On Mon 23 Jul 2007 18:34, Andrew Morton pondered:
> On Mon, 23 Jul 2007 18:15:37 -0400
> Robin Getz <[email protected]> wrote:
> >
> > A single function which does the copy as a loop (existing) is going to
> > be much faster than the overhead of 1024 function calls to copy the
> > last k.
>
> I'd expect the overhead of the (fully-cached) instructions to be
> insignificant compared to the time to write to flash?
Maybe - the time to write a single page on NAND is pretty cheap.
SLC is 200us to program a page.
http://www.st.com/stonline/products/families/memories/fl_nand/nand_large.htm
http://www.st.com/stonline/products/families/memories/fl_nand/nand_small.htm
While you are correct - the majority of the time will be spent in the write to
flash - the overhead of the loop
- call (3 cycles)
- push a few registers on the stack (a cycle for each push),
- pop same registers off the stack (a cycle for each pop).
- return (5 cycles)
(Numbers go up and down depending on your arch - this was for 486)
If you assume three registers need to be pushed/popped - for a 1k buffer that
is 14336 cycles of pure overhead - assuming 500MHz processor (and a perfect
cache), that is 28us - about 14% of the time in overhead that doesn't need to
be there.
Can someone increase the size of the capacitors in their power supply to make
up for this - yes. But larger caps are more money.
> Lots of the kernel is pretty crappy. One of our main tools for fixing
> that is to ensure that new stuff is non-crappy.
No problem - I wasn't advocating putting something that you thought was crappy
into your tree (afterall that would be futile). Other than making the
overhead high - is there a way to make it less crappy?
If the answer is no, and Mike's latest suggestion/patch is OK - that is what I
will live with - and tell people to spend more on their power supply.
-robin
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