Thanks for detailed response. Probably the only way to know what the
overhead is for a specific application is to time it, which is what I'll
do. Thanks again!

Thomas
 

Am Freitag, den 02.11.2007, 08:42 -0400 schrieb Thomas Fletcher:
> 
> > -----Original Message-----
> > From: Thomas Buschmann [mailto:buschmann@amm.mw.tum.de] 
> > Sent: October 31, 2007 5:42 PM
> > To: general-community
> > Subject: resource manager performance
> > 
> > I am just porting some real-time control software to qnx. I 
> > would like to use resource managers for interfacing the 
> > actual controller code with hardware-io (driver) code. 
> > However,  performance is very important for my application. 
> > That's why I'm wondering how using a resource manager 
> > compares with using other IPC methods (e.g. message queues) 
> > performance-wise. 
> > Typical read/write sizes would be only a few bytes, but there 
> > are several reads and writes every msec. I guess the question 
> > is, how high is the overhead of using the resource manager 
> > framework compared to native message passing?
> > Any help would be greatly appreciated.
> 
> A resource manager is really just a (relatively) thin layer on
> top of the native messaging.  It provides a bit of a handshaking
> and authentication framework on top of the standard messaging 
> API as well as a set of standardized messages (ie read/write).
> 
> Once you have established a connection to your resource manager,
> (via open) the fd you work with is the same as a coid that you
> would have received if you had done a ConnectAttach().
> 
> When you read/write on that fd, it is equivalent to performing
> a MsgSend with a standard message header.  You can see the read/write
> implementation here:
> http://community.qnx.com/integration/viewcvs/viewcvs.cgi/trunk/lib/c/1/read.
> c?root=coreos_pub&rev=153052&system=exsy1001&view=markup
> 
> Depending on what else you are doing in your system, performing
> hundreds or thousands of single byte read/writes every second
> is really not very efficient.  You can do it, and the limiting
> factor is going to be the speed of your CPU and the frequency
> of the operations, but you might want to look at different 
> alternatives.
> 
> One alternative is the mq library that uses the asynchronous 
> messaging framework.  There is also an mqueue service that uses
> standard message passing, but your own custom resource manager
> will be more efficient in that case.
> 
> Alternatively to that, it might be more worthwhile for the 
> application to create a shared memory area that the server and
> clients can all access.  It depends on what is being done with 
> the data.
> 
> To get a feel for the overhead, you can use the instrumented kernel
> and the system profiler tooling to see what is going on in the 
> system and measure some of the typical latencies.
> 
> Hope this helps,
>  Thomas
> 
> _______________________________________________
> General
> http://community.qnx.com/sf/go/post2408
> 
-- 
Dipl.-Ing. Thomas Buschmann

Technische Universitaet Muenchen
Lehrstuhl fuer Angewandte Mechanik
Boltzmannstrasse 15, D-85747 Garching

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