website: Add a gem5-20 release post
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Change-Id: I3d8164ed54f4a42192804bf45857eb9eccfad474
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/29312
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
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+---
+layout: post
+title: "gem5-20.0 Released!"
+author: Jason Lowe-Power
+date: 2020-05-21
+categories: project
+---
+
+After much waiting, and a few surprising issues, [gem5-20.0](https://gem5.googlesource.com/public/gem5/+/refs/heads/master) has been released!
+The [master](https://gem5.googlesource.com/public/gem5/+/refs/heads/master) branch of the gem5 repo now points to the gem5-20 release instead of the gem5-19 release.
+
+Thank you to everyone that made this release possible!
+This has been a very productive release with [84 issues](https://gem5.atlassian.net/), about 500 commits, and 30 unique contributors.
+
+[Below](#changelog), I go over the major changes in gem5-20.0 since gem5-19, which can also be found in the [`RELEASE-NOTES.md` file](https://gem5.googlesource.com/public/gem5/+/master/RELEASE-NOTES.md).
+This should cover all of the *major* changes and user-facing changes.
+The hope is that after reading the changelog, you can make updates to your local gem5 changes and then things will *just work*.
+I'm sure this *isn't* going to be that clean, but we can hope.
+
+## Leaderboard
+
+First, I want to look at a quick leaderboard of the most prolific contributors for this release.
+Going forward, I'd like for these leaderboards to be living and automatically updated on the website.
+If you're interested in helping out on this, let me know!
+
+### Commits
+
+There was a total of 510 commits between the v19.0.0.0 tag and the v20.0.0.0 tag.
+gem5 v19.0.0.0 was tagged on February 20th, so this release was over 91 days or over 5.5 commits per day!
+This was quite a breakneck pace!
+
+Having more commits doesn't necessarily mean that you've contributed more.
+However, being on this list of the top 10 committers does mean that you've put a significant amount of your time into gem5.
+For that, the community thanks you!
+
+```
+ 235 Gabe Black
+ 66 Giacomo Travaglini
+ 46 Bobby R. Bruce
+ 24 Nils Asmussen
+ 21 Adrian Herrera
+ 21 Daniel Carvalho
+ 19 Nikos Nikoleris
+ 14 Matthew Poremba
+ 12 Timothy Hayes
+ 9 Ciro Santilli
+```
+
+### Reviewers
+
+Possibly more important than raw commits are the people who take time out of their days to review code.
+Here are the top 10 reviewers for this commit.
+Again, thank you for your work!
+The number of bugs that reviewers find is incredible, and without these people gem5's code quality would suffer.
+
+While there were 510 commits, there were 642 reviews for an average of 1.25 reviews per change.
+The way we count reviews isn't perfect as it misses "re-reviews" and also skips "Maintainer" reviews.
+However, I would like to see this review ratio to be closer to 2 reviews per changeset in future releases.
+
+```
+ 162 Jason Lowe-Power
+ 120 Bobby R. Bruce
+ 76 Nikos Nikoleris
+ 58 Gabe Black
+ 51 Daniel Carvalho
+ 49 Giacomo Travaglini
+ 44 Ciro Santilli
+ 13 Matthew Poremba
+ 10 Chun-Chen Hsu
+ 6 Ayaz Akram
+ 6 Andreas Sandberg
+```
+
+I want to call special attention to this list as there are a number of people who do more reviewing than writing code.
+This is *incredibly* helpful to the community, and we appreciate your contributions even if they aren't adding code!
+
+## Lessons learned
+
+The gem5 community is committed to continually improving its processes based
+on best-practices and feedback from developers and users. In releasing
+gem5 20, our second official release of gem5, we have learned some lessons which
+which we shall take into account when developing future versions of gem5.
+
+The gem5 project abides by a gitflow-inspired development setup where our
+"master" branch contains the latest release of gem5 at its HEAD, with a
+separate "develop" branch used for day-to-day gem5 development. Upon a release
+we create a staging branch from develop. This staging branch is then tested,
+and fixed if necessary, during which time development continues on the develop
+branch. Once all major tests are shown to pass, and no additional bugs can be
+found, the staging branch merged into both the master and develop branches,
+then subsequently deleted, thus completing the release. We have found, by and
+large, this policy works in our favor, though there is room for improvement.
+
+For gem5 20, we had a policy of giving users exactly two weeks
+notice before creating the staging branch, with an additional two weeks given
+to test and fix errors on the staging branch. Our experience
+has revealed the two week notice prior to creating
+the staging was good at focusing the minds of developers, prompting
+prioritization of desired features for the release, and, overall, improving the
+churn of patchsets reviewed through our Gerrit code-review system. However, two
+weeks is, in hindsight, too short a notice for many, and resulted in a large
+Gerrit backlog from those wishing their features to be incorporated into
+gem5 20. In future releases, **we are going to give a full month notice prior
+to creating the staging branch**. Furthermore, we created the staging branch on
+a Friday, with the goal of merging the staging branch into the master and
+develop branches exactly two weeks later (another Friday). We found this led to
+long weeks of hectic changes, fatigue, and bad moral. Friday releases also have
+the daunting prospect of work leaking into the weekend, or complaints that
+aren't resolved until the following Monday. Going forward, **we're going to
+commit to mid-week releases**.
+
+We have also came to realize that our policy of the staging branch existing
+for two weeks is needlessly restrictive. Given only testing, bug fixes, and
+critical last-minute changes are permitted to be incorporated into the
+branch, we see no reason as to why the staging branch should not exist until
+its stability and reliability is sufficiently proven. In future releases, **the
+gem5 staging branch shall exist for as long as it takes for tests to run, and
+bugs exposed by these tests to be fixed**. As development can continue on the
+develop branch, this longer-lived staging branch shall not interfere with other
+gem5 activities.
+
+In testing on our staging branch, we found some bugs exposed by tests we run
+irregularly. In particular, and to our surprise, a simple check of building
+different gem5 ISAs with the set of compilers we officially support revealed
+many compilation errors. **We will therefore aim to regularly test the
+supported compilers** during the development of gem5. We will also **continue
+to improve our testing procedures to run more tests more often**, in particular
+by starting regular nightly builds where larger tests and build processes may
+be run. Finding bugs earlier, as they appear in our development process, will
+save us a lot of work compared to finding and fixing bugs later in the staging
+branch.
+
+Finally, gem5 20 officially supports Python3, as well as being backwards
+compatible with Python2. We have learned that supporting both Python3 and
+Python2 is a laborious task, more so than we had anticipated. Significant
+resources have been deployed to ensure gem5 20 functions with Python3 while still
+functioning for those who continue to use, the now retired, Python2. This level
+of support, however, cannot be maintained indefinitely. **We are therefore keen
+to accelerate our dropping of Python2 support in future releases of gem5**.
+
+We welcome any other feedback on how to improve this process.
+Please feel free to let us know on the [mailing list](http://www.gem5.org/mailing_lists/) or by [opening an issue](https://gem5.atlassian.net/).
+
+## Changelog
+
+### New features
+
+* [gem5-resources repository](https://gem5.googlesource.com/public/gem5-resources/)
+ * This new repository will store all of the *sources* (e.g., code) used to create testing and research resources. This includes disk images, testing binaries, kernel binaries, etc.
+ * Binaries created with the sources are hosted on dist.gem5.org.
+ * Details on the new page for resources: <http://www.gem5.org/documentation/general_docs/gem5_resources>.
+* Memory SimObjects can now be initialized using an image file using the image_file parameter.
+* **[USER-FACING CHANGE]** The m5 utility has been revamped with a new build system based on scons, tests, and updated and more consistent feature support.
+ * To build, now use `scons build/<arch>/out/m5`, not `make`.
+ * [Documentation](http://www.gem5.org/documentation/general_docs/m5ops/) coming soon.
+* Robust support for marshalling data from a function call inside the simulation to a function within gem5 using a predefined set of rules.
+ * Developers can specify an ABI for guest<->simulator calls and then "just call functions".
+ * Unifies pseudo-inst, syscall, and other support.
+ * Code within gem5 has been updated. However, users which added new pseudo-ops may have to update their code.
+* **[PYTHON API CHANGE]** Workload configuration pulled out into its own object, simplifying the System object and making workload configuration more modular and flexible.
+ * All full system config/run scripts must be updated (e.g., anything that used the `LinuxX86System` or similar SimObject).
+ * Many of the parameters of `System` are now parameters of the `Workload` (see `src/sim/Workload.py`).
+ * For instance, many parameters of `LinuxX86System` are now part of `X86FsLinux` which is now the `workload` parameter of the `System` SimObject.
+ * See https://gem5-review.googlesource.com/c/public/gem5/+/24283/ and https://gem5-review.googlesource.com/c/public/gem5/+/26466 for more details.
+* Sv39 paging has been added to the RISC-V ISA, bringing gem5 close to running Linux on RISC-V.
+ * (Some) Baremetal OSes are now supported.
+* Improvements to DRAM model:
+ * Added support for verifying available command bandwidth.
+ * Added support for multi-cycle commands.
+ * Added new timing parameters.
+ * Added ability to interleave bursts.
+ * Added LPDDR5 configurations.
+
+### Removed features
+
+* Support for the ALPHA ISA has been dropped.
+ * All ALPHA ISA code has been removed
+ * Old "rcS" scripts for ALPHA have been removed
+
+### New supported platforms
+
+* Compiling and running gem5 with Python 3 is now fully supported.
+ * Lots of code changes required for this.
+ * There may still be some python code that's not up to date. Please open a [Jira ticket](https://gem5.atlassian.net/) if you find any code that doesn't work with python3.
+* gem5 now supports Ubuntu 20.04.
+* Compiling gem5 with GCC 8 and 9 is now supported.
+* Compiling with clang up to version 9 is now supported.
+
+### Testing improvements
+
+* Scons-based tests have been migrated to the testlib framework.
+ * Tests can now be run with `tests/main.py`, except for the unittests.
+ * Please consult TESTING.md for more information on how these may be run.
+* We are continuing to work on CI tests. Most of the plumbing is there for Google Cloud Build integration. See [the Jira issue](https://gem5.atlassian.net/browse/GEM5-237) for details.
+
+### Other API changes
+
+* **[API CHANGE]** Ruby's prefetcher renamed to RubyPrefetcher.
+ * Any SLICC protocols with prefetchers need to be updated.
+ * Some config scripts for Ruby protocols with prefetchers may need to be updated.
+* **[API CHANGE]** SE mode improvements.
+ * Better support for the mmap and related syscalls.
+ * A new virtual memory area API for tracking SE mode allocations.
+ * When implementing syscalls, the way that guest memory is allocated changes. All code in gem5 is updated, but if there are any external syscalls, they may need be updated.
+* **[COMMAND LINE CHANGE]** The `--disk-image` argument to `fs.py` is now optional.
+ * However, the disk image names *are no longer implied*.
+ * The script still implicitly searches `M5_PATH`, but the name of the disk image must be specified.
+* **[API CHANGE]** SLICC `queueMemory` is now `enqueue`.
+ * All protocol configs must be updated with another message buffer in the memory controllers (directories).
+ * All protocol SLICC files must replace `queueMemoryRead` and `queueMemoryWrite` with `enqueue` to another "special" message buffer named `memQueue`.
+ * This allows finite buffering between the cache controllers and DRAMCtrl.
+* **[API CHANGE]** Added Prefetcher namespace
+ * All prefetchers' names have changed from `*Prefetcher` to `Prefetcher::*`
+ * If you have any prefetchers that are not in the gem5 mainline, your code will likely need to be updated.
+
+### Other changes
+
+* Implemented ARMv8.3-CompNum, SIMD complex number extension.
+* Support for Arm Trusted Firmware + u-boot with the new VExpress_GEM5_Foundation platform
+* Removed author list from source files.
+ * This was originally so future people would know who to contact.
+ * However, it was difficult to maintain and quickly out of date.
+ * Copyright is unchanged.
+* Improvements to gem5's power model.
+* MESI_Three_Level Ruby protocol bugfixes.
+* Ruby functional reads now work in more cases.
+* GCN3 ISA added.
+* Indirect branch stats work correctly now.
