On Thu, Sep 07, 2000 at 03:50:17PM -0400, Tony Mouawad wrote:
> I'm new to the RT world, but I'm curious... what significant challenges are
> there when working with dynamic memory and real time? Just the term
> "dynamic" gives us an indication that the mechanism would be
> undeterministic... but what's worse case here? Why is it impossible to have
> a "real time" dynamic memory manager? At worst, nothing is "real time",
To illustrate:
Thread:
loop:
if(get_a_buffer){
wait for an interrupt
copy data to buffer
dispatch buffer
}
else try_to_recover();
goto loop
Case 1: "get_a_buffer" may sometimes stall the thread
Result: program may stop at any momement.
Case 2: "get_a_buffer" allocates from a pool that is used by other threads and
just returns failure if there is no free buffer.
Result: program may fail at any input load depending on who else
uses the buffer pool.
Case 3: "get_a_buffer" allocates data from a static pool used only by this thread
and freed by the destination.
Result: thread can handle sizeof(buffer_pool)/sizeof(buffer) outstanding
requests. If a dispatched buffer is known to be freed within K time
units and interrupts take less than K time units plus
jitter+interrupt dispatch
+ copy time ... then there will never be a out of buffer failure.
So it's a subtle difference with big implications. In the case of Steve
Papacharalambous
module, things are more complex since he essentially manages a cache of memory and
replenishes this cache from Linux when the system is not busy. I think this is an
especially dangerous method since it makes out-of-memory failures harder to find by
testing. I'm in favor of:
crank the input device up to full speed and see what happens
but to test our program above using a caching memory allocator we need
crank the input device up to full speed, make Linux busy and
also do something to exercise all other modules that may use
the memory allocator so that they use max memory.
All that said and done, however, since implementing a cache memory pool is the work of
maybe
an hour, at best, this discussion is more about philosophy of design than about
anything concrete.
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