8212084: Implement UseGCOverheadLimit for G1 #27936
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Hi all, please review these changes to implement the `UseGCOverheadLimit` functionality for G1 (and make the implementation for Parallel GC have similar output). The `UseGCOverheadLimit` feature prematurely returns `null` from a GC if GC cpu usage limits and heap usage limits are met for some time. This is to avoid a VM limping along if garbage collection gets into an endless cycle of garbage collections or until a "real" OOME is thrown. What is important here is how this works (derived from the Parallel GC implementation): * check overheads at the end of the (initial) garbage collection (before upgrading) to see whether we are over the limits for a successive amount of GCs. * keep doing GCs without actually allocating memory for the allocation request to keep on measuring gc CPU usage. This is important for measuring the correct cpu usage in case of the application being able to free memory on the OOME. Testing: tier1-5 without any OOMEs due to this feature, test case Thanks, Thomas
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Hi all,
please review these changes to implement the
UseGCOverheadLimitfunctionality for G1 (and make the implementation for Parallel GC have similar output).The
UseGCOverheadLimitfeature prematurely returnsnullfrom a GC if GC cpu usage limits and heap usage limits are met for some time. This is to avoid a VM limping along if garbage collection gets into an endless cycle of garbage collections or until a "real" OOME is thrown.What is important here is how this works (derived from the Parallel GC implementation):
Check overheads at the end of the (initial) garbage collection (before upgrading) to see whether we are over the limits for a successive amount of GCs. If so, keep doing GCs without actually allocating memory for the allocation request to keep on measuring gc CPU usage. It is important to measure the correct cpu usage in case of the application being able to free memory on the OOME. Otherwise GC cpu usage will go down again and might result not exceeding the threshold next time, which will reset the gc counter. In that case we may ping-pong between exceeding cpu usage and not all the time.
I do not have an opinion on whether the application trying to handle OOMEs/try to recover is a good idea after having determined that overhead is too large for quite some time now; however this seems to be the only valid reason why after the first time the overhead limit is exceeded Parallel does not just always return
nullafter any subsequent GC.Note that G1 and Parallel measure CPU time differently (e.g. G1 uses the last 10 GCs, and takes concurrent work into account, while Parallel takes the last 32 GCs as "long term cpu usage". Also the memory usage calculation is different, so the overall sensitivity is different. There is nothing that can be done about this imo.
Testing: tier1-5 without any new OOMEs due to this feature (it's enabled by default in release builds like in Parallel), test case
Thanks,
Thomas
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Using
gitCheckout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/27936/head:pull/27936$ git checkout pull/27936Update a local copy of the PR:
$ git checkout pull/27936$ git pull https://git.openjdk.org/jdk.git pull/27936/headUsing Skara CLI tools
Checkout this PR locally:
$ git pr checkout 27936View PR using the GUI difftool:
$ git pr show -t 27936Using diff file
Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/27936.diff