contender

Contender

A toy for demonstrating workload contention.

To install: go install github.com/cockroachlabs/contender@latest

Workload

Contender use a simple key-value table to demonstrate the effects of contention in a CockroachDB workload.

CREATE TABLE IF NOT EXISTS contend (
    id UUID NOT NULL PRIMARY KEY DEFAULT gen_random_uuid(),
    value INT NOT NULL DEFAULT 0
)

The --uniqueIds flag sets the number of unique ids to be chosen, and the --workersPerId flag controls the number of concurrent workers for each id. Each worker executes the following SQL:

BEGIN;
SELECT value FROM contend WHERE id = $1 FOR UPDATE;
UPDATE contend SET value = $1 WHERE id = $2;
COMMIT;

Graphs

The Contender workload is instrumented with metrics that will show the effects of contention:

• Per-worker timing histograms
• Number of retry-loops attempts necessary to commit the transaction
• Current configuration
• Worker throughput
• Number of retryable error codes
• DB connection pool

There is a prometheus configuration to collect metrics. Once you have installed Prometheus, it will be as simple as running prometheus --config.file=prometheus.yaml.

Finally, Install Grafana and import the Grafana dashboard as shown here:

.

You will have to import the datasource for Prometheus and possibly refresh it from within each graph panel.

Experiments to run

• Establish baseline of 1 id and 1 worker, with no think time.
• Increase to 10 ids, each with 1 worker.
• Increase to 2 workers per id. You might start seeing some contention.
• Increase to 10 ids with 10 workers each. This should definitely create contention.
• Enable SELECT FOR UPDATE. This will create explicit locking within CockroachDB which will cut down on the number of retries needed and will improve throughput.
• Enable the use of SAVEPOINT cockroach_restart. This wil use an advanced retry technique to raise the priority of the transaction when a conflict occurs.

Flags

% contender --help
Usage of ./contender:
  -conn string
    	database connection string (default "postgresql://root@localhost:26257/defaultdb?sslmode=disable")
  -http string
    	a bind string for the metrics server (default ":8181")
  -maxConns int
    	the maximum number of open database connections (default 10000)
  -savePoint
    	if true, use advanced retry approach to increase TX priorities
  -selectForUpdate
    	if true, use SELECT FOR UPDATE
  -thinkTime duration
    	the amount of time for workers to sleep during transaction (default 25ms)
  -tolerateErrors
    	set to true to ignore non-retryable errors
  -uniqueIds int
    	the number of unique keys to update (default 1)
  -workersPerId int
    	the number of workers trying to update each id (default 1)