1. Chrome DevTools
Rendering & Memory profiling
Open Academy 2013 Mate Nadasdi - Ustream, Inc

2. WHY DO WE NEED
A DEVTOOL?

3. • Javascript is an interpreted language
• HTML, CSS debugging have to happen in the browser
• Logging, debugging network requests is essential
• Source and the final output could be totally different
• Performance optimization is becoming increasingly important,
especially on mobile

4. Google is leading in new technologies
Native implementation and really fast support
Canary build deliver new features in really short intervals
Deep configuration possibilites
Early experimental access
Rendering / Memory Profiling tools
Why Google Chrome?

5. Red - Yellow - Blue?
Chrome channels
• Stable (Releases in every 6 weeks)
• Beta (1 month before stable, weekly releases)
• Dev (twice weekly)
• Canary (daily)
Chromium

6. Rendering

7. Parsing (HTML, XML, CSS, Javascript)
Layout
Text and graphics rendering
Image Decoding
GPU interaction
Network access
Hardware acceleration
Webkit / Blink rendering engine
Image Source: HTML5Rocks

8. From source to display - The ordinary way
HTML
HTML
Parser
Attachment
CSS ParserStylesheets
Layout
Shared bitmap
DOM tree
CSSOM tree
Render Object
tree
Render Layer
tree
n..1

9. Layout (Reflow) & Repaint
Layout (reflow):
Parts of the render tree needs to be revalidated and node dimensions should be
recalculated.
Repaint:
Some part of the screen needs to be updated because of a reflow, node geometric
change or style change.

10. Possible reflows & repaints
• DOM actions (Add, Remove, Update DOM nodes)
• Hide/show with display: none (reflow & repaint)
• Hide/show with visibility: hidden (repaint only because of no geometric change)
• Adding stylesheet dynamically
• scrolling, resizing the window
• DOM queries can cause a reflow too
• offsetTop/Left/Right/Height
• scrollTop/Left/Right/Height
• clientTop/Left/Right/Height
• getComputedStyle

11. Tips to minimize reflow & repaint
• Try to minimize layout cost with smaller subtree modification
• Detach DOM nodes before huge manipulation
• Do not change styles one by one (use classes instead)
• Group DOM read and write actions to let the browser optimize reflows for you
(DOM queries flush the browser’s queue of changes)
• Cache queried values, do not query them in every case you use it

12. Compositing, You Are Welcome!
• There are more GraphicContexts
• New tree in our forest, Graphic Layer tree
• Composited RenderLayers get their own backing surface
• Upload painted bitmaps to the GPU as textures
• 256x256 tiles
• Different thread for compositing
• Much cheaper then paint

13. Frames & Jank
• 60 Hz = We have got 16.66 ms only! ( 60 Hz = 1 / ~0,016)
• 60FPS is important, because 60Hz is average refresh rate of devices
• Jank: every time you can’t create a frame when your screen refreshes
• vSync - generating new frames only between screen refreshes
• JavaScript timers fails because of inaccuracy and different frame rates
• requestAnimationFrame is a good solution
20ms 20ms 20ms 20ms 20ms 20ms 20ms
Display refresh at 60 hz:
Our frame creation:

14. DEMO! Rock with timeline panel!
Tips:
• Show paint rectangles: Use it to show expensive paints
• Show composited layer borders: Check your GPU compositing layers easily
• Enable continuous page repainting: Easy to find the most expensive layers
• chrome://tracing/: Detailed tracing system to track core functionalities
• Use JavaScript CPU profile with Flame chart to diagnose your yellow pieces.

15. Memory profiling

16. Javascript memory basics
Root object
• Top of the memory management system
• We cannot manipulate
• References global variables
Object variable
• Can reference other variables
Scalar variable
• number, boolean, etc

17. Object sizes
Shallow size
• Memory held directly by the object
• It can be significant for arrays and strings
Retained size
• Indirect memory hold
• A size what will be freed if the object will be terminated
• For primitive types retained size equals shallow size

18. Object’s retaining tree
Root
A
G
C
D
B
H
D object’s retaining tree is a graph, where paths are retaining paths from GC root to A.

19. Garbage
Garbage:
Variables wich are unreachable from the GC root node
Garbage collection:
Finds all garbages and frees it’s memory to the system
Leak:
Object that still has retaining path accidently

20. Possible leaks
• Closures
• Deleting DOM nodes without removing event handlers
• DOM could hold event listeners to objects wich are inactive already
• Cycles: two objects reference to each other when they retain each other

21. DevTools workflow tips
1. Search for possible memory leaks with timeline panel
2. Use heap snapshots to capture object graph
3. Use all four views of the snapshot panel:
• Summary: An overview of our heap grouped by constructor name
• Comparison: Compare two snapshots
• Containment: Structured view of object graph
• Dominators: Find the most dominant points of the heap
4. Use the new Object Allocation Tracker in DevTools experiments
“Memory Lane with Gmail” talk about this new tool on Google IO 2013
https://developers.google.com/events/io/sessions/325547004

22. DEMO!
Tips:
• Check counters during your interaction on the examined page
• Use GC force button to see how it impacts on your memory
• Use it in incognito window, because extension allocated memory will be listed too
• Ignore:
• line in parentheses
• lines in light gray
• GC collects garbage before snapshot execution

23. Use Google Canary as it has all the features I have shown
and more including lots of profiling experiments
https://www.google.com/intl/en/chrome/browser/canary.html

24. Thank you!
@matenadasdi
Special thanks to Paul Irish and Addy Osmani for reviewing these slides.