5 sec boot

[david at lang.hm]

On Sat, 4 Oct 2008, Mitch Bradley wrote:

> Deepak Saxena wrote:
>> On Oct 03 2008, at 07:34, Mitch Bradley was caught saying:
>> If
>>   it is stored deocompressed to begin with on the filesystem, we
>>   can simply read it into mem from flash and run. This would require
>>   a few extra MiB of flash. Granted, reading from flash is
>>   slower than reading from memory, so we may not see a a net
>>   benefit.  Easy enough to test...
>>
>
> Some rules of thumb for OLPC:
>
> a) Memory copy goes at about 500 MB/sec (so can be neglected in this case)
> b) zlib decompression goes at about 3 MB/sec for typical code (2:1
> compression ratio)  (Over the complete filesystem, JFFS2's automatic
> zlib compression gets you about 3:2 space savings, because of various
> overhead factors).
> c) Raw FLASH read time maxes out at 20 MB/sec.  But you don't get that
> speed from the filesystem; JFFS2 is good for between 5 and 10 MB/sec.
>
> Considering all the intricacies of JFFS2, my best guess is that it's
> going to be close to a wash whether the kernel + initrd is stored in
> compressed or uncompressed form.
>
> OTOH, if the kernel + initrd were in a separate partition in e.g. romfs
> format, where OFW could just blast them into memory without doing JFFS2
> node processing, we could probably get close to the 20 MB/sec speed.
>
> Another holdup is the time it takes to do the hash calculation for
> security checking.  That costs a couple of seconds.  Ivan and I worked
> pretty hard to minimize that time, choosing one of the faster hash
> functions.

can you do the hash as you copy it? it should be pretty close to free at 
that point (since the CPU is waiting for memory/flash access it can do the 
hash calculationwhen it would otherwise be stalled)

David Lang