Understanding ZGC details

Simone Bordet simone.bordet at gmail.com
Fri Jul 13 17:35:12 UTC 2018


first of all congrats and thanks for ZGC.

I've been running a benchmark of mine
(https://docs.cometd.org/current/reference/#_benchmarking), a
Servlet-based chat application on top of Jetty, to compare G1,
Shenandoah and ZGC.
The hope is to produce some conference material to spread the word
about ZGC (for which personally I have high expectations).

Meanwhile I'm trying to understand a bit more about ZGC and I have few
questions (I have read the GC handbook from Jones et al., but I'm new
to Linux memory tricks so please pardon my ignorance on the matter; I
have also watched Per's presentation at JFokus 2018).

I don't quite get the mechanism of colored pointers and how it exactly works.
I see that ZGC maps the heap space into 3 views (in
What is the purpose of mapping into 3 views (I get the number 3, it's
for the number of phases; but why cannot they be handled with just 1

Skimming around in the code - not deeply, I admit - seems to be that
pointers to those views are always de-colored before being
dereferenced, so I am a bit puzzled about those 3 views.
Assuming I'm wrong, is the mechanism similar to C4, where a view is
mprotect()-ed on a phase flip?
If so, I am also assuming that this would generate a SEGV that will
run some code that will fix the pointer. Can you please detail where
is the SEGV handling code that does that?

I'm stopping here feeling that I'm offroad already :)

I'm sure if you put me back on the road I'll have more questions.


Simone Bordet
Finally, no matter how good the architecture and design are,
to deliver bug-free software with optimal performance and reliability,
the implementation technique must be flawless.   Victoria Livschitz

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