Need som transcoding explanation

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Need som transcoding explanation

Postby Joksi » Tue Jan 04, 2011 12:08 am

Hi!

I have been using PS3 Media Server for a long time, but never actually had it all clear how it works.
I played around a little bit this evening, and some questions have arised that I hope probably someone here could answer.

I can start mentioning i'm running the latest beta (r412) on Windows Vista, Phenom II X3 270 (2,8Ghz), 4GB memory and gigabit-connection directly from the PC to the PS3.
PS3 Media Server is configured with lossless MPEG2 Mencoder transcoding profile (1-1-1) and multi-core Mencoder experimental build enabled.

1. I really dont have the need for bandwitdh limiting, because im on gigabit, however im curious if gigabit is really needed so I changed to 100Mbit NIC and started experimenting with the AVI 1080P Bunny-sample (have heard it's a pretty good stress test, atleast at some parts in the beginning).
I started out with 0 limiting, which worked on gigabit (pendin between 50 and 200Mbps), but started to stutter on 100 mbit connection. Thats reasonable, and doesnt need explanation. :) However, I do wonder why the CPU starts working harder (ie 70-90% pending instead of 40-60%) when for example limiting to 50, or 80Mbps, in Mencoder settings?

2. I was curious how much the three cores and/or multi-core Mencoder experimental build combination did to up the performance, so I swithced it off one by one. Guess what? Everything played as smootlhy as with it enabled, is this working as it should? With three cores enabled in PS3 Media Server my CPU shows in Vista task-manager 80-90% utilization but with one core selected it shows 40-50% only! But as I said, the Bunny sample do play exactly as smootly at a pretty constant limit of 40-50Mbps showing on PS3 OSD. Explanation please?

The only big difference in PS3 Media Server that I can notice is when on gigabit, without any limiting, the 600MB buffer fills upp pretty rapidly even before the movie starts streaming on the PS3, but, with limiting at 50Mbps and/or disabling extra CPU cores it struggles a bit, allthough it do hold the buffer partly filled (20-40% going up and down).

So my conclusion is, if the majority of movies is not more demanding then this Rabbit sample it should be enough on my setup (even without the extra cores) with only 100Mbps connection and maximum quality settings. What still amazes is my though is that the CPU seem to have to work harder with three cores enabled, then with one core!
Joksi
 
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Re: Need som transcoding explanation

Postby shagrath » Wed Jan 12, 2011 10:56 pm

1. The CPU works harder when limiting the bandwidth because it has to compress harder some frames to fit in the given bandwidth, hence more cpu. With no limit, mencoder -qmin 2 -qscale 1 is able to generate peaks of hundreds of mbs with hard scenes (the ones with a lot of text for example)

2. multicore is not the perfect solution for trancoding. Yes, it's better to use 3x2Ghz instead of only 2, but usually, the encoder badly split the decoding/encoding threads between cores, so it can end up to some bottleneck in a few cases
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Re: Need som transcoding explanation

Postby Joksi » Wed Jan 12, 2011 11:58 pm

1. Does this then mean that a limit of say 50Mbps will give less quality then unlimited bandwitdh on gigabit? (compression)

2. So, should I use three cores in PS3 MS or one core?
Joksi
 
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Re: Need som transcoding explanation

Postby shagrath » Thu Jan 13, 2011 12:40 am

1. No, it will just eat more CPU

2. It's possible that the encoding threads will get less free cycles than the decoding ones (or the opposite, it depends of the scene I guess). I remember having troubles with my dual core on some cases, where the decoding would take different charges on cores, and the encoding thread not taking the rest (but it was long ago, so maybe it's better now)
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