# 4. How To¶

## 4.1. Debug PS-Lite¶

One way to debug is loggining all communications. We can do it by specifying the environment variable PS_VERBOSE:

• PS_VERBOSE=1: logging connection information
• PS_VERBOSE=2: logging all data communication information

For example, first run make test; cd tests in the root directory. Then

export PS_VERBOSE=1; ./local.sh 1 1 ./test_connection


Possible outputs are

[19:57:18] src/van.cc:72: Node Info: role=schedulerid=1, ip=127.0.0.1, port=8000
[19:57:18] src/van.cc:72: Node Info: role=worker, ip=128.2.211.110, port=58442
[19:57:18] src/van.cc:72: Node Info: role=server, ip=128.2.211.110, port=40112
[19:57:18] src/van.cc:336: assign rank=8 to node role=server, ip=128.2.211.110, port=40112
[19:57:18] src/van.cc:336: assign rank=9 to node role=worker, ip=128.2.211.110, port=58442
[19:57:18] src/van.cc:347: the scheduler is connected to 1 workers and 1 servers
[19:57:18] src/van.cc:354: S[8] is connected to others
[19:57:18] src/van.cc:354: W[9] is connected to others
[19:57:18] src/van.cc:296: H[1] is stopped
[19:57:18] src/van.cc:296: S[8] is stopped
[19:57:18] src/van.cc:296: W[9] is stopped


where H, S and W stand for scheduler, server, and worker respectively.

## 4.2. Use a Particular Network Interface¶

In default PS-Lite automatically chooses an available network interface. But for machines have multiple interfaces, we can specify the network interface to use by the environment variable DMLC_INTERFACE. For example, to use the infinite-band interface ib0, we can

export DMLC_INTERFACE=ib0; commands_to_run


If all PS-Lite nodes run in the same machine, we can set DMLC_LOCAL to use memory copy rather than the local network interface, which may improve the performance:

export DMLC_LOCAL=1; commands_to_run


## 4.3. Environment Variables to Start PS-Lite¶

This section is useful if we want to port PS-Lite to other cluster resource managers besides the provided ones such as ssh, mpirun, yarn and sge.

To start a PS-Lite node, we need to give proper values to the following environment variables.

• DMLC_NUM_WORKER : the number of workers
• DMLC_NUM_SERVER : the number of servers
• DMLC_ROLE : the role of the current node, can be worker, server, or scheduler
• DMLC_PS_ROOT_URI : the ip or hostname of the scheduler node
• DMLC_PS_ROOT_PORT : the port that the scheduler node is listening

## 4.4. Retransmission for Unreliable Network¶

It’s not uncommon that a message disappear when sending from one node to another node. The program hangs when a critical message is not delivered successfully. In that case, we can let PS-Lite send an additional ACK for each message, and resend that message if the ACK is not received within a given time. To enable this feature, we can set the environment variables

• PS_RESEND : if or not enable retransmission. Default is 0.
• PS_RESEND_TIMEOUT : timeout in millisecond if an ACK message if not received. PS-Lite then will resend that message. Default is 1000.

We can set PS_DROP_MSG, the percent of probability to drop a received message, for testing. For example, PS_DROP_MSG=10 will let a node drop a received message with 10% probability.