Server Version#: 1.21.3.4014
Player Version#: Any - impacts all players
Hi everyone. I have a Synology DS918+ and am struggling to play Standard 1080p video with H264 encoding when hardware acceleration is enabled AND subtitles are also enabled (when they have to be burned in).
I’ve taken a look at the CPU utilization with hardware acceleration and it only caps around ~40 some percent according to the graph on the Plex interface. Nonetheless, the transcoding “Speed” indicator drops from about 1.5 to around .7-.9 when I enable subtitles.
If I entirely turn off hardware acceleration my CPU utilization is near capped at ~95% however I can at least play a single video smoothly when closed captions are enabled without buffering.
Given hardware acceleration drops my utilization substantially, it’s incredibly frustrating that it somehow winds up with a worse performance. Why would turning off hardware acceleration result in higher CPU utilization but better transcoding performance?
I found threads like the below where others have the same issue and many turn off all hardware acceleration as a workaround - pretty poor solution if text-based subtitles are not available (like for live TV)
So not to be argumentative, but why does everything work fine when ONLY using the CPU with no acceleration? What am I not grasping? Why does it only stutter with acceleration? Shouldn’t that rule out that my CPU isn’t fast enough?
I have exactly the same problem with my DS920+.
I have problem with DTS sound as my new LG Oled CX doesn’t support it and Plex needs to transcode it. It does it smoothly when no subtitles are chosen. When I switch on SRT (external) subtitles it momentarily starts buffering and video freezes every 30 seconds or so - only with HW transcoding. Without HW transcoding - movie plays smoothly but Synology procesor goes to 95-98%.
SRT subtitles with any other movie without DTS sound - everything is OK.
In a few days (right now I’m out of home) I will create new thread for this problem with logs and everything but wanted you to know you are not alone with your problem.
Today I wanted to make tests you mentiond.
But also today I installed 500GB Samsung 970EVO M.2 as SSD cache (only 1 slot, so it’s in read-only cache mode). I bought this SSD for another purpose but decided to try it as additional cache for DS920+.
And the problem disappeared. HW transcoding works like a charm, it transcodes HD (1080p) movie with DTS sound and subtitles enabled on the fly without any buffering, CPU stays around 20%.
I’m a little affraid to remove the SSD to check if it comes back to previous state but I can do it if you insist.
I don’t have previous logs but when HW transcoding was ON, I was able to observe in Plex console info/warning like on attached pictures. Those warnings didn’t appear when HW transcoding was OFF.
A Read-only cache does help. It doesn’t help as much as a Read-Write but it definitely helps. It’s biggest benefit will be with the DB
It increases the perceived i/o operations per second (IOPS) of the drives.
(Drives are read 1 time to put into the SSD; SSD is read many times with zero latency)
Guess what happened - before installing the second SSD to have write/read SSD cache I removed the first SSD cache and uninstalled SSD disk from my Syno just to check if it shows problems as before - and HW transcoding with enabled subtitles still works flawlessly. I mean H264 + DTS + SRT subs = very smooth play, no buffering. Dashboard still shows HW encoding, Syno processor gets at maximum 20%.
I don’t understand it, I haven’t changed anything in my Plex configuration, Syno was restarted before and it didn’t help with HW transcoding / buffering problems.
Nevertheless - for now my problem is gone and I can use my SSD for the purpose I bought it
