File Format Investigation: jimage

[Ioi Lam]

On 2/14/2023 1:50 PM, Brian Goetz wrote:
> Nice analysis and reverse-engineering.  (Also, nice monospace font; is 
> that Roboto?)
>
> I agree with your list of "useful properties"; storing existing JVM 
> artifacts (such as CDS archives) as well as new JVM artifacts (heap 
> object snapshots, AOT) are going to be required, and jimage is a 
> candidate here (though not the only one.)  Similarly, there's an 
> evaluation to be done, for example, whether to update the CDS format 
> to store more things (AOT, more general heap object storage), or to 
> have separate stores for these, or to overhaul the format.  But 
> whichever is chosen, embedding in jimage seems a possibility.
>
> The one assumption that I would quibble with is the suggestion for 
> using jimage as an intermediate format in the condenser pipeline.  It 
> seems credible that separate intermediate and final formats will be 
> preferable to a one-size-fits-all, since the needs of the next 
> condenser in the pipeline at build time are not likely to be the same 
> as those of the VM at runtime, and formats like CDS are optimized for 
> the VM to consume them, not to be, for example, easily queried and 
> updated.
>

FYI,

For debugging purposes, CDS can be generated with a map file that 
describes its contents in detail.

One thing on my todo list is to generate an hprof file as part of the 
CDS dump. You can load it into an hprof viewer to examine the archived 
Java objects.

But I agree that trying to extract individual elements from the CDS 
archive is not easy, and probably not worth doing.

Thanks
- Ioi


> Cheers,
> -Brian
>
>
>
> On 2/14/2023 8:39 AM, Severin Gehwolf wrote:
>> Hi,
>>
>> We have been looking at lib/modules (a.k.a jimage) as a potential
>> candidate format for Project Leyden. Along the way, I've documented how
>> the file is structured[1] and used in its current form. The document
>> discussing jimage in the context of Leyden is here:
>>
>> https://cr.openjdk.org/~sgehwolf/leyden/jimage_file_format_investigation_leyden.pdf
>>
>> Looking forward to your feedback!
>>
>> Thanks,
>> Severin
>>
>> [1]https://cr.openjdk.org/~sgehwolf/leyden/jimage_visualization_1.png
>>
>
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