Okay, my problem seems to be this:
http://en.wikipedia.org/wiki/Direct_memory_access#Cache_coherency
Is there a way to do this without programming kernel-level code? I've never
done this before and don't really know what to do exactly.
I found the option to drop the cache by writing a "1"
to /proc/sys/vm/drop_caches. I used this is my program with this few lines
of code:
FILE * fd;
fd = open("/proc/sys/vm/drop_caches", O_WRONLY);
write(fd, "1", sizeof(char));
close(fd);
It works most of the time, but sometimes I have to write the "1" a few more
times until the cache is really flushed. Perhaps this is only a problem
with another cache when writing to this file.
I have some more questions now:
This is a workaround, but I think no good solution. Is there any nicer way
to drop the cache in C?
Could I get performance problems at another point if I drop the cache too
often (about every sec. later)?
Is there a possibility to drop only the cache for my memory section? Or to
disable caching for this part of memory?
Is there a function like memcpy which is able to read directly from the
memory without looking in the cache?
Thank you for the help!
Nils
Am Freitag, 4. Oktober 2013 23:00:34 UTC+2 schrieb Charles Steinkuehler:
>
> On 10/4/2013 12:13 PM, Nils Zottmann wrote:
> > Has anyone an explanation for this behaviour? Perhaps it is a problem
> with
> > some kind of cache, how could I get around this? I'd like to implement a
> > ring buffer but it would have to be very large if it has to take data of
> > several seconds.
>
> You are encountering caching effects on the ARM side. The proper way to
> deal with this is to set the memory flags appropriately in the MMU,
> typically with kernel-level code that's setting up to do DMA. The PRU
> in this use case basically looks like a "smart" peripheral that can
> modify the contents of memory directly. That is the same as any other
> DMA enabled I/O device, and requires the same care with handling of the
> memory regions.
>
> --
> Charles Steinkuehler
> [email protected] <javascript:>
>
>
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