[mythtv-users] Will moving to DDR-400 memory from DDR-333 improve performance?

Brian J. Murrell brian at interlinx.bc.ca
Tue Oct 31 02:13:45 UTC 2006


On Mon, 2006-30-10 at 16:47 -0500, Chris Pinkham wrote:
> 
> Where are these hundreds of MB you are seeing?
> 
> Here's samples from my 2 backends and my main frontend.  Between all three
> of them I'm barely using 165MB and that's 2 copies of mythbackend and one
> copy of mythfrontend.  Command being run is:
> 
> 	ps auxw | egrep "mythfront|mythback|RSS" | grep -v grep ; echo ; free
> 
> Master backend w/ 3 AverMedia M179 cards:
> 
> USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> mythtv    2074  0.1 11.8 248100 30244 pts/2  S    Oct28   3:27 mythbackend
                           ^^^^^^ ^^^^^
Uhm, these are in KiB you realize yes?
> 
>              total       used       free     shared    buffers     cached
> Mem:        255208     245296       9912          0      31932     111540
              ^^^^^^     ^^^^^^       ^^^^
As are these

> OK, so we're using ~30MB there for mythbackend.

No, you are using 248MB.  Only 30 of it is in actual RAM though.  That
doesn't mean that the process is not not using 248MB.  It's just doesn't
need to access all 248MB at the exact moment in time that you took that
process snapshot.  But take comfort in knowing that at points in time
portions of that 30MB that are in RAM will need to be swapped out to
disk and what is in swap will need to come into RAM.  This causes lots
of disk trashing which of course means recording and playback burps and
so forth.

> 
> Now the slave backend with 2 air2pc ATSC HD cards:
> 
> USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> mythtv   28147  1.4  4.0 206800 20920 pts/2  S    Oct28  42:24 mythbackend
> 
>              total       used       free     shared    buffers     cached
> Mem:        516500     189156     327344          0      18868      63956
> -/+ buffers/cache:     106332     410168
> Swap:       265064          0     265064
> 
> OK, using a whopping 20MB there.

Nope.  206MB.


> USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> mythtv    5717  3.6 22.7 242300 116720 pts/4 S    Oct08 1167:51 mythfrontend
> 
>              total       used       free     shared    buffers     cached
> Mem:        512668     503240       9428          0      60868     169424
> -/+ buffers/cache:     272948     239720
> Swap:       265064      55420     209644
> 
> So, using about 116MB there for a grand total of 166MB on all three boxes.

Correction.  Using 242MB there for a grand total of something like
696MB.

> Just because the VSZ of a program is 260MB doesn't mean it's using that,

Any one time, yes, you are correct.  But that that much of the process
is sitting out in swap just means there is going to be a lot of disk
thrashing when the process does need it.

> look
> at the swap numbers in my 'free' command.  On the slave backend where VSZ is
> 200MB,  I don't have anything in swap at all,

Hrm.  I think that means that the difference between the process size
and RSS is "code" which doesn't need to go to swap since it can be read
from the filesystem when needed.  It will still need to be read from
disk when the portions of "code" that are not in memory are executed.
Whether it has to come from swap or from the filesystem really, makes no
difference in the effect.

> so just because VSZ is 200MB and
> RSS is 20MB it doesn't mean that I'm going to have to swap in 180MB of data
> or program the next time that something wants to use mythbackend.

Certain not 180M, true, but some portion, to be sure.

> Perhaps there are a lot of people out there jumping to conclusions about
> how much memory Myth is actually using.

It's not jumping to conclusions.  It's listening to how more the disk
thrashes and how much more recordings drop frames and how much the
frontend stutters and pauses when things (like mythfilldatabase for
example) start up compared to 0.19.  It is soooo dramatic that it is
that noticeable, yes.  Unlike many, when I upgraded my BE/FE to 0.19 to
0.20 that is all that was upgraded.  No new O/S, no new kernel, etc.
just myth.

b.

-- 
My other computer is your Microsoft Windows server.

Brian J. Murrell
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