Update MEP doc and add MEP template

.github/ISSUE_TEMPLATE/mmtk-enhancememnt-proposal--mep-.md

@@ -0,0 +1,125 @@
+---
+name: MMTk Enhancememnt Proposal (MEP)
+about: Use this template to create an MMTk Enhancement Proposal (MEP).
+title: ''
+labels: MEP
+assignees: ''
+
+---
+
+# TL;DR
+
+This section should use about one to three sentences to summarize the MEP.  As the name "TL;DR" (too
+long, didn't read) suggests, this section should be short enough so that readers (including those in
+a hurry) can get the main idea very quickly without reading through the MEP.
+
+# Goal
+
+What are the goals of the proposal?  This should be succinct.  If there's more than one goal,
+enumerate them in a list.
+
+-   goal 1
+-   goal 2
+-   ...
+
+# Non-goal
+
+Optional.  Use this section to explicitly exclusive goals out of the scope of the current MEP.
+
+-   non-goal 1
+-   non-goal 2
+-   ...
+
+# Success Metric
+
+How do we determine whether the MEP is a success?  This can include improvements in performance,
+safety, readability, maintainability, etc.  Enumerate the success metrics in a list (details should
+be in the *Description* section).
+
+# Motivation
+
+Why should this work be done?  Who is asking for it?
+
+Make sure the readers understand the problem this MEP is trying to address.  You can also mention
+the languages, VMs, or users that need this enhancement.
+
+If there are alternative ways to solve the problem, you can mention them here, but keep in mind that
+there is an *Alternatives* section for adding more details.
+
+# Description
+
+This is the main section of the MEP.  Describe the enhancement in detail.
+
+If you have already made prototype implementations for this MEP, add hyperlinks to the relevant PRs,
+commits, repositories, etc.
+
+If not, describe how you intend to implement this MEP.  You should think about all parts of
+mmtk-core that your MEP may interact with.
+
+This section will be long.  Feel free to add more subsections.
+
+## Impact on Performance
+
+Describe how this MEP will affect the performance.  "This MEP should not have any impact on performance" is still a valid description if it is so.
+
+## Impact on Software Engineering
+
+Describe whether this MEP will make software engineering easier or more difficult.  Will it make the code easier or harder to understand, maintain and/or change?
+
+# Risks
+
+In the following sub-sections, outline the *long-term* risks posed by this MEP and how those risks are mitigated.   **The core
+purpose of the MEP process is to avoid the unintentional introduction of changes that bring
+long-term negative impacts to MMTk**. This section is about identifying and accounting for risks
+associated with such negative outcomes.  It should include the following subsections:
+
+## Long Term Performance Risks
+
+Enumerate possible negative long-term performance impacts of this MEP, and for each explain how that
+risk will be mitigated.    Note: this is *not* about the immediate performance impact of the MEP,
+but about the impact on future work.  For example, this includes identifying changes that may impede
+the future introduction of entirely new algorithms, or entirely new optimizations.
+
+It is OK for us to accept temporary minor performance reduction to make more significant
+improvements possible.  On the contrary, it is bad to make changes to temporarily improve
+performance and make long-term improvements hard or impossible.
+
+## Long Term Software Engineering Risks
+
+Enumerate possible negative long-term software engineering impacts of this MEP, and for each explain
+how that risk will be mitigated.
+
+One of the core goals of MMTk is making GC development easy.  If a developer wants to develop, for
+example, a new GC algorithm, it should be easy to implement it quickly and easily in MMTk.  We don't
+want changes that make this difficult.
+
+## Impact on API
+
+Outline how this MEP will affect APIs, both public and internal.   Enumerate the cases, and for each
+case, explain how the risk of negative consequences will be mitigated and/or justify the change.
+
+# Testing
+
+If applicable, describe the way to reproduce the problem, and the way to check if the change
+actually works.  For MMTk, it includes but is not limited to what (micro or macro) benchmarks to
+use, and which VM binding (with or without changes) to use.
+
+# Alternatives
+
+Optional.  If there are more than one way to solve the problem, describe them here and explain why
+this approach is preferred.
+
+# Assumptions
+
+Optional.  For the design changes of MMTk, this part mainly includes assumptions about, for example,
+the VM's requirements, the GC algorithms we are supporting, the OS/architecture MMTK is running on,
+etc.  If those assumptions no longer hold, we may need to reconsider the design.  Describe such
+concerns in this section.
+
+# Related Issues
+
+Optional.  If there are related issues, PRs, etc., include them here.
+
+Sometimes people create ordinary issues and PRs to fix some problems, and later MMTk developers
+consider that the change is too fundamental and needs a more thorough reviewing process.  If that is
+the case, add hyperlinks to those original issues and PRs.

docs/contribute/mep.md

@@ -1,12 +1,12 @@
 MMTk Enhancement Proposal (MEP)
 ===============================
 
-An MMTk Enhancement Proposal (MEP) is a more formal variant of issue.  It has a special format, and
-will undergo a more thorough review process.  Its goal is helping the MMTk developers making more
-informed decisions.
+An MMTk Enhancement Proposal (MEP) is a formal process for the MMTk team and its developers to
+propose significant design changes, review the impact of the changes and make informed decisions.
+It has a special format, and will undergo a more thorough review process.  Its goal is helping the
+MMTk developers making more informed decisions.
 
-MEP is inspired by the Java Enhancement Proposal, described in https://openjdk.org/jeps/1  However,
-unlike JEP which is more open-ended, MEP is more focused on making design decisions.
+MEP is inspired by the Java Enhancement Proposal, described in https://openjdk.org/jeps/1
 
 # When is MEP required?
 
@@ -18,39 +18,23 @@ applicable to any kind of significant changes, including but not limited to:
 -   Changes to the MMTk core to implement a feature demanded by bindings.
 -   Major refactoring to the MMTk core.
 
-One purpose of MEP is reducing the risks to the future development of MMTk.  Large-scale changes and
-public API changes usually indicate such risks, but these are only indicators, not criteria.  The
-assessment of risks is subjective, and we need to discuss in order to reach consensus.
+**The core purpose of the MEP process is to avoid the unintentional introduction of changes that
+bring long-term negative impacts to MMTk.**  Large-scale changes and public API changes usually
+indicate such risks, but these are only indicators, not criteria.  The assessment of risks is mostly
+subjective, and the MMTk team need to discuss in order to reach consensus.
 
-Note: JEP is also required for things that "require two or more weeks of engineering effort" and/or
-"are in high demand by developers or customers".  We don't judge whether we need MEP based on
-engineering effort or public demand.  Many PRs for MMTk require multiple weeks of work and rigorous
-testing, but most of them can be settled with regular issues and PRs.  We label priorities using
-GitHub issue tags, such as `P-normal`, `P-high`, etc.  If a feature is requested often, and we have
-man power for that, we can raise the priority.
+If a contributor is uncertain if they should submit an MEP for their proposed changes, we encourage
+them to talk with the MMTk team first, or to simply submit a normal PR/issue to get it started. An
+MEP would be requested by the MMTk team if necessary (See the details about this in the section of
+MEP review process).
 
 # Format
 
-A MEP will be posted as a GitHub issue in the `mmtk-core` repository.  It should contain certain
-tags:
-
--   **The `MEP` tag**
-    -   It is used to identify MEPs.
--   **Area (`A-*`)**
-    -   For example, `A-gc-algorithm`.
--   **Category (`C-*`)**
-    -   For example, `C-refactoring`.
-    -   Note that not all MEP are "enhancement" in the sense of `C-enhancement`.  Some MEPs may
-        simply be intended for fixing long-standing hard-to-fix bugs by making non-trivial changes.
--   **Goal (`G-*`)**
-    -   For example, `G-safety`.
-
-We use the format of JEP (https://openjdk.org/jeps/2) as a frame of reference, but deviate from it
-when needed.
+A MEP will be posted as a GitHub issue in the `mmtk-core` repository.  It should contain `MEP` tag.
 
 A MEP should have the following sections:
 
--   TL;DR (summary)
+-   TL;DR
 -   Goal
 -   Non-goal (optional)
 -   Success Metric
@@ -67,18 +51,13 @@ A MEP should have the following sections:
 -   Risks and Assumptions (optional)
 -   Related Issues (optional)
 
-Note: Sections in JEP but not in MEP:
-
--   Dependencies: We have the *Related Issues* section, instead.
-
 # Sections
 
 ## TL;DR
 
-This section should use about one to three sentences to summarize the MEP.  JEP calls it "Summary",
-but we call it "TL;DR" (too long, didn't read) to emphasize that it should be short enough so that
-readers (including those in a hurry) can get the main idea very quickly without reading through the
-MEP.
+This section should use about one to three sentences to summarize the MEP.  As the name "TL;DR" (too
+long, didn't read) suggests, this section should be short enough so that readers (including those in
+a hurry) can get the main idea very quickly without reading through the MEP.
 
 ## Goals