8354358: ZGC: ZPartition::prime handle discontiguous reservations correctly

[xmas92]

Prior to JDK-8350441 the VM would not have started if we received a discontiguous heap reservation with all reservations smaller than the inital heap capacity. Now we crash because ZPartition::prime does not take this into account.

However in contrast to the page cache, the mapped cache makes it trivial to support this scenario. So I propose fixing ZPartition::prime to handle any discontiguous heap reservation.

Can be provoked in a debug build by using ZForceDiscontiguousHeapReservations > 16
java -XX:+UseZGC -XX:ZForceDiscontiguousHeapReservations=17 -Xmx128m -Xms128m --version

Currently running this through testing.

---

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Issue

• JDK-8354358: ZGC: ZPartition::prime handle discontiguous reservations correctly (Bug - P4)

Reviewers

• Stefan Karlsson (@stefank - Reviewer)
• Joel Sikström (@jsikstro - Committer) Review applies to 70b0e923
• Erik Österlund (@fisk - Reviewer) Review applies to 70b0e923

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[openjdk]

@xmas92 This change now passes all automated pre-integration checks.

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8354358: ZGC: ZPartition::prime handle discontiguous reservations correctly

Reviewed-by: stefank, jsikstro, eosterlund

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At the time when this comment was updated there had been 27 new commits pushed to the master branch:

• 51ce312: 8348645: IGV: visualize live ranges
• f8ae1d4: 8354254: Remove the linux ppc64 -minsert-sched-nops=regroup_exact compile flag
• f169fc5: 8353175: Eliminate double iteration of stream in FieldDescriptor reinitialization
• ... and 24 more: https://git.openjdk.org/jdk/compare/d8bed6937311c1613d432bed73983d00dfd2a14a...master

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Webrevs

• 02: Full - Incremental (a33c7e39)
• 01: Full - Incremental (70b0e923)
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[stefank]

Looks good.

As a follow-up, we might want to move the pre-touching so that we don't start and stop threads multiple times.

[jsikstro]

Maybe something like "The partition must have size available in virtual memory when priming"? I'm reading this as the number of vmems, not the size of them combined.

[jsikstro]

Looks good. As you say, this is nicely implemented with features from the Mapped Cache.

[xmas92]

Thanks for the reviews.
/integrate

[openjdk]

Going to push as commit 97e1075.
Since your change was applied there have been 27 commits pushed to the master branch:

• 51ce312: 8348645: IGV: visualize live ranges
• f8ae1d4: 8354254: Remove the linux ppc64 -minsert-sched-nops=regroup_exact compile flag
• f169fc5: 8353175: Eliminate double iteration of stream in FieldDescriptor reinitialization
• ... and 24 more: https://git.openjdk.org/jdk/compare/d8bed6937311c1613d432bed73983d00dfd2a14a...master

Your commit was automatically rebased without conflicts.

[openjdk]

@xmas92 Pushed as commit 97e1075.

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