[crac] RFR: Correct System.nanotime() value after restore

[Ashutosh Mehra]

On Thu, 23 Mar 2023 15:38:35 GMT, Radim Vansa <duke at openjdk.org> wrote:

> There are various places both inside JDK and in libraries that rely on monotonicity of `System.nanotime()`. When the process is restored on a different machine the value will likely differ as the implementation provides time since machine boot. This PR records wall clock time before checkpoint and after restore and tries to adjust the value provided by nanotime() to reasonably correct value.

src/hotspot/share/runtime/os.cpp line 2051:

> 2049:     diff_millis = 0;
> 2050:   }
> 2051:   javaTimeNanos_offset = checkpoint_nanos - javaTimeNanos() + diff_millis * 1000000L;

Can you please explain why isn't `javaTimeNanos_offset = checkpoint_nanos - javaTimeNanos()` sufficient? What am I  missing here?

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PR Review Comment: https://git.openjdk.org/crac/pull/53#discussion_r1146521174