Parallel layout is slower than sequential on my Linux system

[mbrubeck]

Using release builds of Servo on my 4-core, 8-thread Linux laptop, time spent in layout is 25% to 100% longer in parallel mode than in sequential mode (-y 1), across a few different real-world pages (Wikipedia article, Google search results, MySpace page from tp5n). This doesn't appear to be a recent regression; Servo builds from up to a year ago have approximately the same performance in both parallel and sequential mode. The recent update to rayon 0.7.1 did not fix this problem.

One of the biggest deltas is "LayoutMain", which frequently takes 10x to 30x longer in parallel mode than in sequential mode.

Running the same benchmarks on my 4-core iMac running macOS, parallel layout is significantly faster than sequential layout, as expected.

System details:

• Hardware: Lenovo Thinkpad P50
• CPU: Intel(R) Xeon(R) CPU E3-1505M v5 @ 2.80GHz
• OS: Debian 9.0
• CPU governor: "performance"
• no other processes using significant CPU during benchmark runs

[heycam]

This reproduces on my P50 (i7-6820HQ @ 2.70GHz) on Linux Mint 18 with the performance governor.