suspend-to-disk

[Mitch Bradley]

Andres Salomon wrote:
> ...
>
> Implementation questions:
>
> I'm not going to concern myself with our B1s, for they have more ram
> and less nand.  Our B2s have 128MB of ram, and 512MB of nand; B3s and
> up have 256MB of ram, and 1GB of nand.  We need to figure out just
> how much space we'd need to set aside in a snapshot partition for STD.
> I'm not sure what would be required from the OFW side; Mitch?  

First-order swag:

You need 256MB of disk to store 256MB of RAM, in the nothing-fancy scenario.

Second-order swag:

Machine code compresses about 2:1 with gzip, so if the memory were full 
of code, you would need about 128MB of disk for 256 MB of RAM.  OFW 
already includes a gunzip-compatible decompressor which is reasonably 
fast (written in C, runs from cached memory).  It would be easy enough 
to add either LZMA , LZO, or QuickLZ.  It's likely that we could overlap 
the compression/decompression with NAND access, so the speed penalty 
would be minimal.

Third-order swag:

The memory is likely to be significantly more compressible than random 
machine code, due to the likely prevalence of sparsely-encoded data.  So 
maybe the average compression in practice would be 3:1 or 4:1 or even 
better.  But we would might need to allocate space for the worst case.

Fourth-order swag:

It wouldn't be necessary to save read-only code ("text" pages) to the 
save area; just mark those pages "not present" and save the information 
necessary to page them back in.  But that would probably make the resume 
slower, because of the JFFS2 operations necessary to resolve all those 
page-in references.  The firmware part of the resume would be faster, 
but the overall suspend/resume process might take longer (or maybe not; 
you trade not having to write the data out on the way down for having to 
do more work on the way up).

The firmware part of this shouldn't be particularly difficult if we can 
make the save format reasonably straighforward.  It could get more 
difficult if we run into enough examples of "somebody has already done 
this so we have to use their existing solution even though it is massive 
overkill for our situation".