DS128+ -- Transcoding is taking 98% CPU suddently

[Aleridia]

Synology DS218+
CPU : Intel Celeron J3355 dual-core 2.0 GHz
RAM : 10GB in DDR3L

Server Version#: DSM 6.2.3-25426 Update 3
PMS Version#: 1.21.4.4079-1b7748a7b

Plex Media Server Logs_2021-02-26_10-19-40.zip (1.6 MB)

Hello,

I’m facing a problem : plex server take 98% of my nas’ CPU suddently.
So the video is loading every 10-15 minutes, or plex tell me I have no enought CPU to convert this item.
It’s very annoying and I can’t figure out why it’s making this without apparent reason.
Before this, I had no problem with plex and I thought I didn’t make any modification that can trigger this issue.

I’m connected in local network so I don’t think the problem is related to my network settings.

When I’m trying to watch an episode :

The processus and services in the nas :

My plex settings :

I’ve tried to test different configurations (Without direct stream, disable video transcoding) and it doesn’t solve my problem.

If I forgot to provide some information, please tell me and I will add them asap.

Do I need to get Plex Pass to hardware transcoding ?
Or the problem is caused by the video properties ?

Thanks in advance for your help

[FordGuy61]

The video is HEVC. Other than Safari, web browsers do not support HEVC video, so Plex must transcode the video.

You do not have a Plex Pass, so all transcoding is performed by the CPU. The Celeron CPU will struggle with 1080p material, especially HEVC, as it requires more resources to transcode than H.264.

You also have subtitles enabled. This puts an additional load on the CPU, as it has to burn them into the video stream. Note that this process always occurs on the CPU, even if hardware acceleration is enabled.

Option 1: Use a Plex client that direct plays HEVC video.

If you are using a Windows or Mac system, use the Plex for Windows/Mac application instead of a web browser. Plex for Win/Mac direct plays most content, including HEVC video. Note that this essentially transfers the transcoding from the server to the client. You may see the client CPU utilization increase. You may still experience buffering/stalling if the client has a underpowered CPU.

Option 2: Use Hardware Accelerated Transcoding (Plex Pass required)

You can also purchase a Plex Pass and enable hardware accelerated transcoding. This will shift the transcoding process to the Intel HD Graphic 500 embedded GPU.

Note that subtitles will always be a problem when using Celeron based NAS. As mentioned above, subtitles are added to a video stream by the CPU, not the GPU (an Intel driver limitation). The process is also single threaded. The J3455 has a low single thread rating, so it will struggle when burning subtitles into the video stream.

The end result is that the DS218+ can easily transcode 1080p video using hardware acceleration, but you may still see buffering when subtitles are enabled and the video is transcoding.

You can try a Plex Pass via the monthly subscription ($4.99 USD). If it does not help you can cancel the subscription. If you wish to keep the Plex Pass you can consider the other purchase options if desired (yearly, lifetime, etc).

---

References:

Browser HEVC Compatibility

Plex for Windows/Mac

• Download
• Documentation

Plex Media Server Documentation

• Using Hardware Accelerated Streaming
• What kind of CPU do I need for my server? - See Passmark ratings for software transcoding
• Plex NAS Compatibility List - See spreadsheet listing NAS transcode capability.

Intel J3455

• Specifications
• Passmark Score

Plex Media Server on Synology

• Installation & Setup of Plex Media Server on Synology
• Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s

[Aleridia]

Hello,

Thanks for you fast reply.

I understand now why I takes 100% CPU.

I think I will use the plex client if it transcode on the client side.

I’m a little sad that the 218+ isn’t really good for burning subtitles.

Thank you again for your reply, you helped me a lot to understand how plex works and why it’s not complety compatible with my configuration.

Have a good day !

[skwor01]

It can burn at least 1 subtitle (srt) if you are using hardware encoding for the video. My TS-453Be (also a J3455) with Plex Pass has done 7 streams one doing subtitles, 4 video trans-coding and several more audio trans-coding, running at 85-ish percent. But yes subtitles puts a hurt on the CPU. Without the subtitles I have seen it run 12 streams and still only be at 70-ish percent with various (I think 6) video encodings.

In my opinion the lifetime Plex Pass is worth it for HW transcoding if one plans on using Plex and supporting servers for any serious length of time.

See below (notice I also have A LOT of different devices using it - Sony TV, Samsung TV, IPAD, Plex Web, 2 Samsung android phones one an S8+ the other an S9 and a Roku). This was all on 2/26/21 Plex FW 1.21.2.3943, QNAP FW 4.5.1.1495 :

[ChuckPa]

@Aleridia

All of the J-series CPUs can handle HW transcoding of the 2160p HEVC video. That’s never a concern.

Subtitle burning is their pain point. It’s a 2200 Passmark CPU (about 500 passmarks per core) which just isn’t fast enough for any of the higher bitrate video streams.

If you’re careful / lower bitrate video, you’ll be ok.

The best choice, when possible, is to stream the SRT to the player without burning in (player setting).

If subtitle burning is required (PGS, VOBSUB, or DVDRIP subs), handling it on the player side is best unless you are willing to invest in an i5 / i7 class NAS.

[skwor01]

OK, I seriously need to add one more post to this. I just found the Subtitle setting “Burn Subtitles” and switched it from “Automatic” to “Only image formats.” It made a HUGE difference, the server was burning SRTs when it did not have to.

This kind of needs promoted and better explained on the support pages. After switching it to image only I was able to run 3 streams all doing SRT subtitles and my server load was only 35%. That is with the J3455 CPU

@ChuckPa maybe the server is burning subtitles in automatically when it does not have to, especially text based ones? On my screenshots from 2/26/21 above the one srt was responsible for 35-40% of the server work. I had my friend who was playing it on 2/26 just do the exact same thing today and it only pulled 15% load after switching that setting to “Only image formats.”

[ChuckPa]

Verify DEBUG logging on, VERBOSE logging off.
Start Playback
Play 20 seconds
Stop Playback
Wait 20 seconds
Download logs ZIP file
Attach please.

I will show what it’s doing. We can proceed from there.

[skwor01]

LOL I knew you would want logs. It will have to wait as I have to coordinate with my friend and I start a power plant outage tomorrow which will have me working 6 days a week for about 4 weeks. Sorry.

[ChuckPa]

You mean you forgot?

[skwor01]

Yes i did and yes i feel stupid!