Enhance GC slides

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diff --git a/website/content/slides/10_vm_gc/_index.md b/website/content/slides/10_vm_gc/_index.md
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@@ -16,6 +16,83 @@ outputs: ["Reveal"]
 5. Optimizations
 
 ---
+## Theory reminders
+
+1. [Theory](https://www.slideshare.net/eelcovisser/garbage-collection-69688448)
+0. [Barriers](https://www.cs.kent.ac.uk/pubs/2010/3011/content.pdf)
+0. [Lua GC](http://www.inf.puc-rio.br/~roberto/talks/gc-lua.pdf)
+0. [V8 GC](http://jayconrod.com/posts/55/a-tour-of-v8-garbage-collection)
+0. [Generational GC](https://www.ps.uni-saarland.de/courses/gc-ws01/slides/generational_gc.pdf)
+0. [Java Z GC](http://cr.openjdk.java.net/~pliden/slides/ZGC-FOSDEM-2018.pdf)
+0. [LuaJIT GC](http://wiki.luajit.org/New-Garbage-Collector)
+
+---
+## Tri-colour marking
+
+In the core of concurrent and incremental garbage collectors
+
+- _white_ - not used, _dead_ object
+- _black_ - used, _alive_ object, whose references have been marked
+- _gray_ - used, _alive_ object, whose references have yet to be marked
+
+> references are the objects that are referenced by the current object
+
+---
+### Invariants
+
+- Weak:
+
+  > All white objects pointed to by a black object are reachable from some grey
+  > object through a chain of white objects.
+
+- Strong:
+
+  > There are no pointers from black objects to white objects.
+
+---
+### Escapes
+
+In the incremental or concurrent GC, while the marking phase is running, the
+mutator can change the references of a black object and break the above
+invariants.
+
+---
+### Barriers
+
+Barriers are code that gets executed by the mutator every time it mutates the
+heap. They will enforce that the invariants are being kept.
+
+- read
+- write
+  - deletion
+
+---
+#### Read Barrier
+
+Ensure the invariants by not allowing getting a _white_ object out of a _gray_
+one
+
+    def read(object, field):
+        if is_grey(object):
+            shade(object)
+        return object[field]
+
+Not used, since _read_ operations are more than _write_ operations in a program
+and having a slower _read_ is worse than having a slower _write_
+
+---
+#### Write Barrier
+
+Ensure the invariants by not allowing to set a _white_ object as a reference
+inside a _black_ object
+
+    def write(object, field, value):
+        object[field] = value
+        if is_black(object):
+            shade(value)
+
+
+---
 ## Disclaimer
 
 > This is how GC can be implemented. Everything is subject to change.
@@ -146,5 +223,6 @@ Where to store whether an object is *dead* or *alive*
 - `bit` per pointer
 
 
+
 ---
 # ?