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Thread: Bad performance while recording

  1. #1
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    Dec 2007
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    Bad performance while recording

    I have the following system:

    - GigaByte MA69GM-S2H mainboard with onboard ATI Radeon XPRESS 1250;
    - Athlon 64 X2 4200+;
    - 2 GB DDR2-667 dual channel Kingston memory;
    - Samsung Spinpoint T166 (HD501LJ) 500 GB SATA II harddisk;
    - Samsung SH-S203B DVD writer;
    - Terratec Cinergy 1200 DVB-C.

    I have done a fresh install of Mythbuntu 7.10. After sorting out problems with the video driver (using fglrx now), scanning channels etc. I have got it working, but some problems are still unsolved.

    The biggest problem is that when watching live TV, the performance is not good enough to prevent frame skipping. There also seems to be relatively heavy disk access. While playing recordings however, everything's very smooth and there hardly seems to be any disk access.

    I have already moved the DVD writer and the harddisk to differen SATA channels, to no avail.

    There has been a bug report about slow SATA performance (see https://bugs.launchpad.net/ubuntu/+s...20/+bug/119730), I'm not sure if this could have something to do with it. When I get the drive settings for /dev/sda with hdparm, it shows it is not using any DMA mode. When I attempt to switch it into DMA mode, it fails and I get the message that it reverts to PIO. But then there are also reports that say this can't be set at all for SATA drives.

    My questions are therefore:
    - Is the harddisk performance a likely cause for this problem?
    - Is there anything I can do to solve or work around this?

    Thanks for any help!

  2. #2
    Join Date
    Oct 2006
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    Ubuntu 12.04 Precise Pangolin

    Re: Bad performance while recording

    hard disk performance will absolutely cause issues like dropped frames.
    until you get to the bottom of the possible hard disk issues, there is one thing
    that comes to mind--

    you could try increasing the buffer to see if that helps... i know i had to increase
    it for best performance streaming HD over firewire.

    the buffer setting is in frontend setup: tv settings-->general-->general (advanced)

    by default the HD ringbuffer size is set @ 9600. i recommend you adjust it up a
    step at a time, testing livetv as you go. if you find a setting that stops dropped
    frames, stop there-- setting the bufffer too high can be as bad as too low.

  3. #3
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    Re: Bad performance while recording

    Thanks for the suggestion. I have tried several settings, but in my case it doesn't seem to have any effect on the frame rate.

  4. #4
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    Re: Bad performance while recording

    Is "MythBuntu" a distribution or a Meta-Package?

    If it's a distribution, does it ship w/ the Real-Time Kernel?

    If it's a Meta-Package, try installing the Real-Time Kernel to see if that helps your situation.

  5. #5
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    Re: Bad performance while recording

    Check your SATA settings in BIOS. Sometimes you can get a little more performance by tweaking.

    This might be related to another post with throughput limits hitting 33 MB/sec going through a gigabit ethernet. The motherboard may have a bottleneck in the SATA/IDE controller that is causing slowdowns due to excessive interrupts. During simple playback this might not be a problem due to buffering.

    Post the output of:

    >sudo hdparm -tT /dev/sda

  6. #6
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    Re: Bad performance while recording

    Yes, MythBuntu is a distro and I'm not sure about the real-time kernel, but it must be optimised for multimedia.

    My timings look good:

    /dev/sda:
    Timing cached reads: 2190 MB in 2.00 seconds = 1095.94 MB/sec
    Timing buffered disk reads: 246 MB in 3.00 seconds = 81.96 MB/sec

    But that doesn't necessarily men anything. It took me about 8 seconds to copy a 218 MB file.

    I have already looked at the bios, but there are no user configurable settings for SATA.

    Strange thing is that the framerate looks good for a fraction of a second until disk activity kicks in.

  7. #7
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    Re: Bad performance while recording

    Does it use the Ubuntu repos?
    If so you can install the realtime kernel packages - it works wonders for doing audio recording - I would assume it would do the same for recording realtime video.

  8. #8
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    Re: Bad performance while recording

    *UPDATE
    I did some research for you, and trolled through some mailing list archives, and it seems that Mythbuntu doesn't ship w/ the RT kernel by default, but it is a reccomended tweak, so simply "apt-get install linux-rt" - it'll keep your old kernel in place as an option in grub should you want to go back.

  9. #9
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    Re: Bad performance while recording

    218 MB in 8 seconds is ~27.25 MB/s even though the drive can support 82 MB/sec, which is excellent. So obviously there is a bottleneck.

    The framerate in the beginning is being fed by your high memory throughput: 1,095 MB/sec until it asks for more from the disk then it's 1/100 that speed.

    The real-time kernel will help reduce interrupts at the expense of a super-laggy mouse and keyboard, but that won't cure a throughput problem.

    Who knows what the required throughput needed for HD content? 720p vs 1080p?
    I'm sure it's greater than 33 MB/s.

  10. #10
    Join Date
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    Re: Bad performance while recording

    Quote Originally Posted by tgalati4 View Post
    Who knows what the required throughput needed for HD content? 720p vs 1080p?
    I'm sure it's greater than 33 MB/s.
    it depends on the bitrate of the stream itself, which may be the problem here.

    check to see what the recording bitrate is set at- you'll find it in setup-->tv settings-->recording profiles-->live tv.
    i suggest cranking down the bitrate a little at a time to see if and where the dropped
    frames stop happening. if this helps, my guess is the impact on the quality of
    the livetv video will be negligible.
    Last edited by majoridiot; December 19th, 2007 at 06:58 PM. Reason: typo

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