Jank Busters Part One

Jank, or in other words visible stutters, can be noticed when Chrome fails to render a frame within 16.66ms (disrupting 60 frames per second motion). As of today most of the V8 garbage collection work is performed on the main rendering thread, c.f. Figure 1, often resulting in jank when too many objects need to be maintained. Eliminating jank has always been a high priority for the V8 team [1, 2, 3]. In this blog post we will discuss a few optimizations that were implemented between M41 and M46 which significantly reduce garbage collection pauses resulting in better user experience.

Figure 1: Garbage collection performed on the main thread.

A major source of jank during garbage collection is processing various bookkeeping data structures. Many of these data structures enable optimizations that are unrelated to garbage collection. Two examples are the list of all ArrayBuffers, and each ArrayBuffer’s list of views. These lists allow for an efficient implementation of the DetachArrayBuffer operation without imposing any performance hit on access to an ArrayBuffer view. In situations, however, where a web page creates millions of ArrayBuffers, (e.g., WebGL-based games), updating those lists during garbage collection causes significant jank. In M46, we removed these lists and instead detect detached buffers by inserting checks before every load and store to ArrayBuffers. This amortizes the cost of walking the big bookkeeping list during GC by spreading it throughout program execution resulting in less jank. Although the per-access checks can theoretically slow down the throughput of programs that heavily use ArrayBuffers, in practice V8's optimizing compiler can often remove redundant checks and hoist remaining checks out of loops, resulting in a much smoother execution profile with little or no overall performance penalty.

Another source of jank is the bookkeeping associated with tracking the lifetimes of objects shared between Chrome and V8. Although the Chrome and V8 memory heaps are distinct, they must be synchronized for certain objects, like DOM nodes, that are implemented in Chrome's C++ code but accessible from JavaScript. V8 creates an opaque data type called a handle that allows Chrome to manipulate a V8 heap object without knowing any of the details of the implementation. The object's lifetime is bound to the handle: as long as Chrome keeps the handle around, V8's garbage collector won't throw away the object. V8 creates an internal data structure called a global reference for each handle it passes back out to Chrome through the V8 API, and these global references are what tell V8's garbage collector that the object is still alive. For WebGL games, Chrome may create millions of such handles, and V8, in turn, needs to create the corresponding global references to manage their lifecycle. Processing these huge amounts of global references in the main garbage collection pause is observable as jank. Fortunately, objects communicated to WebGL are often just passed along and never actually modified, enabling simple static escape analysis. In essence, for WebGL functions that are known to usually take small arrays as parameters the underlying data is copied on the stack, making a global reference obsolete. The result of such a mixed approach is a reduction of pause time by up to 50% for rendering-heavy WebGL games.

Most of V8’s garbage collection is performed on the main rendering thread. Moving garbage collection operations to concurrent threads reduces the waiting time for the garbage collector and further reduces jank. This is an inherently complicated task since the main JavaScript application and the garbage collector may simultaneous observe and modify the same objects. Until now, concurrency was limited to sweeping the old generation of the regular object JS heap. Recently, we also implemented concurrent sweeping of the code and map space of the V8 heap. Additionally, we implemented concurrent unmapping of unused pages to reduce the work that has to be performed on the main thread, c.f. Figure 2.

Figure 2: Some garbage collection operations performed on the concurrent garbage collection threads.

The impact of the discussed optimizations is clearly visible in WebGL-based games, for example Turbolenz' Oort Online demo. The following video compares Chrome M41 to M46


We are currently in the process of making more garbage collection components incremental, concurrent, and parallel, to shrink garbage collection pause times on the main thread even further. Stay tuned as we have some interesting patches in the pipeline.

Posted by the jank busters: Jochen Eisinger, Michael Lippautz, and Hannes Payer
2:36:00 AM

World Top 10 September 2015 (Updated)



Models - Record Edition

Thanks mostly to another outstanding performance of the chinese market, more than 47.000 plug-ins were sold last month, which not only represents a 13% increase over the last record (42k last June), but also a significant 52% increase regarding the same month last year, pulling the YTD sales to 334k units, meaning a 54% increase over the same period in 2014, if sales keep up this pace, we should see sales reach some 500.000 units. Yep, half a million EV's in one year, this year.

And to think sales in 2012 were at around 150.000...

Thanks to the UK and its domestic market, the Outlander PHEV (4.829 units) managed to get its second Monthly Best Seller Month in three months, on the other hand, the once dominant Nissan Leaf hasn't reached the highest place in podium since May.

This brilliant performance from the Mitsu Outlander PHEV allowed it to recover the #3 from the BYD Qin, which has seen its sales dive to just 2.115 units, its worst performance since February, that drop has more to do with BYD preferring to make the higher margin Tang SUV (3.044 units, already #6 in the September ranking), than to lower Qin demand. 

Speaking of models with outstanding performances outside the YTD Top 10, better even than the Tang performance was the sales surge of the Kandi Panda EV, shooting to #5 in September, thanks to a record 3.301 units registered.

Looking at the rest of the YTD ranking, the other notable change is the Golf GTE dropping another position to #10 (It was #8 in July), changing positions with the BAIC E-Series EV.

A final word on the BEV / PHEV breakdown, currently pure electrics are winning 60% / 40%, with plug-in hybrids recovering 2% share regarding last year.


PlWORLD Sep. YTD%'14Pl
1Nissan Leaf4.257 35.948111
2Tesla Model S4.65432.995103
3Mitsubishi Outlander PHEV4.82928.79192
4
5
6
BYD Qin
BMW i3
Renault Zoe
2.115
3.361
1.665
26.156
18.040
11.782
8
5
4
7
6
9
7Volkswagen e-Golf1.10311.220321
8Chevrolet Volt (1)1.11110.61234
9BAIC E-Series EV2.10010.344316
10Volkswagen Golf GTE1.19710.231333

TOTAL 47.598
334.500



Manufacturers -  Hot Race for Third Spot

This was the fifth consecutive month that BYD won the Best Selling Manufacturer title, again with a new personal record (6.013 units), the chinese carmaker is closing in to the leader Nissan, having recovered 1.433 units in September, there are now only 1.916 units separating them, so if all continues without major changes, BYD should surpass Nissan and become new Best Selling Manufacturer in November. 

In the race for #3, Tesla is still ahead, but with only 500 units separating it from Mitsubishi, a lot can still happen until the end of the year.

In the low end of the manufacturers ranking, things are pretty leveled, with only 96 sales separating the 12.924 units of the #9 Kandi (Back to the Top 10 after a nine months absence), from the 12.828 units of Chevrolet (#10) and 149 units ahead of the #11 Zotye (12.775), with the #12 BAIC also not that far off (10.533).


PlWORLDSep.YTD%'14Pl
1Nissan4.58038.888121
2BYD6.01336.972117
3Tesla4.66033.001103
4Mitsubishi5.32232.501102
5
6
7
Volkswagen
BMW
Renault 
2.596
3.843
2.428
23.932
22.263
17.568
7
7
5
11
9
8
8Ford1.79915.33855
9Kandi4.05112.924410
10Chevrolet1.273 12.82846





(1) - Includes Holden Volt, Opel and Vauxhall Ampera;


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