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| author | Gina Dimino <gdimino@google.com> | 2021-07-15 23:27:11 +0000 |
|---|---|---|
| committer | Gerrit Code Review <noreply-gerritcodereview@google.com> | 2021-07-15 23:27:11 +0000 |
| commit | 72e0ae8d8207042452f75874221103f4f6b3014a (patch) | |
| tree | 7fa97b880b85be7de5b713d233cdc407c20b88cd /en/devices/architecture/kernel/squashfs.html | |
| parent | e55b3193405187e091a4ac7730ef04360ec04504 (diff) | |
| parent | fc5fc0e74df003b0ee454d3418b88cd722282c49 (diff) | |
| download | source.android.com-master.tar.gz | |
Diffstat (limited to 'en/devices/architecture/kernel/squashfs.html')
| -rw-r--r-- | en/devices/architecture/kernel/squashfs.html | 114 |
1 files changed, 0 insertions, 114 deletions
diff --git a/en/devices/architecture/kernel/squashfs.html b/en/devices/architecture/kernel/squashfs.html deleted file mode 100644 index 93b97ad0..00000000 --- a/en/devices/architecture/kernel/squashfs.html +++ /dev/null @@ -1,114 +0,0 @@ -<html devsite> - <head> - <title>Optimizing SquashFS at the Kernel Level</title> - <meta name="project_path" value="/_project.yaml" /> - <meta name="book_path" value="/_book.yaml" /> - </head> - <body> - <!-- - Copyright 2017 The Android Open Source Project - - 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. - --> - - <p> - SquashFS is a compressed read-only filesystem for Linux. The file system is - read-only by design and thus suitable for use on the system partition. Many - Android devices may benefit from using this file system for their system - partition, for example, the following: - </p><ul> - <li>Devices with a low capacity storage such as Android Watch. - <li>Devices with a slow flash storage (compression reduces the number of block - I/Os).</li></ul> - <p> - Unfortunately the performance of SquashFS lags behind ext4. - </p> - <h2 id="optimizations">Optimizations</h2> - <p> - The following optimizations have been implemented to improve the performance of - SquashFS. - </p> - <h3 id="reduce-the-memory-usage-and-memcpy">Reduce the memory usage and - memcpy</h3> - <p> - When reading a block (default 128K), SquashFS tries to grab all the pages - covering this block. - </p> - <p> - If a single page is up-to-date or locked, it falls back to allocating a full - block, submitting a read request, and then copying its content to the pages. - </p> - <p> - This approach is very ineffective; after some time the page cache is likely to - contain pages that are up-to-date even if the adjacent pages are not. - </p> - <p> - The code is now able to handle blocks with holes (=missing pages). This - improves performance in the following ways: - </p><ul> - <li>Reduces the number of <code>memcpy</code> calls - <li>Decreases memory allocations</li></ul> - <h3 id="asynchronous-reads">Asynchronous reads</h3> - <p> - SquashFS still uses the deprecated <code>ll_rw_block()</code> function. There - are two problems with this approach: - </p><ul> - <li>As the name implies, the function waits for the reads to complete before - returning. This is redundant since <code>.readpage()</code> already waits on the - page's lock. Moreover, we need an asynchronous mechanism to efficiently - implement <code>.readpages()</code>. - <li>Merging the read requests entirely depends on the I/O scheduler. - <code>ll_rw_block()</code> simply creates one request per buffer. SquashFS has - more information than the I/O scheduler about what should be merged. Moreover, - merging the request means that we rely less on the I/O scheduler.</li></ul> - <p> - For these reasons, the <code>ll_rw_block()</code> function has been replaced - with <code>submit_bio()</code>. - </p> - <h3 id="readpages-prefetching">Readpages (prefetching)</h3> - <p> - SquashFS does not implement <code>.readpages()</code>, so the kernel repeatedly - calls <code>.readpage()</code>. - </p> - <p> - Now that our read requests are asynchronous, the kernel can truly prefetch pages - using its asynchronous read-ahead mechanism. - </p> - <h3 id="optimize-reading-uncompressed-blocks">Optimize reading uncompressed - blocks</h3> - <p> - Modern systems such as Android contain a lot of files that are already - compressed. As a consequence, the image contains a lot of blocks that can't be - compressed. - </p> - <p> - SquashFS handles compressed and uncompressed blocks using the same logic: when - asked to read a single page, it actually reads a full block (default 128k). - While this is necessary for compressed blocks, it is just a waste of resources - for uncompressed blocks. - </p> - <p> - Instead of reading a full block, SquashFS now just reads what is advised by the - readahead algorithm. - </p> - <p> - This greatly improves the performance of random reads. - </p> - <h2 id="code">Code</h2> - <p> - SquashFS code optimizations are available in AOSP: - </p><ul> - <li><a - href="https://android-review.googlesource.com/#/q/topic:squashfs">https://android-review.googlesource.com/#/q/topic:squashfs</a></li></ul> - </body> -</html> |