[mythtv-users] AMD Athlon 64 CPUs and MythTV Compiling

Brad Templeton brad+myth at templetons.com
Tue Dec 21 04:07:19 UTC 2004

On Tue, Dec 21, 2004 at 12:06:25AM +0000, Tom Hughes wrote:
> In message <200412202353.41701.doveleys at nildram.co.uk>
>           Giles Jones <doveleys at nildram.co.uk> wrote:
> > On Monday 20 Dec 2004 23:40, Brad Templeton wrote:
> > > Sorry, but as I pretty clearly said,the whole clock speed thing _does_
> > > matter a bunch on this very specific application -- mpeg encode and decode.
> > 
> > It's down to the code. If the code doesn't use 64-bit maths then you don't
> > quite get the advantage of a 64-bit CPU. Not to mention you really need to be
> > running a 64-bit version of Linux.
> But you do get the advantage of an extra eight registers, which makes
> a huge difference to most code as the x86 architechure is very short on
> registers which gives optimisers a very hard time.
> I've seen programs which don't do much 64 bit maths go 50% faster just
> by recompiling for x64-64.

So, for those running Athlon-64s, what sort of timings are you getting
with the myth internal player compiled for 64 bit and for 32 bit, compared
to the regular athlon xp and the P4?

In the end, the best way to tell is just to try it out.  Some code will make
happy use of extra registers or wider registers.  Some doesn't.  Some even
gets slower, because it keeps reading and writing 64 bits when all it needs
is 16 or 32.   Plus programs get bigger, and bigger program equals slower
depending on cache performance.

Of course, with the right coding, it seems that a 64 bit mpeg decoder
written for ia64 could do well, but I am not sure we have one around.

Of course, if it were not for the xvmc problems in myth, the simplest answer
would be to use xvmc and get a nice, cool, slower chip like a Sempron 2000
which is dirt cheap -- and I think could handle things with xvmc doing the
work.  (Of course it would be much slower at transcodes and commercial flags.)

Right now commercial flag is pretty slow if set at low CPU.  On the Athlon
3000 it seems to run at about realtime -- 3 hours to flag a 3 hour show.

Presumably on the wishlist is the ability to start a commercial flag thread
immediately upon starting recording, and almost as importantly, to make
the results available as they are generated and confirmed, instead of
waiting until its all done.  If you have the CPU, you could then watch
shows closer to airtime and have them flagged.

I am actually surprised at how unreliable the flagging is for me, others
have reported much better, some even have said they turn on automatic
skip, which would be a nightmare for me.   Perhaps HDTV is harder?
(It should be easier, since commercial breaks today always contain
most of the commercials in 4:3 format.)

More information about the mythtv-users mailing list