5 sec boot
david at lang.hm
david at lang.hm
Sun Oct 5 12:32:59 EDT 2008
On Sat, 4 Oct 2008, Mitch Bradley wrote:
> Deepak Saxena wrote:
>> On Oct 03 2008, at 07:34, Mitch Bradley was caught saying:
>> 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
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)
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