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Ytk-mp4j is a fast, user-friendly, cross-platform, multi-process, multi-thread collective message passing java library for distributed machine learning. It's similar to MPI but it has several important features:

  1. Supports multi-process, multi-thread message passing at the same time. So you needn't combine MPI with OpenMP to use multi-process, multi-thread. Serial codes can easily be paralleled and distributed.
  2. Supports most primitive types and Java Object(double, float, long, int, short, byte, String, Object).
  3. Supports any object message passing as long as you implement Serializer interface of Kryo.
  4. Supports not only array data container, but also map container(used for sparse communication).
  5. Using map container and Object type, you can realize complex operations(e.g. Set Intersection, Set Union, List concat…).
  6. Supports data compression that can can be used to reduce communication traffic.

Ytk-mp4j uses master-slave mode, only one master, one or more slaves(workers). Responsibilities of master are:

  1. Coordinates and synchronizes all communication of slaves.
  2. Receives logs from slaves and prints.

Communicative slaves are embedded in real workers, which can lodge with any kinds of workers(e.g. reduce of hadoop, executors of spark, …).

Ytk-mp4j implements state-of-art collective algorithms descrited in [1,2].

Master Startup

You can use shell command below to start master directly.

nohup java -server -Xmx600m -classpath .:lib/*:config -Dlog4j.configuration=file:config/log4j_master.properties com.fenbi.mp4j.comm.CommMaster ${slave_num} ${master_port} > log/master_startup.log 2>&1 & echo $! > master_${master_port}.pid

You can also assemble codes below into your project.

CommMaster master = null;
try {
    int nWorker = Integer.parseInt(args[0]);
    int rpcPort = Integer.parseInt(args[1]);
    LOG.info("worker:" + nWorker);
    LOG.info("port:" + rpcPort);
    master = new CommMaster(nWorker, rpcPort);
    master.start();

    LOG.info("server hostname:" + master.getHostName());
    LOG.info("server hostport:" + master.getHostPort());
} catch (IOException e) {
    LOG.error("IO exception", e);
} finally {
    int code;
    if (master == null) {
        code = 1;
        LOG.error("master start error, failed!");
    } else {
        code = master.stop();
    }
    LOG.info("exit code:" + code);
    System.exit(code);
}  

Usage of Slave

  • In multi-process environment, use code template below to start multi-process communication:
int errorCode = 0;
ProcessCommSlave comm = null;
try {
    comm = new ProcessCommSlave(loginName, hostName, hostPort);
    // ...
    // array = comm.allreduceArray(array, ......)
    // ...

} catch (Exception e) {
    errorCode = 1;
    LOG.error("existed exception!", e);
    if (comm != null) {
        try {
            comm.exception(e);
        } catch (Mp4jException e1) {
            LOG.error("comm send exception message error!", e);
        }
    }
} finally {
    try {
        comm.close(errorCode);
    } catch (Mp4jException e) {
        errorCode = 1;
        LOG.error("comm close exception!", e);
    }
}

return errorCode == 0;
  • In multi-process, multi-thread environment, use code template below to start multi-process, multi-thread communication(ThreadCommSlave supports multi-process and multi-thread,it contains all of functions of ProcessCommSlave, so you can just use ThreadCommSlave.):
int errorCode = 0;
ThreadCommSlave comm = null;
try {
    comm = new ThreadCommSlave(loginName, threadNum, hostName, hostPort);
    final ThreadCommSlave finalComm = comm;

    Thread []threads = new Thread[threadNum];
    for (int t = 0; t < threadNum; t++) {
        final int tidx = t;
        threads[t] = new Thread(t + "") {
            @Override
            public void run() {
                finalComm.setThreadId(tidx);
                try {
                    // ...
                    // array = comm.allreduceArray(array, ......)
				  // ...
                } catch (Exception e) {
                    try {
                        finalComm.exception(e);
                        finalComm.close(1);
                    } catch (Mp4jException e1) {
                        LOG.error("comm send exception message error!", e);
                    }
                    System.exit(1);
                }
            }
        };
        threads[t].start();
    }

    for (int t = 0; t < threadNum; t++) {
        threads[t].join();
    }
} catch (Exception e) {
    errorCode = 1;
    LOG.error("existed exception!", e);
    if (comm != null) {
        try {
            comm.exception(e);
        } catch (Mp4jException e1) {
            LOG.error("comm send exception message error!", e);
        }
    }
} finally {
    try {
        comm.close(errorCode);
    } catch (Mp4jException e) {
        errorCode = 1;
        LOG.error("comm close exception!", e);
    }
}

return errorCode == 0;

Collective Communications

Collective communication is a method of communication which involves participation of all processes(all threads). Ytk-mp4j doesn't contain point-to-point communication(using collective comunication can realize it, but it is not recommended). A communication operation in ytk-mp4j usually contains an Operand. In reduction operation, it also contains a Operator.

Slaves

Each process will be assigned a special number—rank which is encoded with lexicographical order of hostname from 0 to slaveNum - 1. Use getRank() to get rank. Use getSlaveNum() to get the total number of processes.

Operand

Ytk-mp4j provides 8 predefined Operands, which almost meets all your needs. If you want to define a new Operand, you can extend Operand abstract class. compress is to decide whether the communication uses compression(default is false). To create a ObjectOperand, you must provide a serializer of Kryo and class type for your object. Kryo is a fast and efficient object graph serialization framework for java. ytk-learn uses Kryo to serialize and compress objects. More details about predefined Operands see Operands.

Using KryoUtils.getDefaultSerializer(Class type) function, you can get serializer easily for most simple objects.

Operand Support Java Types How to Create?
DoubleOperand single double, double array, Double object Operands.DOUBLE_OPERAND(boolean compress)
FloatOperand single float, double array, Float object Operands.FLOAT_OPERAND(boolean compress)
LongOperand single long, long array, Long object Operands.LONG_OPERAND(boolean compress)
IntOperand single int, int array, Integer object Operands.INT_OPERAND(boolean compress)
ShortOperand single short, short array, Short object Operands.SHORT_OPERAND(boolean compress)
ByteOperand single byte, type array, Byte object Operands.BYTE_OPERAND(boolean compress)
StringOperand String object Operands.STRING_OPERAND(boolean compress)
ObjectOperand Object Operands.OBJECT_OPERAND(Serializer serializer, Class type, boolean compress)
Operator(reduction operation)

Different Operands support different Operators. Ytk-mp4j provides some predefined reduction operations. The reduction operations should be commutative and associative. You can implement interface of different operands to get custom reduction operation.More details about predefined Operands see Operators.

Operand Predefined Reduction IOperators Interface
DoubleOperand Operators.Double.SUM/MAX/MIN/PROD/FLOAT_MAX_LOC/FLOAT_MIN_LOC IDoubleOperator
FloatOperand Operators.Float.SUM/MAX/MIN/PROD IFloatOperator
LongOperand Operators.Long.SUM/MAX/MIN/PROD/BITS_AND/BITS_OR/BITS_XOR/INT_MAX_LOC/INT_MIN_LOC ILongOperator
IntOperand Operators.Int.SUM/MAX/MIN/PROD/BITS_AND/BITS_OR/BITS_XOR IIntOperator
ShortOperand Operators.Short.SUM/MAX/MIN/PROD/BITS_AND/BITS_OR/BITS_XOR IShortOperator
ByteOperand Operators.Byte.SUM/MAX/MIN/PROD/BITS_AND/BITS_OR/BITS_XOR IByteOperator
StringOperand null IStringOperator
ObjectOperand null IObjectOperator
Special Operators

Set union, intersection and List concat(partial) are commutative and associative. Those operations are often used. More details see ThreadCommSlave, ProcessCommSlave.

Collective Operations

gather: Before the gather, each process node owns a piece of the data. After the gather, root process owns the entire data.

allgather: Before the allgather, each process node owns a piece of the data. After the allgather, all processes own all of the data.

broadcast: Before the broadcast, only root process owns entire data. After the broadcast, all processes own all of the data.

scatter: Before the scatter, only root process owns entire data. After scatter, each process owns a piece of the data.

reduce: Before the reduce, each process owns the data x(i). After the reduce, only root process owns the data of (x(0) + x(1) + … + x(p-1)). "+" is a general reduction operation.

allreduce: Identical to the reduce, except all the processes own the data of reduced.

reducescatter: Identical to the reduce, except each process owns a piece of reduced result.

Rpc-based allreduce

When the array size for allreduce is very small, using the allreduce algorithm in [1,2] may be not efficient, because most of time spend in network connection, so rpc-based allreduce is better for this situation.

More details of collective operations, see ThreadCommSlave, ProcessCommSlave.

Container

Ytk-mp4j supports not only array data container in which data are arranged in different processes/threads in order, but also supports map data container which is more powerful and flexible. But it is not efficient, and the order of data in map data container will not be emphasized. In ThreadCommSlave, all threads in the same process shared the same result(reduce memory use and gc) when using map container, and if you want to modify in different threads, you must clone a duplicate ahead of modification.

Information

Master can receive information from slaves then print them out. Ytk-mp4j provides 4 different information-sending interfaces: info, debug, error, exception. More details see ThreadCommSlave, ProcessCommSlave.

Barrier

ProcessCommSlave provides barrier interface to synchronize all processes. Likewise, ThreadCommSlave also provides barrier interface to synchronize all processes and all threads, and provides threadBarrier interface to synchronize all threads in special process.

More Test Cases

In the package com.fenbi.mp4j.check, almost all interfaces have a detailed use case. The bin/comm_cluster_error_check.sh is an integrated test script, and you can use it to test all interfaces.

Applications

Your serial programs can be easily parallelized and distributed with ytk-mp4j.

ytk-learn is a general machine learning library which uses ytk-mp4j to realize distributed version:

  1. Uses allreduce to aggregate the count of instances.
  2. Uses allreduceMap to count frequency of occurrence of all features.
  3. Uses allreduceArray/reduceScatterArray to realize distributed L-BFGS optimizer.
  4. Uses allredueMap to realize distributed weighted approximate quantile.
  5. Uses allreduceArray/reduceScatterArray ... to realize distributed GBDT.

…..

Communication Complexity

Complexity of Process Collective Communications(Using GitHub with MathJax to render Latex)
Communication Connection Transfer Computation Total
gather $ \lfloor lg(p) \rfloor \alpha$ $\frac{p-1}{p}n\beta$ 0 $ \lfloor lg(p) \rfloor \alpha + \frac{p-1}{p}n\beta$
scatter $ \lfloor lg(p) \rfloor \alpha$ $\frac{p-1}{p}n\beta$ 0 $ \lfloor lg(p) \rfloor \alpha + \frac{p-1}{p}n\beta$
allgather $(p-1)\alpha$ $\frac{p-1}{p}n\beta$ 0 $(p-1)\alpha + \frac{p-1}{p}n\beta$
reduce-scatter $(p-1)\alpha$ $\frac{p-1}{p}n\beta$ $\frac{p-1}{p}n\gamma$ $(p-1)\alpha + \frac{p-1}{p}n\beta +\frac{p-1}{p}n\gamma$
broadcast $ (\lfloor lg(p) \rfloor + p-1)\alpha$ $2\frac{p-1}{p}n\beta$ 0 $ (\lfloor lg(p) \rfloor + p-1)\alpha + 2\frac{p-1}{p}n\beta $
reduce $ (\lfloor lg(p) \rfloor + p-1)\alpha$ $2\frac{p-1}{p}n\beta$ $\frac{p-1}{p}n\gamma$ $ (\lfloor lg(p) \rfloor + p-1)\alpha + 2\frac{p-1}{p}n\beta + \frac{p-1}{p}n\gamma $
allreduce $2(p-1)\alpha$ $2\frac{p-1}{p}n\beta$ $\frac{p-1}{p}n\gamma$ $2(p-1)\alpha + 2\frac{p-1}{p}n\beta + \frac{p-1}{p}n\gamma $

where $\alpha$ is network connection latency, $\beta$ is transfer time per byte, $n$ is the number of bytes transferred, $\gamma$ is the computation cost per byte for performing the reduction operation.

Experiments

We test the costs of collective communications in ytk-mp4j with different double array sizes and numbers of slaves using a 1 Gigabit Ethernet in milliseconds.

slaves(#2)

gather scatter allgather reduce-scatter broadcast reduce allreduce
100000 6 6 6 7 12 12 12
1000000 48 43 45 38 95 105 110
5000000 190 185 210 230 410 400 430
10000000 397 355 445 447 850 840 955
50000000 1780 1751 2261 2508 4310 4482 4744
100000000 3526 3555 4667 4611 8002 8061 9547
500000000 17604 17659 22565 21375 40683 40323 46224
1000000000 34228 34464 47179 47361 80846 78442 93624

slaves(#4)

gather scatter allgather reduce-scatter broadcast reduce allreduce
100000 12 8 10 11 19 20 23
1000000 57 56 63 65 118 121 130
5000000 271 285 307 289 557 626 603
10000000 538 534 630 618 1149 1237 1230
50000000 2610 2599 3455 3616 6268 6124 6964
100000000 5223 5266 6601 7322 11973 11894 13283
500000000 25849 26390 33883 34759 60338 60016 63912
1000000000 51396 52517 65308 62918 117354 118999 129174

slaves(#6)

gather scatter allgather reduce-scatter broadcast reduce allreduce
100000 13 13 14 14 24 24 24
1000000 62 62 75 84 155 146 159
5000000 315 380 326 331 705 630 679
10000000 602 762 681 731 1533 1561 1529
50000000 2949 3530 3890 3846 7442 6837 7657
100000000 5770 6934 7584 7408 14734 15791 15226
500000000 28572 34762 37372 39021 72570 76837 77544
1000000000 56946 57042 74376 69005 140572 126827 143813

slaves(#8)

gather scatter allgather reduce-scatter broadcast reduce allreduce
100000 16 11 15 14 24 25 28
1000000 69 67 92 85 152 155 183
5000000 323 332 332 345 666 685 681
10000000 645 648 741 749 1449 1431 1641
50000000 3068 3048 4276 4459 7470 7595 8534
100000000 6050 6143 8716 8931 14800 14561 16332
500000000 30021 30479 40019 38916 70190 69255 77593
1000000000 59974 61426 75464 74495 136574 134260 154090

All collective communications of 1e9 double array size

Integration with Maven

<dependency>
  <groupId>com.fenbi.mp4j</groupId>
  <artifactId>ytk-mp4j</artifactId>
  <version>0.0.1</version>
</dependency>

Reference

  1. Thakur, Rajeev, Rolf Rabenseifner, and William Gropp. "Optimization of collective communication operations in MPICH." The International Journal of High Performance Computing Applications 19.1 (2005): 49-66.
  2. Faraj, Ahmad, Pitch Patarasuk, and Xin Yuan. "Bandwidth efficient all-to-all broadcast on switched clusters." Cluster Computing, 2005. IEEE International. IEEE, 2005.

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