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| author | Android Partner Docs <noreply@android.com> | 2017-08-22 10:41:24 -0700 |
|---|---|---|
| committer | Clay Murphy <claym@google.com> | 2017-08-22 15:01:44 -0700 |
| commit | f16c42333aa6b2de30a344dd68246d4a33d93e7d (patch) | |
| tree | 311af599312cacb21c888aeae828cae59b0d64a1 /en/devices/architecture/kernel/squashfs.html | |
| parent | 04426e67ca3ee557a0083f9b3c6ba789021bd7a0 (diff) | |
| download | source.android.com-f16c42333aa6b2de30a344dd68246d4a33d93e7d.tar.gz | |
Docs: Changes to source.android.com
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PiperOrigin-RevId: 166080694
Change-Id: I2d3a8d77fa6a66c2099f13ba2e864545328fd17a
Diffstat (limited to 'en/devices/architecture/kernel/squashfs.html')
| -rw-r--r-- | en/devices/architecture/kernel/squashfs.html | 114 |
1 files changed, 114 insertions, 0 deletions
diff --git a/en/devices/architecture/kernel/squashfs.html b/en/devices/architecture/kernel/squashfs.html new file mode 100644 index 00000000..93b97ad0 --- /dev/null +++ b/en/devices/architecture/kernel/squashfs.html @@ -0,0 +1,114 @@ +<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> |