0:3) * div)

# Evolution of the average number of customers in the system
plot(mm23.env, "resources", "usage", "server", items="system") +
  geom_hline(yintercept=mm23.N)
#> Warning: 'plot.simmer' is deprecated.
#> Use 'plot(get_mon_resources(x))' instead.
#> See help("Deprecated")

Queueing Networks

Let us consider the following network of queues (example taken from Grotto Networking):

A network of queues.

A network of queues.

There are three exponential generators (\(\lambda_1\), \(\lambda_2\), \(\lambda_4\)) that inject messages with exponential size of mean 100 bytes. There are four M/D/1 queues with a deterministic rate equal to 220 bytes/s. There is a 25% probability that messages from \(\lambda_1\) are dropped before the second queue, and a fork 35/65% for the output of the second queue to reach queues 3 and 4. We are interested, for instance, in the accumulated waiting time for messages entering queue 1 and exiting queue 3 or 4.

The original author of this example, Greg M. Bernstein, simulated this problem with this Python code. We’ll start by setting up the main constants and a couple of functions to set the message size and seize an M/D/1 queue:

mean_pkt_size <- 100        # bytes
lambda1 <- 2                # pkts/s
lambda3 <- 0.5              # pkts/s
lambda4 <- 0.6              # pkts/s
rate <- 2.2 * mean_pkt_size # bytes/s

# set an exponential message size of mean mean_pkt_size
set_msg_size <- function(.)
  set_attribute(., "size", function() rexp(1, 1/mean_pkt_size))

# seize an M/D/1 queue by id; the timeout is function of the message size
md1 <- function(., id)
  seize(., paste0("md1_", id), 1) %>%
  timeout(function() get_attribute(env, "size") / rate) %>%
  release(paste0("md1_", id), 1)

The next step is to set up the three points of attachment for our generators:

to_queue_1 <- trajectory() %>%
  set_msg_size() %>%
  md1(1) %>%
  leave(0.25) %>%
  md1(2) %>%
  branch(
    function() (runif(1) > 0.65) + 1, continue=c(F, F),
    trajectory() %>% md1(3),
    trajectory() %>% md1(4)
  )

to_queue_3 <- trajectory() %>%
  set_msg_size() %>%
  md1(3)

to_queue_4 <- trajectory() %>%
  set_msg_size() %>%
  md1(4)

Finally, we prepare and run the simulation environment with the resources and generators required:

env <- simmer()
for (i in 1:4) env %>% 
  add_resource(paste0("md1_", i))
env %>%
  add_generator("arrival1_", to_queue_1, function() rexp(1, lambda1), mon=2) %>%
  add_generator("arrival3_", to_queue_3, function() rexp(1, lambda3), mon=2) %>%
  add_generator("arrival4_", to_queue_4, function() rexp(1, lambda4), mon=2) %>%
  run(4000)
#> simmer environment: anonymous | now: 4000 | next: 4000.34545681367
#> { Resource: md1_1 | monitored: TRUE | server status: 1(1) | queue status: 6(Inf) }
#> { Resource: md1_2 | monitored: TRUE | server status: 0(1) | queue status: 0(Inf) }
#> { Resource: md1_3 | monitored: TRUE | server status: 0(1) | queue status: 0(Inf) }
#> { Resource: md1_4 | monitored: TRUE | server status: 0(1) | queue status: 0(Inf) }
#> { Generator: arrival1_ | monitored: 2 | n_generated: 8160 }
#> { Generator: arrival3_ | monitored: 2 | n_generated: 2025 }
#> { Generator: arrival4_ | monitored: 2 | n_generated: 2427 }

In the analysis, we filter arrivals from generator 1 that reach queues 3 and 4, and we examine the average waiting time and the total number of messages:

res <- get_mon_arrivals(env, per_resource = TRUE) %>%
  dplyr::select(name, resource) %>%
  dplyr::filter(resource %in% c("md1_3", "md1_4"))
arr <- get_mon_arrivals(env) %>%
  dplyr::mutate(waiting_time = end_time - (start_time + activity_time),
                generator = regmatches(name, regexpr("arrival[[:digit:]]", name))) %>%
  dplyr::left_join(res) %>%
  dplyr::group_by(generator, resource)
#> Joining, by = "name"

dplyr::summarise(arr, average = sum(waiting_time) / n())
#> # A tibble: 5 x 3
#> # Groups:   generator [?]
#>   generator resource average
#>   <chr>     <chr>      <dbl>
#> 1 arrival1  md1_3      7.87 
#> 2 arrival1  md1_4      7.43 
#> 3 arrival1  <NA>       6.40 
#> 4 arrival3  md1_3      0.834
#> 5 arrival4  md1_4      0.455
get_n_generated(env, "arrival1_") + get_n_generated(env, "arrival4_")
#> [1] 10587
dplyr::count(arr)
#> # A tibble: 5 x 3
#> # Groups:   generator, resource [5]
#>   generator resource     n
#>   <chr>     <chr>    <int>
#> 1 arrival1  md1_3     3915
#> 2 arrival1  md1_4     2156
#> 3 arrival1  <NA>      2081
#> 4 arrival3  md1_3     2024
#> 5 arrival4  md1_4     2426