Benchmarking Pre-Alpha x265
We got our hands on the early code and were able to run some preliminary numbers. Again, the developers say their code is still very early. It’s currently x86-only (albeit with support for advanced instruction sets like AVX2), lacks B-frame support (preventing it from achieving maximum compression), and doesn’t yet include some of the optimizations taken from x264 (like look-ahead and rate control). However, the features it does offer are good enough for some Core i7-4770K benchmarking as a precursor to what you might see in an upcoming processor review.
In the interest of transparency, we're using the following benchmark command: x265 --input Kimono1_1920x1080_24.yuv --width 1920 --height 1080 --rate 24 -f 240 -o q24_Kimono1.out --rect --max-merge 1 --hash 1 --wpp --gops 4 --tu-intra-depth 1 --tu-inter-depth 2 --no-tskip, with quantization parameters between 24 and 42. It's notable that we're employing GOP (Group Of Picture)-level parallelism to keep our quad-core -4770K busy. I'm also adding the --cpuid switch to control the instruction sets being used.
The first observation we’re able to make is that x265 currently sees the most benefit from optimizations related to SSE3 and then SSE4.1. Gains attributable to AVX and AVX2 are still relatively small. According to the lead developer, the one part of the encoder utilizing AVX2, motion search, isn’t particularly performance-critical.
The x265 team is hoping to achieve real-time 1080p30 encoding on a Xeon-based server with 16 physical cores by next month. At this early stage, however, we’re still under 4 FPS using a quad-core -4770K.
Performance scales up as the quantization parameter changes, affecting quality. Naturally, as you approach the maximum QP of 51 (we’re only testing up to 42), bit rates drop and the encoder’s speed increases.
Although QP changes affect performance and encode time, the bit rate does not vary, with one exception. For some reason, we observed a very slightly higher rate with AVX2 enabled, which is why you see the red line peeking out above the other overlapped results.
You can draw conclusions about video quality two ways: objectively, using mathematical models, or subjectively, by looking at two clips and creating your own evaluation. PSNR, or peak signal-to-noise-ratio, is the most commonly-used objective technique describing the ratio between the reference image (in our case a raw YUV file) and error introduced by compressing it. This isn’t a perfect representation of quality, but, in general, a higher PSNR corresponds to an output more representative of the original.
Again, we see consistency across each instruction set, except for AVX2, which dips a bit at the highest QPs.