# Use `parfor`

to Speed Up Monte-Carlo Code

This example shows how to speed up Monte-Carlo code by using `parfor`

-loops. Monte-Carlo methods are found in many fields, including physics, mathematics, biology, and finance. Monte-Carlo methods involve executing a function many times with randomly distributed inputs. With Parallel Computing Toolbox, you can replace a `for`

-loop with a `parfor`

-loop to easily speed up code.

This example runs a simple stochastic simulation based on the dollar auction. Run multiple simulations to find the market value for a one dollar bill using a Monte-Carlo method. In this example, the dollar auction is treated as a black-box function that produces outputs that depend on random processes. To find out more about the model, see The Dollar Auction. To see how to speed up Monte-Carlo code in general, see Use a parfor-loop to Estimate Market Value.

### The Dollar Auction

The dollar auction is a non-zero-sum game first introduced by Martin Shubik in 1971. In the game, players bid for a one dollar bill. After a player makes a bid, every other player can choose to make a bid higher than the previous bidder. The auction ends when no more players decide to place a bid. The highest bidder receives the one dollar bill, however, unlike a typical auction both the highest and second-highest bidder give their bid to the auctioneer.

#### Stochastic Model

You can model games similar to the dollar auction using a stochastic model. The state (current bid and number of active players) can be modeled using Markov processes, and therefore outcomes (market value) can be expected to have well-defined statistics. The outcomes are drawn from a conditional distribution, and therefore the dollar auction is ideal for Monte-Carlo analysis. The market value is influenced by the following factors:

Number of players, (

`nPlayers`

)Actions players take

In this example, the following algorithm determines what actions players take (bidding or dropping out) depending on the state.

Set the bid to the previous bid plus

`incr`

.Select a player at random from players who are not the previous bidder.

If no bids have previously been placed, go to 8.

If the previous bid is less than 1, generate a random number between 0 and 1. If the random number is less than

`dropoutRate`

, go to 7.Calculate how much money

`gain`

can be gained if the player makes a winning bid.Calculate how much money

`loss`

the player loses if they are the second highest bidder. If`gain`

is greater than`loss`

, go to 8.The player drops out. Remove the player from the set of players, then go to 9.

The player places a bid.

If there are 2 or more players remaining, go to step 1.

The supporting function `dollarAuction`

simulates a dollar auction. To view the code, see `dollarAuction.m`

. The function takes three inputs: `nPlayers`

, `incr`

, and `dropoutRate`

. Set each of the values.

nPlayers = 20; incr = 0.05; dropoutRate = 0.01;

Run a random scenario by executing the `dollarAuction`

function. Store the outputs `bids`

and `dropouts`

.

[bids,dropouts] = dollarAuction(nPlayers,incr,dropoutRate);

As the game continues, some players place bids and some drop out. If the bid exceeds 1, the players are locked in a "bidding war" until only one player remains.

The table `dropouts`

contains two variables: `Player`

, a unique number assigned to each player; `Epoch`

, the round of bidding when `Player`

dropped out. Use `findgroups`

to group `dropouts.Epoch`

, and use `splitapply`

to get the number of players who drop out in each of the unique rounds in `dropouts.Epoch`

.

[G,epochs] = findgroups(dropouts.Epoch); numberDropouts = splitapply(@numel,dropouts.Epoch,G);

Initially, there are no dropouts. Add this information to `epochs`

and `numberDropouts`

by prepending `1`

and `0`

.

epochs = [1;epochs]; numberDropouts = [0;numberDropouts];

Use `nPlayers`

and `cumsum`

to calculate the number of players remaining from `numberDropouts`

. Calculate the bids using `incr`

and `epochs`

. Use `stairs`

to plot the bid against the cumulative sum of `numberDropouts`

.

playersRemaining = nPlayers - cumsum(numberDropouts); stairs(incr*epochs,playersRemaining) xlabel('Bid') ylabel('Number of players remaining')

### Estimate Market Value Using Monte-Carlo Methods

You can estimate the market value of the bill with value `origValue`

by using Monte-Carlo methods. Here, you produce a Monte-Carlo model and compare the speed with and without Parallel Computing Toolbox. Set the number of trials `nTrials`

used to randomly sample the outcomes.

nTrials = 10000;

You can sample the possible outcomes by executing the supporting function `dollarAuction`

multiple times. Use a `for`

-loop to produce `nTrials`

samples, storing the last bid from each trial in `B`

. Each time you run the `dollarAuction`

function, you get different results. However, when you run the function many times, the results you produce from all of the runs will have well-defined statistics.

Record the time taken to compute `nTrials`

simulations. To reduce statistical noise in the elapsed time, repeat this process five times, then take the minimum elapsed time.

t = zeros(1,5); for j = 1:5 tic B = zeros(1,nTrials); for i = 1:nTrials bids = dollarAuction(nPlayers,incr,dropoutRate); B(i) = bids.Bid(end); end t(j) = toc; end forTime = min(t)

forTime = 21.4323

Use `histogram`

to plot a histogram of the final bids `B`

. Use `xline`

to overlay the plot with the original value (one dollar) and the average market value given by `mean`

.

histogram(B); origLine = xline(1,'k','LineWidth',3); marketLine = xline(mean(B),'k--','LineWidth',3); xlabel('Market value') ylabel('Frequency') legend([origLine, marketLine],{'Original value','Market value'},'Location','NorthEast')

With the given algorithm and input parameters, the average market value is greater than the original value.

#### Use `parfor`

-loop to Estimate Market Value

You can use Parallel Computing Toolbox to easily speed up your Monte-Carlo code. First, create a parallel pool with four workers using the `'Processes'`

profile.

`p = parpool('Processes',4);`

Starting parallel pool (parpool) using the 'Processes' profile ... Connected to parallel pool with 4 workers.

Replace the `for`

-loop with a `parfor`

-loop. Record the time taken to compute `nTrials`

simulations. To reduce statistical noise in the elapsed time, repeat this process 5 times then take the minimum elapsed time.

t = zeros(1,5); for j = 1:5 tic parfor i = 1:nTrials bids = dollarAuction(nPlayers,incr,dropoutRate); B(i) = bids.Bid(end); end t(j) = toc; end parforTime = min(t)

parforTime = 5.9174

With four workers, the results indiciate that the code can runs over three times faster when you use a `parfor`

-loop.

#### Produce Reproducible Results with Random Numbers in `parfor`

-loops

When you generate random numbers in a `parfor`

-loop, each run of the loop can produce different results. To create reproducible results, each iteration of the loop must have a deterministic state for the random number generator. For more information, see Repeat Random Numbers in parfor-Loops.

The supporting function `dollarAuctionStream`

takes a fourth argument, `s`

. This supporting function uses a specified stream to produce random numbers. To view the code, see `dollarAuctionStream.m`

.

When you create a stream, substreams of that stream are statistically independent. For more information, see `RandStream`

. To ensure that your code produces the same distribution of results each time, create a random number generator stream in each iteration of the loop, then set the `Substream`

property to the loop index. Replace `dollarAuction`

with `dollarAuctionStream`

, then use `s`

to run `dollarAuctionStream`

on a worker.

Record the time taken to compute `nTrials`

simulations. To reduce statistical noise in the elapsed time, repeat this process five times, then take the minimum elapsed time.

t = zeros(1,5); for j = 1:5 tic parfor i = 1:nTrials s = RandStream('Threefry'); s.Substream = i; bids = dollarAuctionStream(nPlayers,incr,dropoutRate,s); B(i) = bids.Bid(end); end t(j) = toc; end parforTime = min(t)

parforTime = 8.7355

### Scale Up from Desktop to Cluster

You can scale your code from your desktop to a cluster with more workers. For more information about scaling up from desktop to a cluster, see Scale Up from Desktop to Cluster.

Use `delete`

to shut down the existing parallel pool.

delete(p);

Compute the supporting function `dollarAuctionStream`

in a `parfor`

-loop. Run the same `parfor`

-loop with different numbers of workers, and record the elapsed times. To reduce statistical noise in the elapsed time, run the `parfor`

-loop five times, then take the minimum elapsed time. Record the minimum times in the array `elapsedTimes`

. In the following code, replace `MyCluster`

with the name of your cluster profile.

workers = [1 2 4 8 16 32]; elapsedTimes = zeros(1,numel(workers)); % Create a pool using the 'MyCluster' cluster profile p = parpool('MyCluster', 32);

Starting parallel pool (parpool) using the 'MyCluster' profile ... Connected to the parallel pool (number of workers: 32).

for k = 1:numel(workers) t = zeros(1,5); for j = 1:5 tic parfor (i = 1:nTrials, workers(k)) s = RandStream('Threefry'); s.Substream = i; bids = dollarAuctionStream(nPlayers,incr,dropoutRate,s); B(i) = bids.Bid(end); end t(j) = toc; end elapsedTimes(k) = min(t); end

Analyzing and transferring files to the workers ...done.

Calculate the computational speedup by dividing `elapsedTimes(1)`

by the times in `elapsedTimes`

. Examine strong scaling by plotting the speedup against the number of workers.

speedup = elapsedTimes(1) ./ elapsedTimes; plot(workers,speedup) xlabel('Number of workers') ylabel('Computational speedup')

The computational speedup increases with the number of workers.