On 05/29/2012 02:38 PM, ali hagigat wrote:
We know that Xenomai can create hard real time tasks and it means that
the real time tasks will be executed up to a specific time. My
question is about that time. How much is that time?(100 micro seconds,
150 usec, or...)
Do you mean maximum latency? If so, you only have to run the "latency"
test program on your board (/usr/xenomai/bin/latency for a default install).
With modern hardware, 100 us on x86 is considered a bug.
I know that it may depend on the architecture and the software platform.
I want to know how can i determine that time for my x86 32-bit PC
system? and besides what major factors are effecting the time?
Assuming time == latency, major arch-independent factors include bus
locking, MMU handling (e.g. see issues with VIVT caches on ARM), cache
artifacts caused by concurrent linux activity in general (which may
cause real-time code to be evicted during idle phases by non-rt code).
x86-wise, there is some more issues to keep an eye on:
- Hyperthreading (not nice to -rt requirements)
- SMI sources (causing IRQ-disabling BIOS code to be invoked directly by
special interrupts not going through the programmable interrupt
controller, so we can't block them, and don't know when they happen).
- Some graphic card adapters (causing hw stalls to favor their own
throughput)
- Some graphic card drivers, most often proprietary, disabling
interrupts for too long.
- Some cycle-hungry x86 cache invalidation instructions (wbinv).
- The legacy x86 PIT (i8254) when used, which goes through a legacy ISA
bus, as expensive as ~1.5-2.5 us for each I/O access (you need at least
two of them to program the next clock tick). Fortunately, you won't need
it on a modern x86 platform, unless really unlucky.
Oh well, other that that, x86 is almost fine for -rt stuff.
Any answer will be much appreciated.
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--
Philippe.
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