JFFS2 as root FS

[Vipin Malik]

> Now I am having some other problems :-(
> First it takes about 1 min 30 sec to mount my root FS. Is that expected? I
> was hoping for a much shorter
> mount time, 10-20 sec
> The root partition is 15 MB and df reports:
>
> Filesystem           1k-blocks      Used Available Use% Mounted on
> /dev/mtdblock1           15104      6580      8524  44% /mnt/mtdblock1

Just for reference, my AMD SC520 -100MHz (a 100MHz 486+) with a 8MB, 57% full
JFFS2 f/s takes ~19 seconds to mount.
The flash is connected on a 32bit bus. Even if it takes 700us (for max wait
states on my system) per quad word read (32bits), the entire flash can be
read in 1.46 seconds.

Even if your flash is connected x8 wide, your flash read times should not be
more than 10-12seconds.

Obviously, as my flash read time is only ~2 seconds of my 19s fs mount time,
the processor speed plays quite a significant role in the mount process.

To test this theory, I upped the speed to 133MHz and then it took ~15seconds
to mount.
The increase in clock speed was 33%, the mount was (19-15)/15 = 26% faster.
However, if I remove the "constant" portion of the two mounts- the flash read
which is not any faster, then the increase in
mount speed  = ((19-1.46) - (15-1.46))/ (15 - 1.46) = 29.5% faster, almost
linear with the increase with clock speed!
(This also tells me that the code is executing primarily out of cache).

If you say, that your system takes ~90seconds to mount. Let's say that your
flash read time is ~10 seconds (for 16MB), then if all else is
ok, your processor should be ~80/17.5 = 4.5 times slower than mine. This
assumes equal amount of processing. But your processor is doing about
(6580/4636 = 140%) more
processing than mine. Assuming linear overhead, then the new factor is ~57/18
= 3.1X slower. (your flash usage is 6580 blocks, mine is 4636 blocks).

Now you say that you are using a MPC869T. I did not find a 869T on the web. I
presume that you meant 860T. At what MHz?

The 860T is claimed to have a performance of ~66 Dhrystone MIPS at 50MHz. I
downloaded Dhrystone 2.1 and that give me 51 mips at 100Mhz (and 67.7at
133MHz).

However, my 486 has a 16KB combined cache and a 66MHz SDRAM bus. The FLASH is
not cached, so the entire cache can be used for the instruction execution.
The 860T has only 4KB of instruction cache. Under some circumstances (when
the code fits in the cache in my processor and not in yours), the SC520 will
behave like a 100MHz machine while the PPC will be limited to the bus clock
speed. What is you bus clock speed (and your core clock speed).

If your BUS clock is ~25-33 MHz, it's not unbelievable that your processor
runs ~2-3X slower, specially if your processor core is running at <50MHz.
Also remember that execution from cache is significantly faster than
execution from EDO DRAM on the bus.

Something to think about. Of course it could be something completely
different, like a bug in David's code ;)

Vipin



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