# Configuring Tez
You can customize Tez by setting values using the `tez-site` configuration classification, which configures settings in the `tez-site.xml` configuration file. For more information, see [TezConfiguration](https://tez.apache.org/releases/0.8.2/tez-api-javadocs/configs/TezConfiguration.html) in the Apache Tez documentation. To change Hive or Pig to use the Tez execution engine, use the `hive-site` and `pig-properties` configuration classifications as appropriate. Examples are shown below.
## Example configuration
**Example: Customizing the Tez root logging level and setting Tez as the execution engine for Hive and Pig**
The example `create-cluster` command shown below creates a cluster with Tez, Hive, and Pig installed. The command references a file stored in Amazon S3, `myConfig.json`, which specifies properties for the `tez-site` classification that sets `tez.am.log.level` to `DEBUG`, and sets the execution engine to Tez for Hive and Pig using the `hive-site` and `pig-properties` configuration classifications.
Linux line continuation characters (\$1) are included for readability. They can be removed or used in Linux commands. For Windows, remove them or replace with a caret (^).
```
aws emr create-cluster --release-label emr-7.13.0 \
--applications Name=Tez Name=Hive Name=Pig --ec2-attributes KeyName=myKey \
--instance-type m5.xlarge --instance-count 3 \
--configurations https://s3.amazonaws.com/amzn-s3-demo-bucket/myfolder/myConfig.json --use-default-roles
```
Example contents of `myConfig.json` are shown below.
```
[
{
"Classification": "tez-site",
"Properties": {
"tez.am.log.level": "DEBUG"
}
},
{
"Classification": "hive-site",
"Properties": {
"hive.execution.engine": "tez"
}
},
{
"Classification": "pig-properties",
"Properties": {
"exectype": "tez"
}
}
]
```
**Note**
With Amazon EMR version 5.21.0 and later, you can override cluster configurations and specify additional configuration classifications for each instance group in a running cluster. You do this by using the Amazon EMR console, the AWS Command Line Interface (AWS CLI), or the AWS SDK. For more information, see [Supplying a Configuration for an Instance Group in a Running Cluster](https://docs.aws.amazon.com/emr/latest/ReleaseGuide/emr-configure-apps-running-cluster.html).
## Tez asynchronous split opening
When there is a large number of small files in the table path, and a query attempts to read them all, each small file that corresponds to each individual split gets combined under one Tez *grouped split*. A single mapper then processes the single Tez grouped split. Since the execution is synchronous, each individual split under the grouped split gets processed one by one. This requires `RecordReader` objects to synchronously process the splits.
Amazon EMR 6.15.0 introduces configurations that you can specify to asynchronously open the input splits in a Tez grouped split. The feature was initiated by [TEZ-4397](https://issues.apache.org/jira/browse/TEZ-4397), but had regressions in OSS Hive. EMR Hive fixed the regressions and additional bugs in Hive ACID table. This improvement results in faster performance of read queries when there are a large number of input splits in a single Tez Grouped Split.
| Name | Classification | Description |
| --- | --- | --- |
| `tez.grouping.split.init.threads` | `tez-site` | Specifies the number of daemon threads that Tez uses to pre-initiate the `RecordReaders` and open splits. For ACID tables, the maximum supported value of `tez.grouping.split.init.threads` is `1`. |
| `tez.grouping.split.init.recordreaders` | `tez-site` | Specifies the number of `RecordReaders` to keep pre-initialised by the daemon threads. This can help when Tez grouped split contains a large number of `InputSplits`. Initialization of `RecordReaders` to process those input splits can be done asynchronously with daemon threads instead of sequential processing. |
Configuration notes:
| Subject matter | Details |
| --- | --- |
| Recommended configuration settings | It is recommended to set the above configuration settings to the values you want in both `hive-site` and `tez-site`. |
| Matching values | The values of the configuration settings should be the same in both `hive-site` and `tez-site`. |
| LLAP recommendation | It isn't recommended to use this feature when LLAP is enabled. |
### Benchmarking for Tez asynchronous split opening
We used the following environments and configurations for benchmarking the Tez asynchronous split opening capability:
+ **Benchmark environment** – Amazon EMR cluster with 1 primary node that uses m5.16xlarge, and 16 core nodes that use m5.16xlarge.
+ **Benchmark configurations** – To simulate the scenario for benchmarking where a large number of input splits are in a single Tez grouped split, `tez.grouping.split-count` is set to `1`.
+ **Table used for benchmarking** – The table contains 200 partitions, with each partition containing a single file. The benchmark is done for when that table contains CSV files, and when that table contains parquet files. Hive query for benchmarking: `SELECT COUNT(*)` from the table ten times, and take the average runtime.
+ **Configurations to enable Tez async split opening** – As follows:
+ `tez.grouping.split.init.threads` = `4`
+ `tez.grouping.split.init.recordreaders` = `10`
| Dataset | Feature disabled (baseline) | Feature enabled | Improvement |
| --- | --- | --- | --- |
| CSV dataset | 90.26 seconds | 79.20 seconds | 12.25% |
| Parquet dataset | 54.67 seconds | 42.23 seconds | 22.75% |