CPU Features
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Note
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CPU Features are available since zulu17.46, zulu21.30 (October 2023).
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CPU features differ from host to host as newer CPUs offer support for newer instructions, and some optional features may be removed over time. The JVM (Java Virtual Machine) depends on CPU features (such as AVX512, SSE3, etc.) of each computer it is running on. The more CPU features JVM can use, the better performance it can provide. JVM normally uses all the CPU features it can find on the CPU where it has started.
CRaC unfortunately cannot reconfigure an already running JVM to stop using some of the CPU features when being restored on a CPU which is missing those features.
Generic Target
You can specify the most generic CPU target by providing -XX:CPUFeatures=generic for checkpoint. The VM uses only features available on every x86-64 CPU.
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Note
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The generic target prevents using CPU features that are available, thus it brings performance impact.
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To avoid performance impact associated with generic, you can follow the advice printed by the VM and specify an exact feature set as detected on the target machine.
Specifying CPU Features
You may create a snapshot (-XX:CRaCCheckpointTo=PATH) on a newer computer and restore it (-XX:CRaCRestoreFrom=PATH) on an older computer.
In such case you may see, during restore, a variant of this error message:
$JAVA_HOME/bin/java -XX:CRaCRestoreFrom=cr
You have to specify -XX:CPUFeatures=0x21421801fcfbd7,0x3e6 together with -XX:CRaCCheckpointTo when making a checkpoint file; specified -XX:CRaCRestoreFrom file contains CPU features 0x4ff7fff9dfcfbf7,0x3e6; missing features of this CPU are 0x4de3de79c000020,0x0 = 3dnowpref, adx, avx512f, avx512dq, avx512cd, avx512bw, avx512vl, sha, avx512_vpopcntdq, avx512_vpclmulqdq, avx512_vaes, avx512_vnni, clflushopt, clwb, avx512_vbmi2, avx512_vbmi, rdpid, fsrm, gfni, avx512_bitalg, pku, ospke, avx512_ifma
If you are sure it will not crash you can override this check by -XX:+UnlockExperimentalVMOptions -XX:+IgnoreCPUFeatures .
<JVM exits here>
Just follow the advice from the complaining system running -XX:CRaCRestoreFrom=PATH when configuring the initial system creating the snapshot (-XX:CRaCCheckpointTo=PATH).
$JAVA_HOME/bin/java -XX:CRaCCheckpointTo=cr \
-XX:CPUFeatures=0x21421801fcfbd7,0x3e6 \
-jar target/spring-boot-0.0.1-SNAPSHOT.jar
Merging CPU Features
If you restore the CRaC image on computers with different CPU features, you may need to use logical and (&) of the suggested -XX:CPUFeatures option.
# Computer A used for an image restore:
You have to specify -XX:CPUFeatures=0x21421801fcfbd7,0x3e6 together with -XX:CRaCCheckpointTo when making a checkpoint file; [...]
# Computer B used for an image restore:
You have to specify -XX:CPUFeatures=0x4b03c643c9869,0x173 together with -XX:CRaCCheckpointTo when making a checkpoint file; [...]
# Compute common minimal set of features of computers A and B:
python -c 'print(hex(0x21421801fcfbd7 & 0x4b03c643c9869)+","+hex(0x3e6 & 0x173));'
0x18003c9841,0x162
# Computer used for the image snapshot:
$JAVA_HOME/bin/java -XX:CRaCCheckpointTo=cr -XX:CPUFeatures=0x18003c9841,0x162 -jar target/spring-boot-0.0.1-SNAPSHOT.jar
Command Line Options
-XX:CPUFeatures
Native
-XX:CPUFeatures=native is the default. You may want to use it to override another -XX:CPUFeatures option specified earlier on your command line.
Generic
If the CRaC image is restored on computers with different CPU features, the most simple solution is to just run Java with:
$JAVA_HOME/bin/java -XX:CRaCCheckpointTo=cr \
-XX:CPUFeatures=generic \
-jar target/spring-boot-0.0.1-SNAPSHOT.jar
Unfortunately, in this case, the run has suboptimal performance. Still, the difference may not be even measurable. To obtain an optimal performance, check Merging CPU Features for more info.
Ignore
If you have any problems with this CPU features handling code, you can disable it completely by using:
$JAVA_HOME/bin/java -XX:CRaCCheckpointTo=cr \
-XX:CPUFeatures=ignore \
-jar target/spring-boot-0.0.1-SNAPSHOT.jar
Please provide feedback to the CRaC project in such a case.
-XX:+ShowCPUFeatures
You may use the option -XX:+ShowCPUFeatures to easily detect parameters for the -XX:CPUFeatures option on the computer intended to run the -XX:CRaCRestoreFrom=PATH option. Using --version as in this example, is not mandatory but otherwise the CPU features may scroll away.
$JAVA_HOME/bin/java -XX:+ShowCPUFeatures --version
This machine's CPU features are: -XX:CPUFeatures=0x4ff7fff9dfcfbf7,0x3e6
CPU features being used are: -XX:CPUFeatures=0x4ff7fff9dfcfbf7,0x3e6
openjdk 22-internal 2024-03-19
OpenJDK Runtime Environment (fastdebug build 22-internal-adhoc.azul.crac-git)
OpenJDK 64-Bit Server VM (fastdebug build 22-internal-adhoc.azul.crac-git, mixed mode)
-XX:+IgnoreCPUFeatures
In some cases, you can see the following error:
$JAVA_HOME/bin/java -XX:CRaCRestoreFrom=cr
You have to specify -XX:CPUFeatures=0x21421801fcfbd7,0x3e6 together with -XX:CRaCCheckpointTo when making a checkpoint file; specified -XX:CRaCRestoreFrom file contains CPU features 0x4ff7fff9dfcfbf7,0x3e6; missing features of this CPU are 0x4de3de79c000020,0x0 = 3dnowpref, adx, avx512f, avx512dq, avx512cd, avx512bw, avx512vl, sha, avx512_vpopcntdq, avx512_vpclmulqdq, avx512_vaes, avx512_vnni, clflushopt, clwb, avx512_vbmi2, avx512_vbmi, rdpid, fsrm, gfni, avx512_bitalg, pku, ospke, avx512_ifma
If you are sure it will not crash you can override this check by -XX:+UnlockExperimentalVMOptions -XX:+IgnoreCPUFeatures .
<JVM exits here>
You may be sure the missing CPU feature is not really required for the run of JVM. You can enforce JVM to run even in such a case with -XX:+IgnoreCPUFeatures. Logically, JVM may still crash by segmentation fault (on UNIX) or other fatal error (on MS-Windows) later during its run, due to the missing CPU feature(s). You have been warned!
$JAVA_HOME/bin/java -XX:CRaCRestoreFrom=cr \
-XX:+UnlockExperimentalVMOptions \
-XX:+IgnoreCPUFeatures