Re: [Xenomai-help] Xenomai and futexes - Native API optimized for user space only applications

[Jan Kiszka <jan.kiszka@domain.hid>]

[-- Attachment #1: Type: text/plain, Size: 2394 bytes --]

M. Koehrer wrote:
> Hi everybody,
> 
> I am using Xenomai for a high performance real time simulation system. 
> All of the simulation code is executed within user space. One application is running that consists
> of several Xenomai real time threads.
> For performance reasons I am always using the latest PC technology available (which is currently
> Pentium D or Core2Duo PCs, we are thinking even of using quad-core CPUs).
> There has to be a kind of thread synchronisation e.g. when accessing shared data.
> Hardware I/O is done via rtnet or via PCI I/O boards that work in user space aswell (PCI memory
> mapped into the user space).
> This is done e.g. by using semaphores, mutexes et.c
> I like the Xenomai-native skin as it provides a very clear API that is easy to use.
> However, for a user space only application it is a performance killer, that all API calls
> lead to a mode switch from user to kernel space. Each API call takes about 1-2 microseconds (us)
> on my PC which is really expensive.
> Especially when inter process communication is used to protect the access to shared data
> it is mostly the case that the calling thread does not have to wait. In this situation there is no
> need for a context switch. The API call did not lead to a rescheduling of the available tasks.
> And for this the required 1-2 us do really hurt.
> 
> Thus my question/proposal is if there is a plan to use a "variant" of the native API that is optimized for
> user space only applications. In this case e.g. futexes can be used. If there is a need to
> reschedule to another task it is fine to "invest" the 2us but it can be avoided mostly which should
> increase the overall performance dramatically.

Your analysis is not wrong. But before going into any detail may I ask
you for on clarification: What do you want to optimise *primarily*, the
average execution time of your RT threads in order to gain more CPU time
for non-RT Linux jobs *or* the worst case execution time of your RT threads?

> This would lead to a library where a big part of the functionality is handled directly in the library 
> (in user space). Currently the skin passes the (user space) API call via a Xenomai System call 
> to the kernel space to execute there the actual functionality.
> 
> Thanks for any feedback on this proposal.
> 
> Regards
> 
> Mathias
> 

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 250 bytes --]