dannycjones commented on code in PR #4205: URL: https://github.com/apache/hadoop/pull/4205#discussion_r858597582
########## hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/prefetching.md: ########## @@ -0,0 +1,192 @@ +<!--- + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. See accompanying LICENSE file. +--> + +# S3A Prefetching + +This document explains the `S3PrefetchingInputStream` and the various components it uses. + +This input stream implements prefetching and caching to improve read performance of the input +stream. +A high level overview of this feature was published in +[Pinterest Engineering's blog post titled "Improving efficiency and reducing runtime using S3 read optimization"](https://medium.com/pinterest-engineering/improving-efficiency-and-reducing-runtime-using-s3-read-optimization-b31da4b60fa0). + +With prefetching, the input stream divides the remote file into blocks of a fixed size, associates +buffers to these blocks and then reads data into these buffers asynchronously. +It also potentially caches these blocks. + +### Basic Concepts + +* **Remote File**: A binary blob of data stored on some storage device. +* **Block File**: Local file containing a block of the remote file. +* **Block**: A file is divided into a number of blocks. +The size of the first n-1 blocks is same, and the size of the last block may be same or smaller. +* **Block based reading**: The granularity of read is one block. +That is, either an entire block is read and returned or none at all. +Multiple blocks may be read in parallel. + +### Configuring the stream + +|Property |Meaning |Default | +|--- |--- |--- | +|`fs.s3a.prefetch.enabled` |Enable the prefetch input stream |`true` | +|`fs.s3a.prefetch.block.size` |Size of a block |`8M` | +|`fs.s3a.prefetch.block.count` |Number of blocks to prefetch |`8` | + +### Key Components + +`S3PrefetchingInputStream` - When prefetching is enabled, S3AFileSystem will return an instance of +this class as the input stream. +Depending on the remote file size, it will either use +the `S3InMemoryInputStream` or the `S3CachingInputStream` as the underlying input stream. + +`S3InMemoryInputStream` - Underlying input stream used when the remote file size < configured block +size. +Will read the entire remote file into memory. + +`S3CachingInputStream` - Underlying input stream used when remote file size > configured block size. +Uses asynchronous prefetching of blocks and caching to improve performance. + +`BlockData` - Holds information about the blocks in a remote file, such as: + +* Number of blocks in the remote file +* Block size +* State of each block (initially all blocks have state *NOT_READY*). +Other states are: Queued, Ready, Cached. + +`BufferData` - Holds the buffer and additional information about it such as: + +* The block number this buffer is for +* State of the buffer (Unknown, Blank, Prefetching, Caching, Ready, Done). +Initial state of a buffer is blank. + +`CachingBlockManager` - Implements reading data into the buffer, prefetching and caching. + +`BufferPool` - Manages a fixed sized pool of buffers. +It’s used by `CachingBlockManager` to acquire buffers. + +`S3File` - Implements operations to interact with S3 such as opening and closing the input stream to +the remote file in S3. + +`S3Reader` - Implements reading from the stream opened by `S3File`. +Reads from this input stream in blocks of 64KB. + +`FilePosition` - Provides functionality related to tracking the position in the file. +Also gives access to the current buffer in use. + +`SingleFilePerBlockCache` - Responsible for caching blocks to the local file system. +Each cache block is stored on the local disk as a separate block file. + +### Operation + +#### S3InMemoryInputStream + +For a remote file with size 5MB, and block size = 8MB, since file size is less than the block size, +the `S3InMemoryInputStream` will be used. + +If the caller makes the following read calls: + +``` +in.read(buffer, 0, 3MB); +in.read(buffer, 0, 2MB); +``` + +When the first read is issued, there is no buffer in use yet. +The `S3InMemoryInputStream` gets the data in this remote file by calling the `ensureCurrentBuffer()` +method, which ensures that a buffer with data is available to be read from. + +The `ensureCurrentBuffer()` then: + +* Reads data into a buffer by calling `S3Reader.read(ByteBuffer buffer, long offset, int size)`. +* `S3Reader` uses `S3File` to open an input stream to the remote file in S3 by making + a `getObject()` request with range as `(0, filesize)`. +* The `S3Reader` reads the entire remote file into the provided buffer, and once reading is complete + closes the S3 stream and frees all underlying resources. +* Now the entire remote file is in a buffer, set this data in `FilePosition` so it can be accessed + by the input stream. + +The read operation now just gets the required bytes from the buffer in `FilePosition`. + +When the second read is issued, there is already a valid buffer which can be used. +Don’t do anything else, just read the required bytes from this buffer. + +#### S3CachingInputStream + +If there is a remote file with size 40MB and block size = 8MB, the `S3CachingInputStream` will be +used. + +##### Sequential Reads + +If the caller makes the following calls: + +``` +in.read(buffer, 0, 5MB) +in.read(buffer, 0, 8MB) +``` + +For the first read call, there is no valid buffer yet. +`ensureCurrentBuffer()` is called, and for the first `read()`, prefetch count is set as 1. + +The current block (block 0) is read synchronously, while the blocks to be prefetched (block 1) is +read asynchronously. + +The `CachingBlockManager` is responsible for getting buffers from the buffer pool and reading data +into them. This process of acquiring the buffer pool works as follows: + +* The buffer pool keeps a map of allocated buffers and a pool of available buffers. +The size of this pool is = prefetch block count + 1. +If the prefetch block count is 8, the buffer pool has a size of 9. +* If the pool is not yet at capacity, create a new buffer and add it to the pool. +* If it’s at capacity, check if any buffers with state = done can be released. +Releasing a buffer means removing it from allocated and returning it back to the pool of available +buffers. +* If there are no buffers with state = done currently then nothing will be released, so retry the + above step at a fixed interval a few times till a buffer becomes available. +* If after multiple retries there are still no available buffers, release a buffer in the ready state. +The buffer for the block furthest from the current block is released. + +Once a buffer has been acquired by `CachingBlockManager`, if the buffer is in a *READY* state, it is +returned. +This means that data was already read into this buffer asynchronously by a prefetch. +If it’s state is *BLANK,* then data is read into it using Review Comment: we can move out the comma or drop it here ```suggestion If it’s state is *BLANK* then data is read into it using ``` ########## hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/prefetching.md: ########## @@ -0,0 +1,192 @@ +<!--- + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. See accompanying LICENSE file. +--> + +# S3A Prefetching + +This document explains the `S3PrefetchingInputStream` and the various components it uses. + +This input stream implements prefetching and caching to improve read performance of the input +stream. +A high level overview of this feature was published in +[Pinterest Engineering's blog post titled "Improving efficiency and reducing runtime using S3 read optimization"](https://medium.com/pinterest-engineering/improving-efficiency-and-reducing-runtime-using-s3-read-optimization-b31da4b60fa0). + +With prefetching, the input stream divides the remote file into blocks of a fixed size, associates +buffers to these blocks and then reads data into these buffers asynchronously. +It also potentially caches these blocks. + +### Basic Concepts + +* **Remote File**: A binary blob of data stored on some storage device. +* **Block File**: Local file containing a block of the remote file. +* **Block**: A file is divided into a number of blocks. +The size of the first n-1 blocks is same, and the size of the last block may be same or smaller. +* **Block based reading**: The granularity of read is one block. +That is, either an entire block is read and returned or none at all. +Multiple blocks may be read in parallel. + +### Configuring the stream + +|Property |Meaning |Default | +|--- |--- |--- | +|`fs.s3a.prefetch.enabled` |Enable the prefetch input stream |`true` | +|`fs.s3a.prefetch.block.size` |Size of a block |`8M` | +|`fs.s3a.prefetch.block.count` |Number of blocks to prefetch |`8` | + +### Key Components + +`S3PrefetchingInputStream` - When prefetching is enabled, S3AFileSystem will return an instance of +this class as the input stream. +Depending on the remote file size, it will either use +the `S3InMemoryInputStream` or the `S3CachingInputStream` as the underlying input stream. + +`S3InMemoryInputStream` - Underlying input stream used when the remote file size < configured block +size. +Will read the entire remote file into memory. + +`S3CachingInputStream` - Underlying input stream used when remote file size > configured block size. +Uses asynchronous prefetching of blocks and caching to improve performance. + +`BlockData` - Holds information about the blocks in a remote file, such as: + +* Number of blocks in the remote file +* Block size +* State of each block (initially all blocks have state *NOT_READY*). +Other states are: Queued, Ready, Cached. + +`BufferData` - Holds the buffer and additional information about it such as: + +* The block number this buffer is for +* State of the buffer (Unknown, Blank, Prefetching, Caching, Ready, Done). +Initial state of a buffer is blank. + +`CachingBlockManager` - Implements reading data into the buffer, prefetching and caching. + +`BufferPool` - Manages a fixed sized pool of buffers. +It’s used by `CachingBlockManager` to acquire buffers. + +`S3File` - Implements operations to interact with S3 such as opening and closing the input stream to +the remote file in S3. + +`S3Reader` - Implements reading from the stream opened by `S3File`. +Reads from this input stream in blocks of 64KB. + +`FilePosition` - Provides functionality related to tracking the position in the file. +Also gives access to the current buffer in use. + +`SingleFilePerBlockCache` - Responsible for caching blocks to the local file system. +Each cache block is stored on the local disk as a separate block file. + +### Operation + +#### S3InMemoryInputStream + +For a remote file with size 5MB, and block size = 8MB, since file size is less than the block size, +the `S3InMemoryInputStream` will be used. + +If the caller makes the following read calls: + +``` +in.read(buffer, 0, 3MB); +in.read(buffer, 0, 2MB); +``` + +When the first read is issued, there is no buffer in use yet. +The `S3InMemoryInputStream` gets the data in this remote file by calling the `ensureCurrentBuffer()` +method, which ensures that a buffer with data is available to be read from. + +The `ensureCurrentBuffer()` then: + +* Reads data into a buffer by calling `S3Reader.read(ByteBuffer buffer, long offset, int size)`. +* `S3Reader` uses `S3File` to open an input stream to the remote file in S3 by making + a `getObject()` request with range as `(0, filesize)`. +* The `S3Reader` reads the entire remote file into the provided buffer, and once reading is complete + closes the S3 stream and frees all underlying resources. +* Now the entire remote file is in a buffer, set this data in `FilePosition` so it can be accessed + by the input stream. + +The read operation now just gets the required bytes from the buffer in `FilePosition`. + +When the second read is issued, there is already a valid buffer which can be used. +Don’t do anything else, just read the required bytes from this buffer. + +#### S3CachingInputStream + +If there is a remote file with size 40MB and block size = 8MB, the `S3CachingInputStream` will be +used. + +##### Sequential Reads + +If the caller makes the following calls: + +``` +in.read(buffer, 0, 5MB) +in.read(buffer, 0, 8MB) +``` + +For the first read call, there is no valid buffer yet. +`ensureCurrentBuffer()` is called, and for the first `read()`, prefetch count is set as 1. + +The current block (block 0) is read synchronously, while the blocks to be prefetched (block 1) is +read asynchronously. + +The `CachingBlockManager` is responsible for getting buffers from the buffer pool and reading data +into them. This process of acquiring the buffer pool works as follows: + +* The buffer pool keeps a map of allocated buffers and a pool of available buffers. +The size of this pool is = prefetch block count + 1. +If the prefetch block count is 8, the buffer pool has a size of 9. +* If the pool is not yet at capacity, create a new buffer and add it to the pool. +* If it’s at capacity, check if any buffers with state = done can be released. Review Comment: I noticed Yetus formats these `’` weirdly. Can we switch to standard single quote where-ever `’` occurs? -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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