More details on testing

Author: rtyley

README.md

@@ -1,9 +1,28 @@
 # aws-sdk-async-response-bytes
 
+_Reducing memory consumption of the [AWS SDK for Java v2](https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/home.html) when loading an S3 object into memory_
+
+This repo is for evaluation of different approaches to reducing memory consumption of the AWS SDK Java v2 [async](https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/asynchronous.html)
+method
+[`AsyncResponseTransformer.toBytes()`](https://github.com/aws/aws-sdk-java-v2/blob/69f7191252c26b351f7fb1c5f031948dac43e4c9/core/sdk-core/src/main/java/software/amazon/awssdk/core/async/AsyncResponseTransformer.java#L204-L206).
+The resulting PR is https://github.com/aws/aws-sdk-java-v2/pull/4355.
+
+Approaches under consideration for reducing memory usage:
+
+* Change A: Optimise byte storage, comes in two successive parts:
+  * [**A1**](https://github.com/rtyley/aws-sdk-async-response-bytes/pull/8): Use the Content Length to initialise the `ByteArrayOutputStream` with **a byte array of the right size**.
+  * [**A2**](https://github.com/rtyley/aws-sdk-async-response-bytes/pull/10): Use a simple fixed-size byte array in preference to a `ByteArrayOutputStream`
+* Change [**B**](https://github.com/rtyley/aws-sdk-async-response-bytes/pull/12): Avoid performing a byte array copy when creating the `ResponseBytes` instance
+
+Approaches A & B can be considered individually, but work much better when combined.
+
+## Automated Memory-Consumption tests
+
 The test script [`findMinMem.sh`](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/ab9307fee6d4965cce0cba60c439035a22cf2d98/findMinMem.sh)
 repeatedly downloads a [**258 MB**](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/eb84c15259f947e91c442eb0b8eda68b8cdbefbb/src/main/java/com/madgag/aws/sdk/async/responsebytes/KnownS3Object.java#L7)
-object from S3 into memory, while varying the amount  of Java heap memory allocated with `-Xmx`,
-to find the amount of memory necessary for the download to consistently succeed.
+object from S3 into memory, while varying the amount of Java heap memory allocated with `-Xmx`,
+to find the amount of memory necessary for the download to consistently succeed with the given
+approach.
 
 These are the resulting memory requirements found, in MB, for all possible permutations
 of the fixes:
@@ -14,4 +33,11 @@ of the fixes:
 | Include B | [787](https://github.com/rtyley/aws-sdk-async-response-bytes/actions/runs/6058093770#summary-16439898197)  | [657](https://github.com/rtyley/aws-sdk-async-response-bytes/actions/runs/6058097434#summary-16439905403) | [357](https://github.com/rtyley/aws-sdk-async-response-bytes/actions/runs/6058098624#summary-16439907698) |
 
 From the results, it's clear that we need all 3 changes (A1, A2, & B) in order
-to get the lowest memory usage.
+to get the lowest memory usage.
+
+### Test details
+
+* JVM invocation: [forked](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/3d8e86870a86e035189d6f88b4e41e42be72ca82/build.sbt#L1) (for each invocation of `GetObject`)
+* Invocation repetition count for success: [20](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/3d8e86870a86e035189d6f88b4e41e42be72ca82/build.sbt#L17)
+* SdkAsyncHttpClient: [`AwsCrtAsyncHttpClient`](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/3d8e86870a86e035189d6f88b4e41e42be72ca82/src/main/java/com/madgag/aws/sdk/async/responsebytes/Main.java#L39) ([the AWS CRT-based HTTP client](https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/http-configuration-crt.html))
+* Download integrity: [SHA-256 check against known hash](https://github.com/rtyley/aws-sdk-async-response-bytes/blob/main/src/main/java/com/madgag/aws/sdk/async/responsebytes/Main.java#L30)