Someone threw a number into the discussion and said enabling the MMU would
cause performance losses of about 30-40 percent, thus if we switch to Linux at
all it would have to be ucLinux.
uCLinux does not mean "without MMU". uCLinux does support several
architectures with MMU.
The numbers are certainly not relevant, as the degradation due to
MMU-usage depends on multiple influences.
Hardware:
Maybe the CPU in fact is faster if the MMU is disabled. I am using a
NIOS CPU that can be configured with and without MMU (here in fact the
MMU hardware is not implemented in the FPGA design). While two years ago
(as Altera was up to create the MMU design) I was told the MMU would
reduce the maximum usable clock frequency by 50 % not that the design is
mature, the degradation seems to be 20 % at most. Maybe there are delays
due to additional hazards when the CPU needs to wait for memory access.
I have no numbers on that yet.
I don't suppose the clock specification varies regarding enabling the
MMU with "Hardware"-CPUs, but there might be additional hazards when
enabling it
Software:
Of course programming the MMU registers need to be updated with any
process switch. So this is only relevant if you do many process switches
and this depends heavily on the application you do.
MMU-design:
It's known that the MMU-design with ARM based chips is bad in that the
cache is between the CPU and the MMU instead of between the MMU and the
RAM (as e.g. with the x86 CPUs). This is why you need to flush the cache
whenever the MMU registers are updated. This makes the task switch very
slow.
-Michael
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