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<font size="4"><font face="monospace">Do you have any data on the
relative distribution of the three buckets? </font></font><br>
<br>
<div class="moz-cite-prefix">On 1/23/2023 1:50 PM, Dan Heidinga
wrote:<br>
</div>
<blockquote type="cite" cite="mid:CAJq4Gi4X0bOL7pv5e2iTTiAbBSEuQxN6wvDSmz9g_a0-wPTckg@mail.gmail.com">
<div dir="ltr">Picking up on the Class Init discussion from before
the holidays, here's the next piece looking at how static fields
can be characterized:
<div><br>
</div>
<div>The next major step in our investigation was to try to
replace the GraalVM Substitutions with the IODH pattern. Our
assumption at the time was that we could group class
initializations into two buckets - one for build time init and
one for runtime init.</div>
<div><br>
</div>
<div>Unfortunately, as we worked on adapting the Substitutions,
we found that a third bucket needed to be introduced. This is
the "reinitialize bucket" for classes that needed to be
initialized at build time and then reinitialized at runtime as
they hold values that are specific to the current system.</div>
<div><br>
</div>
<div>Classes also unfortunately don't neatly stay in one
bucket. Many of the classes in the "runtime init" bucket can
be dragged into the "reinitialize" bucket if their
initialization is triggered by a use in the "build time init"
bucket. </div>
<div><br>
</div>
<div>The characterization of a given class is often
application-specific - in *this* app, class Foo can be runtime
initialized but in *that* app Foo needs to be reinitialized.</div>
<div><br>
</div>
<div>Here's an example that shows how the runtime bucket can
easily be moved to the reinitialization bucket:</div>
<div><br>
</div>
<div>java.nio.Bits has fields, like PAGE_SIZE, which can be
converted to use the IODH pattern provided the IODH helper
class is only initialized at runtime. If it is initialized
earlier (ie: build time), then it must be in the
reinitialization bucket. Any value that is calculated from
PAGE_SIZE may also need to be reinitialized. </div>
<div><br>
</div>
<div>Alternatively, some kind of constraint that prevents the
app from running could be registered that ensures the page
size is consistent between build and runtime envs.</div>
<div><br>
</div>
<div>Bits also has fields like COUNT which represents the number
of reserved direct memory allocations. This field (and
related fields) could be grouped into an IODH helper class but
has the same characteristics as PAGE_SIZE in that it belongs
in the "runtime" bucket but may be dragged into the
"reinitialization" bucket by build time use. (ie: a library
used at build time that temporarily creates a DirectByteBuffer
to speed up processing of some json file and then discards the
DBB. Initialization of Bits' IODH helper will still have
occurred).</div>
<div><br>
</div>
<div>Similarly we may need constraints (or raise errors?) if
COUNT is non-zero when transitioning from between phases
(build -> runtime).</div>
<div><br>
</div>
<div>See appendix B [0] for more examples.</div>
<div><br>
</div>
<div>The use of the IODH pattern to group static fields provides
two benefits:</div>
<div>1) it group fields having similar lifetimes together and
allows them to benefit from delayed initialization (ie: init
groups rather than all fields)</div>
<div><br>
</div>
<div>2) It makes it easier to reset the fields as smaller
groupings are being reset (smaller blast radius).</div>
<div><br>
</div>
<div>The IODH pattern doesn't help to define the constraints
under which a class may be reused as we still need developer
intent to be expressed to know what are the constraints. </div>
<div><br>
</div>
<div>It also doesn't help to track the uses of the fields in the
IODH helper class as once they've been read and used, the
dependency information is lost.</div>
<div><br>
</div>
<div>It's still a useful tool, but doesn't solve all use cases.</div>
<div><br>
</div>
<div>--Dan</div>
<div><br>
</div>
<div><br>
</div>
<div>[0] <a href="http://cr.openjdk.java.net/~heidinga/leyden/ClassInitPlan.pdf" moz-do-not-send="true" class="moz-txt-link-freetext">http://cr.openjdk.java.net/~heidinga/leyden/ClassInitPlan.pdf</a></div>
</div>
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