[svlug] GNU Denemo chokes a dual core 2 GB server

[Roland Krause]

Christian,

--- Christian Einfeldt <einfeldt at gmail.com> wrote:

> hi
> 
> Here is the next issue for our public middle school project.  And it
> is
> actually part of a recurring theme, it seems:  why is our linux lab
> lagging,
> when most people think that it should be able to handle what we are
> throwing
> at it.  More specifically, why are we experiencing delays in playing
> music
> compositions using GNU Denemo under Edubuntu over a GB network that
> should
> be performing better, and what can we do to solve the problem without
> spending any money?
> 
> As most of you know from reading this list, I am a volunteer
> supporting a
> public middle school with FOSS.  We have a brand new dual core server
> with 2
> GB of RAM whose only purpose in life is to support 24 chubby clients
> running
> edubuntu.  

2GB is *not* a lot of memory when you run 24 clients. But in any case
you need to start monitoring how much memory is in use when the clients
are running. Simply put - when your server pages you are dead
performance wise. 


> The thin clients have at least 256 MB of RAM, and most of
> them
> have swap space on their hard drive also, although some of them are
> true
> thin clients in that the hd has no swap and is basically a vestige. 

Again, not necessarily a lot of memory if you run heavy X apps such as
GNOME.

> I am
> told that we have a gigabyte network.  The server feeds the signal to
> the
> first 18 clients through one GB switch, and then that switch feeds
> another
> identical switch with services about 6 more clients.

This is likely a nonmanaged switch correct? These typically don't have
a very wide backplane, which means you are dividing you backplane
bandwidth by two times the number of clients. Worse, for non managed
switches the slowest network connection dictates the speed. If you have
a printer on such a network or a defective or semi-defective network
card you are typically - you guessed it - dead :-) 

A cheap thing to do is to buy a second GiG-E card for your server - or
since you can't spend any money I can give you one (contact me in
separate mail and tell me where to send it) - then spread you clients
to both switches. 

Further your problem might be disk access, since you are likely dealing
with music files. One easily underestimates how slow all ATA based
solutions actually are. A server that serves home directories usually
needs Ultra 360 SCSI disks. GNOME and other desktops do a multitude of
near atomic file operations and are very heavy on disk utilization. I
used to run centralized home directories but try to avoid that because
it's actually cost prohibitive.

> 
> Our problem is that when we use GNU Denemo to play the short little
> compositions that the students write in class, we can have only one
> student
> play his / her composition at a time or the system is choked, meaning
> that
> it lags.  More specifically, all of the students find that their
> GNOME
> desktops are almost completely non-responsive to the mouse.  The
> mouse still
> moves, but programs launch very slowly for about 5 mins, at which
> time the
> choke point is presumably cleared, and then one student can again
> play his /
> her composition.  It might be possible for, say, 2 students or maybe
> even
> three students to play their music; but we don't know where that
> stress
> point is.
> 
No idea about the particular GNU program that you are running but I
would start monitoring disk IO, client memory and server memory in that
order. Also I would monitor all network cards for dropping packages. It
almost sounds like you have defective network cards.

Good luck
Roland