Mimir is a new implementation of MapReduce over MPI. Mimir inherits the core principles of existing MapReduce frameworks, such as MR-MPI, while redesigning the execution model to incorporate a number of sophisticated optimization techniques that achieve similar or better performance with significant reduction in the amount of memory used.
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Mimir is a memory-efficient and scalable MapReduce implementation over MPI for supercomputing systems. The inefficient memory use of MapReduce-MPI drives the design of this new library.

The code is still under development. If you find any problems, please contact us at taufer@utk.edu.


  1. Tao Gao, Yanfei Guo, Boyu Zhang, Pietro Cicotti, Yutong Lu, Pavan Balaji, and Michela Taufer. On the Power of Combiner Optimizations in MapReduce over MPI Workflows. IEEE International Conference on Parallel and Distributed Systems (ICPADS). Sentosa, Singapore, December 2018.

  2. Tao Gao, Yanfei Guo, Boyu Zhang, Pietro Cicotti, Yutong Lu, Pavan Balaji, and Michela Taufer. Mimir: Memory-Efficient and Scalable MapReduce for Large Supercomputing Systems. The 31st IEEE International Parallel and Distributed Processing Symposium (IPDPS) 2017.

  3. Tao Gao, Yanfei Guo, Yanjie Wei, Bingqiang Wang, Yutong Lu, Pietro Cicotti, Pavan Balaji, and Michela Taufer. Bloomfish: A Highly Scalable Distributed K-mer Counting Framework. IEEE International Conference on Parallel and Distributed Systems (ICPADS) 2017.


  • A C++ compiler: supports C++ 11
  • MPI implementation: supports MPI 3.0

Getting Started

Mimir can be build and install from release tarball as follow.

make install

Programming with Mimir

Mimir implements the MapReduce programming model. Here we use wordcount as an example to show how to program with Mimir.

The MapReduce algorithm as the map and the reduce functions.

void map(Readable<char *, void> *input, Writable<char *, uint64_t> *output, void *ptr)
    char *line = NULL;
    while (input->read(&line, NULL) == true) {
        char *saveptr = NULL;
        char *word = strtok_r(line, " ", &saveptr);
        while (word != NULL) {
            if (strlen(word) < 1024) {
                uint64_t one = 1;
                output->write(&word, &one);
                nwords += 1;
            word = strtok_r(NULL, " ", &saveptr);

void reduce(Readable<char *, uint64_t> *input, Writable<char *, uint64_t> *output, void *ptr);
    char *key = NULL;
    uint64_t val = 0;
    uint64_t count = 0;
    while (input->read(&key, &val) == true) {
        count += val;
    output->write(&key, &count);

After defining the map and reduce function, we can start creating the runtime environment. Since Mimir is built on top of MPI, the first step is setup the MPI environment.


Next, the user need to create a MimirContext. The MimirContext is the runtime abstraction of the MapReduce job.

MimirContext<char *, uint64_t, char *, void> *ctx
    = new MimirContext<char *, uint64_t, char *, void>(input, output,
                                                       MPI_COMM_WORLD, NULL)

The MimirContext template has four template parameters. These parameters describes the types of the keys and values. In the example above, the input and output keys are C string type and the output value is uint64_t type. Note that the types here must match the ones used in the map and the reduce function. Next the constructor takes four arguments which represents the lists of the input and output files, the group of processes involved in the Mimir job (represented as MPI communicator) and the optional pointer to the combiner function.

In this example, we take the first command line arguments as the output file and the rest arguments as input files.

std::string output = argv[1];
std::vector<std::string> input;()
for (int i = 2; i < argc; i++) {

Once the Mimir context is created, we can start processing the input by calling the map and the reduce function was follow. Note the user-defined map and reduce functions are passed here as function pointers.

nunique = ctx->reduce(reduce, NULL, true, "text");

After executing the MapReduce job, the runtime can be cleaned up by simply destroying the mimir context. The application would also need to destroy the MPI runtime at the end of it.

delete context;

Running a Mimir Application

The user can run a Mimir application in the same way as they do with any regular MPI application. That is launching the application using the launcher provided by the MPI library or the cluster. The example above can be launched simply as follow.

mpiexec -n 16 ./wordcount output.txt input0 input1 ...

This will start the wordcount example with 16 processes.

Configuration Parameters

Mimir provides environment variables to tune the library parameters.


  • MIMIR_COMM_SIZE (default: 64M) --- communication bufer size
  • MIMIR_PAGE_SIZE (default: 64M) --- data buffer unit size
  • MIMIR_DISK_SIZE (default: 64M) --- disk I/O buffer size
  • MIMIR_BUCKET_SIZE (default: 1M) --- hash bucket size used by reduce and combine phases
  • MIMIR_MAX_RECORD_SIZE (default: 1M) --- maximum length of any <key,value> pair


  • MIMIR_SHUFFLE_TYPE (default: a2av) --- a2av: MPI_Alltoallv; ia2av: MPI_Ialltoallv
  • MIMIR_MIN_COMM_BUF (default: 2) --- if the shuffle type is ia2av, it sets the min communication buffer count
  • MIMIR_MAX_COMM_BUF (default: 5) --- if the suffle type is ia2av, it sets the max communication buffer count
  • MIMIR_READ_TYPE (default: posix) --- read type (posix; mpiio)
  • MIMIR_WRITE_TYPE (default: posix) --- write type (posix; mpiio)
  • MIMIR_DIRECT_READ (default: off) --- direct read
  • MIMIR_DIRECT_WRITE (default: off) --- direct write


  • MIMIR_WORK_STEAL (default: off) --- enable/disable work stealing
  • MIMIR_MAKE_PROGRESS (default: off) --- enable/disable aggressive progress pushing during nonblocking communication
  • MIMIR_BALANCE_LOAD (default: off) --- enable/disable load balancing
  • MIMIR_BIN_COUNT (default: 1000) --- number of bins per process
  • MIMIR_BALANCE_FACTOR (default: 1.5) --- the balance factor
  • MIMIR_BALANCE_FREQ (default: 1) --- load balancing frequency
  • MIMIR_USE_MCDRAM (default: off) --- if use MCDRAM when there is MCDRAM
  • MIMIR_LIMIT_POWER (default: off) --- enable/disable power capping
  • MIMIR_LIMIT_SCALE (default: 1.0) --- power capping percentage (e.g. 1.0 means no power capping)

Stat & Debug

  • MIMIR_OUTPUT_STAT (default: off) --- output stat file
  • MIMIR_OUTPUT_TRACE (default: off) --- output trace file
  • MIMIR_STAT_FILE (default: NULL) --- stat file name
  • MIMIR_DBG_ALL (default: off) --- enable/disable debug message

Notes for Developers

For developers working on the project, please follow these steps to build the library.

git clone https://github.com/TauferLab/Mimir.git
cd Mimir

make install