Re: The idea of disabling interrupts
- From: karthikbalaguru <karthikbalaguru79@xxxxxxxxx>
- Date: Mon, 3 Dec 2007 01:17:38 -0800 (PST)
On Nov 30, 2:01 pm, ssubbarayan <ssu...@xxxxxxxxx> wrote:
On Nov 30, 1:11 pm, karthikbalaguru <karthikbalagur...@xxxxxxxxx>
wrote:
On Nov 29, 2:15 pm, Arlet Ottens <usene...@xxxxxxxxxx> wrote:
karthikbalaguru wrote:
If you're sharing between two tasks, it depends. Generally, if you
disable interrupts for a short period, and you don't call any OS/library
functions, it can be a useful method. Exactly how long you can disable
interrupts depends on your environment and real-time requirements.
Thx for the info. Interesting :):)
Thinking over this, i have a query -
Is it possible to do sharing of data(Critcal Section) wihtout diabling
of interrupts and also
without the use of these RTOS features like semaphore/Mutex/
MessageQueues/Pipes/Mailboxes ?
Sometimes you can use atomic instructions, if your CPU has them. For
instance, the ARM has the SWP instruction that can be used to solve some
concurrency problems.
The other possibility is to not use a preemptive RTOS. You can either
use a non-preemptive (cooperative) OS, or not use an OS at all. I've
implemented quite a few embedded systems that just had a polling loop,
and a bunch of ISRs. In some cases, the ISRs did the bulk of the work,
so there wouldn't be any concurrency issues, and the timing was guaranteed.
Thx for the info.
I thought of such a similar idea - The idea of using flags w.r.t
interrupts.
Enable a flagA and manipulate the part A
(shared data) in task code and as flagA is enabled, manipulate the
part B
in the ISR. And vice versa for part B(Shared Data).
Though this replaces the idea of disabling interrupts,
this is time consuming.
Karthik Balaguru- Hide quoted text -
- Show quoted text -
Hi,
While flags may seem to resemble in one way like a semaphore in its
way of operation,there are subtle differences and advantages on using
semaphore then a flag.
1)A flag may get modified any time and may introduce write/read
problems and that too while dealing with interrupts you always dont
know when your flag will be required.
Multually exclusive task manipulation with exclusive flags
can make a good design.
2)If your idea is to share data between 2 tasks(strictly 2 tasks
only),flag might be useful
True.
With careful design , we can extend this design.
For example in assembly level programming, we handle many flags
with ease with proper correlation and care taken while implementation.
Similar scenario applies here.
3)The overhead you experience with your OS structures may be high
compared to your own implementation,but I would not use my own
structures when its available from OS.Further I believe the overheads
of using these OS features are too minimal to cause a major problem
for your product or application,provided your talking about RTOS .You
can request the OS vendor for time bench marks for these operations if
you still doubt.
But, i think that using RTOS resouces will consume
the microprocessor resources a bit more in the
intialisation/creation stuffs. Further, making it independent of
the RTOS functions, makes efficient programming by making
it work only for the desired functionalities that we need.
4)But when you are going to share data between multiple
tasks,controlling access to the flag gets a nightmare.It really needs
great thinking and effort to ensure only the right task accesses the
flag at right time.Suppose two tasks try to access data at same
time,you would not be able to decide which one to be given preference
to access the flag.This would well be solved by a semaphore.AFAIK
atleast with vxworks and OS20 you can specify during semaphore
creation the access controllability for tasks accessing the resource
using semaphores.Belive such feature would be available in most of
RTOS
Accepted.
But, creation of large number of tasks has its own drawbacks.
Context switch time will be adding to the response time.
Even large number of semaphores can cause all kind of
potential bugs . (Wrong Correlation between semaphore and task etc).
5)Interrupt locks are best for short duration codes and same case
applies for taskLocks.Always as a thumb rule,locks should be for
minimal period which could be depending on your application.
True.
As long it is short it is good. But, really bad if we use it while
performing some calculations / lenghy operations as it might add
up to the response time.
Karthik Balaguru
.
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