[mythtv-users] optimum for HDRingBufferSize?

[John P Poet]

On Tue, Apr 29, 2008 at 11:29 AM, Udo van den Heuvel <udovdh at xs4all.nl> wrote:
> Kevin Kuphal wrote:
>  > On Mon, Apr 28, 2008 at 11:03 AM, Udo van den Heuvel <udovdh at xs4all.nl
>  > <mailto:udovdh at xs4all.nl>> wrote:
>  >     I am streaming 3 lowish bitrate DVB-T channels to SATA disk on a VIA
>  >     Epia EN12000. What is the optimum value for HDRingBufferSize to use just
>  >     enough RAM to smoothly stream stuff and avoid hiccups, harddisk
>  >     thrashing, etc?
>  >     18800?
>  >
>  > The default?  I've never had to change it and I'm using two AverMedia
>  > A180 HD cards with 2 simultaneous recordings each.
>
>  Then why would it be configurable?
>  Is my machine as fast as yours w.r.t. CPU and/or disk?
>
>  I am just looking for some way to verify if my setting is OK or a way to
>  calculate or find out the best setting that causes the least chance of
>  problems.

It depends on how much *available* RAM you have.  If you have 4GB in
your system, you should be able to really crank up the size of the
ringbuffer.  If you only have 1GB, then the default may be right on
the edge.  It really depends on what all your computer is being asked
to do at the same time -- how much RAM is being consumed by all the
running processes, and how much disk I/O those processes are doing.

If you set the ringbuffer size too high, such that you exceed the
available RAM, then your computer will start swapping -- and that
would be disastrous!  If you set the ringbuffer size to low, then you
can loose data because your computer is not fast enough to get it
written to the hard drive before then next packet of data arrives.

Note that the ringbuffer will not save you if your computer is just
plain too slow to processes the data packets.  What the ringbuffer
does, is give your computer time to weather short bursts of high
activity by other processes.  The larger the ringbuffer, the longer
mythbackend can be starved for disk I/O before it has to drop some of
the data.

Hope that helps.

John
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