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Buffering issue

Any issues that are tvOS specific
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swineone
Posts: 4
Joined: 16 Jun 2018, 02:25

Buffering issue

Post by swineone » 16 Jun 2018, 02:43

Hi, I'm a new user, having downloaded MrMC a couple of hours ago, and I'm having an issue, which I hope someone can help me with.

My setup: Apple TV 4K, MrMC paid version.

I also have an AirPort Time Capsule which I've connected via SMB to MrMC -- if there's a better way, please let me know. I'm trying to watch a fairly high-bitrate MKV (a 4K HDR movie) which is in the Time Capsule's hard drive. Although I believed the Time Capsule would be fast enough to stream it, and on average it seems to be, at times during the movie when the bitrate increases to 100 Mbps or beyond, it seems like the buffer empties and the movie starts to stutter.

On another thread I saw the suggestion about POSTing {"jsonrpc":"2.0","method":"Input.ShowCodec","id":1} to the web server. There I can see the buffer emptying, confirming my suspicions.

I started messing around with buffering settings and so on. It appears 200 MB is the max in-memory value (trying to set it to any value beyond 200 doesn't save the setting when I return to the menu), and also tried setting it to 0 for using the disk cache rather than memory.

In the displayed debug info, there is a "forward" field. I understand this is some kind of extra buffer beyond the regular VQ and AQ buffers. Unfortunately, it usually stays at 0 B. At one point, after unplugging my Ethernet cable, which is connected directly to the Time Capsule, and configuring WiFi, I managed to get this "forward" field to start increasing while the movie was paused -- at one point it reached a few GB, this with the buffer set to 0, i.e. disk cache mode. Unfortunately, I kept playing around with some more settings and now it is always stuck at 0 B, regardless of the connection type (WiFi or Ethernet) and whether the buffer setting is 200 or 0. I stress that, right now, I can't get this buffer to move beyond 0 B even with the movie paused, which ensures there's no risk that the buffer is being consumed faster than it can be replenished.

Without this extra buffer, the movie is unplayable for me as it stutters too often. I'm not sure if there's a setting I'm missing somewhere to enable this extra buffer, or perhaps I've stumbled across a bug. Either way, I could really use some help getting this buffer to reliably go beyond 0 B. If I don't fix it, I'll have to cope with stutters in most or all high-bitrate content I try playing with MrMC, not only this movie.

Thanks in advance for any help.

cosmoxl
Posts: 1498
Joined: 12 Dec 2015, 14:16

Re: Buffering issue

Post by cosmoxl » 16 Jun 2018, 03:52

I'm sure they'll want to see a log :) https://wiki.mrmc.tv/index.php?title=Debug_log

swineone
Posts: 4
Joined: 16 Jun 2018, 02:25

Re: Buffering issue

Post by swineone » 16 Jun 2018, 04:56

Well it appears that after disabling buffering and then reenabling it for all filesystems, it's working again. I'll report if the problem comes up again and this doesn't fix it. I speculate this may have to do with me trying to set invalid values for the buffer size (> 200 MB), but this is just wild speculation with no facts to back it up.

cosmoxl
Posts: 1498
Joined: 12 Dec 2015, 14:16

Re: Buffering issue

Post by cosmoxl » 16 Jun 2018, 13:45

I'd be more concerned with why you're needing buffering anyway. The time capsule and the ATV4k are gigabit. Streaming 100mbit/s shouldn't be a problem.

swineone
Posts: 4
Joined: 16 Jun 2018, 02:25

Re: Buffering issue

Post by swineone » 16 Jun 2018, 17:29

cosmoxl wrote:
16 Jun 2018, 13:45
I'd be more concerned with why you're needing buffering anyway. The time capsule and the ATV4k are gigabit. Streaming 100mbit/s shouldn't be a problem.
In principle I'd agree. However, the hard drive on the Time Capsule is slow (4200 RPM I think?), I'm doing writes on the drive at the same time, there may be bitrate peaks a little beyond 100 Mbps for the video and don't forget the audio stream adds up a little to the bitrate. To top it off, SMB is not the most efficient protocol.

While troubleshooting, it occurred to me that the Ethernet connection could have fallen back to 100 Mbps, but I have no idea how to check, and anyway the 802.11ac WiFi is not performing any better. The Time Capsule and Apple TV 4K are about a feet away from each other so the WiFi connection between them is easily worth at least half a gigabit. Thus it is clear to me that the issue is not network bandwidth, but rather a hard drive speed limitation compounded by the SMB overhead.

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pecinko
Posts: 200
Joined: 02 Dec 2015, 10:55

Re: Buffering issue

Post by pecinko » 16 Jun 2018, 19:09

swineone wrote:
16 Jun 2018, 17:29
While troubleshooting, it occurred to me that the Ethernet connection could have fallen back to 100 Mbps, but I have no idea how to check, and anyway the 802.11ac WiFi is not performing any better. The Time Capsule and Apple TV 4K are about a feet away from each other so the WiFi connection between them is easily worth at least half a gigabit. Thus it is clear to me that the issue is not network bandwidth, but rather a hard drive speed limitation compounded by the SMB overhead.
From my experience, high bitrate movie + playing while writing to TimeCapsule is pretty much "mission impossible" for smooth playback experience.

swineone
Posts: 4
Joined: 16 Jun 2018, 02:25

Re: Buffering issue

Post by swineone » 07 Jan 2019, 12:45

Just to add some closure to this thread.

After posting this I basically didn’t watch any more movies on my Apple TV, but on the last few days I started doing so again. Thought I would use my 2018 MacBook Pro as the file server instead, since its SSD with 2 to 3 GB/s read speed shouldn’t be a bottleneck. Weirdly, there were still buffering issues.

By this point I was sure there were bottlenecks elsewhere in the network. I was ready to point the finger at SMB or its implementation in the Mac and/or MrMC, but I found an app called “Network Speed Tester”, for testing LAN (not internet) speed — I installed the server on my Mac and the tvOS client. Tests showed I was getting about 90 Mbps out of my wired connection on the Apple TV. Switched to WiFi and instantly got a few hundred Mbps (802.11ac Time Capsule quite close to the Apple TV). So there was my bottleneck.

Digging a little more, I discovered that the network cable connecting the Apple TV to the Time Capsule until now was unable to negotiate a gigabit connection — I confirmed this connecting it to my Mac, where it only showed a 100 Mbps connection, whereas just replacing the cable made the connection Gigabit. Therefore, this cable was the problem all along.

It’s just too bad the Apple TV hides the negotiated link speed from you, since this would have easily solved my problem back then. Anyway, hope this helps anyone who’s going through similar issues.

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davilla
Team MrMC
Posts: 3860
Joined: 26 Oct 2015, 17:01

Re: Buffering issue

Post by davilla » 07 Jan 2019, 14:19

heh, it's always the cable :) 1st thing I go for when dealing with transport speed issues.

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