Lambdas stop invoking after a period of time

Here's my setup:

A Python 3.6 lambda function, which I want to keep pre-warmed at a certain concurrency level (say, 10). The lambda's initialization is painful enough that I don't want to inflict this cost on visitors at random. I call these lambdas "workers"

A Node lambda function which runs every 5 minutes to try to pre-warm 10 instances. It uses the Event invocation type for 9 of them, and RequestResponse for 1. There's only either one or zero of this lambda running at any one time. I call this a "warmer".

I followed the guidelines at [https://www.jeremydaly.com/lambda-warmer-optimize-aws-lambda-function-cold-starts/], namely:

• Don’t ping more often than every 5 minutes
• Invoke the function directly (i.e. don’t use API Gateway to invoke it)
• Pass in a test payload that can be identified as such
• Create handler logic that replies accordingly without running the whole function

Here's a problem: this works great for several minutes. Then, as I watch the logs, I start to get timeouts from my worker lambda invocations. The timeouts quickly take over all the invocations that the warmer is trying to launch.

Now, no worker lambdas are prewarmed any more. But the warmer keeps on trying, on a Cloudwatch event cron schedule, suffering 100% timeouts. Finally, Lambda stops trying to launch my worker lambdas at all. It feels like some aspect of Lambda's getting its state scrambled. The only way to recover is to re-deploy the lambda. That buys me another hour with pre-warmed lambdas working.

Questions:

• How do I get visibility into why my worker lambdas start timing out, and then become completely non-responsive?
• What is the definition of a "Concurrent Execution"? On the main Lambda dashboard it shows me this chart of them. Yet, it seems to have more than twice as many Concurrent Executions as I'm requesting.

Here's the warmup lambda code (Node):

// warmer
"use strict";

/** Generated by Serverless WarmUP Plugin at ${new Date().toISOString()} */
const aws = require("aws-sdk");
aws.config.region = "${this.options.region}";
const lambda = new aws.Lambda({httpOptions: {timeout: 60000}});
const functionNames = ${JSON.stringify(functionNames)};
const delay = ms => new Promise(res => setTimeout(res, ms))
const concurrency = 10;
module.exports.warmUp = async (event, context, callback) => {
  console.log("Warm Up Start");
  const invokes = await Promise.all(functionNames.map(async (functionName) => {

    let invocations = [];

    try {
      for(let i=1;i <= concurrency;i++){
          let params = {
            FunctionName: functionName,
            InvocationType: (i===concurrency)?'RequestResponse': 'Event',
            LogType: 'None',
            Qualifier: process.env.SERVERLESS_ALIAS || "$LATEST",
            Payload: JSON.stringify({
              source: 'serverless-plugin-warmup',
              '__WARMER_INVOCATION__': i,
              '__WARMER_CONCURRENCY__': concurrency,
              '__WARMER_REQUESTED__': new Date().toISOString(),
            })
          };

          invocations.push(lambda.invoke(params).promise())
      }
      return await delay(75).then(Promise.all(invocations.map(p => p.catch(e => e)))
        .then(results => console.log('results', results))
        .catch(e => {
          console.log(e);
          return e;
        }
        ))
    } catch (e) {
      console.log(\`Warm Up Invoke Error: \${functionName}\`, e);
      return false;
    }
  }));

  console.log(\`Warm Up Finished\`);

}

And here's the worker lambda (Python):

    source = event.get('source')
    if source == 'serverless-plugin-warmup':
        time.sleep(0.05)
        print(event)
        return lambda_gateway_response(200, {"status": "lambda warmup"})