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Video Decoding

I’ve had a saga of getting 4K monitors to work well. My latest issue has been video playing, the dreaded mplayer error about the system being too slow. My previous post about 4K was about using DisplayPort to get more than 30Hz scan rate at 4K [1]. I now have a nice 60Hz scan rate which makes WW2 documentaries display nicely among other things.

But when running a 4K monitor on a 3.3GHz i5-2500 quad-core CPU I can’t get a FullHD video to display properly. Part of the process of decoding the video and scaling it to 4K resolution is too slow, so action scenes in movies lag. When running a 2560*1440 monitor on a 2.4GHz E5-2440 hex-core CPU with the mplayer option “-lavdopts threads=3” everything is great (but it fails if mplayer is run with no parameters). In doing tests with apparent performance it seemed that the E5-2440 CPU gains more from the threaded mplayer code than the i5-2500, maybe the E5-2440 is more designed for server use (it’s in a Dell PowerEdge T320 while the i5-2500 is in a random white-box system) or maybe it’s just because it’s newer. I haven’t tested whether the i5-2500 system could perform adequately at 2560*1440 resolution.

The E5-2440 system has an ATI HD 6570 video card which is old, slow, and only does PCIe 2.1 which gives 5GT/s or 8GB/s. The i5-2500 system has a newer ATI video card that is capable of PCIe 3.0, but “lspci -vv” as root says “LnkCap: Port #0, Speed 8GT/s, Width x16” and “LnkSta: Speed 5GT/s (downgraded), Width x16 (ok)”. So for reasons unknown to me the system with a faster PCIe 3.0 video card is being downgraded to PCIe 2.1 speed. A quick check of the web site for my local computer store shows that all ATI video cards costing less than $300 have PCI3 3.0 interfaces and the sole ATI card with PCIe 4.0 (which gives double the PCIe speed if the motherboard supports it) costs almost $500. I’m not inclined to spend $500 on a new video card and then a greater amount of money on a motherboard supporting PCIe 4.0 and CPU and RAM to go in it.

According to my calculations 3840*2160 resolution at 24bpp (probably 32bpp data transfers) at 30 frames/sec means 3840*2160*4*30/1024/1024=950MB/s. PCIe 2.1 can do 8GB/s so that probably isn’t a significant problem.

I’d been planning on buying a new video card for the E5-2440 system, but due to some combination of having a better CPU and lower screen resolution it is working well for video playing so I can save my money.

As an aside the T320 is a server class system that had been running for years in a corporate DC. When I replaced the high speed SAS disks with SSDs SATA disks it became quiet enough for a home workstation. It works very well at that task but the BIOS is quite determined to keep the motherboard video running due to the remote console support. So swapping monitors around was more pain than I felt like going through, I just got it working and left it. I ordered a special GPU power cable but found that the older video card that doesn’t need an extra power cable performs adequately before the cable arrived.

Here is a table comparing the systems.

2560*1440 works well 3840*2160 goes slow
System Dell PowerEdge T320 White Box PC from rubbish
CPU 2.4GHz E5-2440 3.3GHz i5-2500
Video Card ATI Radeon HD 6570 ATI Radeon R7 260X
PCIe Speed PCIe 2.1 – 8GB/s PCIe 3.0 downgraded to PCIe 2.1 – 8GB/s

Conclusion

The ATI Radeon HD 6570 video card is one that I had previously tested and found inadequate for 4K support, I can’t remember if it didn’t work at that resolution or didn’t support more than 30Hz scan rate. If the 2560*1440 monitor dies then it wouldn’t make sense to buy anything less than a 4K monitor to replace it which means that I’d need to get a new video card to match. But for the moment 2560*1440 is working well enough so I won’t upgrade it any time soon. I’ve already got the special power cable (specified as being for a Dell PowerEdge R610 for anyone else who wants to buy one) so it will be easy to install a powerful video card in a hurry.

3 comments to Video Decoding

  • Paul

    Mentioning the i5-2500 and PCIe2/3 issues made me think about age…
    And after checking your i5-2500 with wikipedia, it seems this processor only supports PCIe2
    But as you calculated: PCIe2 should still be plenty fast for your requirements.
    A …thing.. I noticed when examining my setup under windows, is that my radeon 7870 (sometimes?) only gets recognized as 8x instead of 16x – on an AsRock AB350 board. The card is still fine for most older games and I only use 1920×1080 anyway. Also, I don’t know a way to check the current use of PCIe-Lanes in Linux.
    But if you are running PCIe2 (due to old CPU) and not the maximum of lanes (for whatever reason), you would be in the range of bandwidth troubles.

  • Paul

    I re-read your post and figured I just use your commands… my debian says for the 7870 (supposedly running in PCIe3):
    LnkCap: Port #0, Speed 8GT/s, Width x16, ASPM L0s L1, …
    LnkSta: Speed 5GT/s, Width x16, …
    So on linux, my link _width_ seems fine but as with yourself the link _speed_ seems off.

  • Paul: Thanks for that information, that makes some things clearer. The line “LnkSta: Speed 5GT/s (downgraded), Width x16 (ok)” shows that it’s using 16 lanes and the expected speed for PCIe 2.0.

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