Q: Cramfs Vs. Ubifs

[Ricard Wanderlof]

On Thu, 14 Jun 2012, Nathan Lynch wrote:

>
>> Can you please elaborate why accessing the flash while main "real-time
>> application" is running it is not a problem. I also read in
>> free-electrons tutorial on embedded linux such recommendation for
>> using file system such as cramfs, ubifs, which also access the flash
>> during execution.
>> Why is it not a problem for embedded linux ? From my experience with
>> other embedded OS, it is not recommended to access flash during
>> execution.

I think the question to ask is 'why is it a problem'? If you can meet your 
real time requirements even if there is latency due to parts of the 
application being fetched (i.e. paged in) runtime, it is not a problem.

If you do rely on your application having millisecond response times, then 
you have quite stringent real time requirements, and you may need to lock 
your application in memory to avoid latency due to paging. You may also 
have to look at configuring the Linux kernel so that it doesn't introduce 
any latency due to task switching. And there may be other issues in that 
case, with paging from mass storage being but one of them.

For reference, the company I work for makes network cameras, running 
embedded Linux, which is definitely a real time application (streaming 
live video), but our requirements are such that we have not needed to take 
measures such as mlockall in order to keep applications in memory at all 
times.

So I wouldn't say as a general rule that accessing flash memory during 
execution is a problem, it all depends on what your requirements are.

> On Thu, 2012-06-14 at 08:37 +0300, Ran Shalit wrote:
>
> Did you read the man page for mlockall?  It does seem to address your
> concern as I understand it.
>
> (This has become off-topic for the MTD list IMO.)

Principally, yes, but since MTD devices tend to be used primarily in 
embedded systems, and they are often the ones with the most real time 
intensive requirements, there is a certain relevancy even if it is not 
directly related to mtd.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30