I have Kafka engine table, MV and destination MT table.
Kafka table create query:
```
CREATE TABLE abc.Kafka_TestSource (...) 
ENGINE = Kafka 
SETTINGS kafka_broker_list = '...'
, format_avro_schema_registry_url = '...'
, kafka_topic_list = '...'
, kafka_group_name = 'Test_KafkaToCH'
, kafka_format = 'AvroConfluent'
, kafka_num_consumers = 1
, kafka_thread_per_consumer = 0
```

Due to Grafana consumer group stats dashboard it reads the topic with average speed ~11k records per second.
I recreate Kafka table with `kafka_num_consumers = 9` (kafka topic has 9 partitions) and it still reads with the same speed ~11k rps, **system.metrics** also shows that only 1 pool is being used:
```
SELECT
    m.metric,
    m.value
FROM system.metrics AS m
WHERE m.metric LIKE 'Background%'

Query id: 831a9285-e91b-45a4-8a1b-6b536d6860fe

┌─metric──────────────────────────────────┬─value─┐
│ BackgroundMergesAndMutationsPoolTask    │     4 │
│ BackgroundFetchesPoolTask               │     0 │
│ BackgroundCommonPoolTask                │     0 │
│ BackgroundMovePoolTask                  │     0 │
│ BackgroundSchedulePoolTask              │     0 │
│ BackgroundBufferFlushSchedulePoolTask   │     0 │
│ BackgroundDistributedSchedulePoolTask   │     0 │
│ BackgroundMessageBrokerSchedulePoolTask │     1 │
└─────────────────────────────────────────┴───────┘
```

If I set `kafka_thread_per_consumer = 1` then **BackgroundMessageBrokerSchedulePoolTask** in **system.metrics** raises up to 9 and read speed in Grafana raises up to 100 records per second:
![pic](https://github.com/sokolov-alexey/pics/blob/main/ch_kafka.PNG?raw=true)
1. `kafka_num_consumers = 1, kafka_thread_per_consumer = 0`
2. `kafka_num_consumers = 9, kafka_thread_per_consumer = 0`
3. `kafka_num_consumers = 9, kafka_thread_per_consumer = 1`

Why `kafka_num_consumers` doesn't work without `kafka_thread_per_consumer`?

ClickHouse 21.11.4.14.
conf.set("client.software.version", VERSION_DESCRIBE);
    conf.set("auto.offset.reset", "earliest");     
    
    
    
    size_t default_queued_min_messages = 100000; 
    conf.set("queued.min.messages", std::max(getMaxBlockSize(),default_queued_min_messages));
    updateConfiguration(conf);
    
    conf.set("enable.auto.commit", "false");       
    conf.set("enable.auto.offset.store", "false"); 
    conf.set("enable.partition.eof", "false");     
    
    auto consumer = std::make_shared<cppkafka::Consumer>(conf);
    consumer->set_destroy_flags(RD_KAFKA_DESTROY_F_NO_CONSUMER_CLOSE);
    
    if (thread_per_consumer)
    {
        auto& stream_cancelled = tasks[consumer_number]->stream_cancelled;
        return std::make_shared<ReadBufferFromKafkaConsumer>(consumer, log, getPollMaxBatchSize(), getPollTimeoutMillisecond(), intermediate_commit, stream_cancelled, topics);
    }
    return std::make_shared<ReadBufferFromKafkaConsumer>(consumer, log, getPollMaxBatchSize(), getPollTimeoutMillisecond(), intermediate_commit, tasks.back()->stream_cancelled, topics);
}
size_t StorageKafka::getMaxBlockSize() const
{
    return kafka_settings->kafka_max_block_size.changed
        ? kafka_settings->kafka_max_block_size.value
        : (getContext()->getSettingsRef().max_insert_block_size.value / num_consumers);
}
size_t StorageKafka::getPollMaxBatchSize() const