Make ZGC run often

Question

ZGC runs not often enough. GC logs show that it runs once every 2-3 minutes for my application and because of this, my memory usage goes high between GC cycles (as high as 90%). After GC, it drops to as low as 20%.

How to increase GC run's frequency to run more often?

Answer 1

-XX:ZCollectionInterval=N - set maximum gap between collections to N seconds.
-XX:ZUncommitDelay=M - set the delay until unused memory is returned to the OS to M seconds.

Answer 2

Presently (as of JDK 17), ZGC's primary strategy is to wait until the last possible moment of the heap filling up and then do a collection. Its goals are

• Avoid unnecessary CPU load by running GC only when it's necessary.
• Start the GC early enough so that it will finish before the heap actually fills up (since the heap filling up would be bad, leading to a temporary application stall).

It does this by measuring how fast your app is allocating memory, how long the GC takes to run, and predicting at what point it should start the GC. You can find the exact algorithm in the source code.

ZGC also exposes some knobs for running GC more often (ie, proactively), but honestly I don't find them terribly effective. You can find more info in my other answer. G1 does a better job of being proactive, but whether that's good or not depends on your use-case. (It sounds like you care more about throughput than memory usage, so I think you should prefer ZGC's behavior.)

However, if you find that ZGC is making mistakes in predicting when the heap will fill up and that your application really is hitting stalls, please share that info here or on the ZGC mailing list.

Answer 3

Before tuning the GC, I would recommend to investigate why this is happening. Might have some issue/bug in your application.

[Some notes about GC]

• -XX:ZUncommitDelay=M (Check if it is supported by your linux kernel)
• -XX:+ZProactive: Enables proactive GC cycles when using ZGC. By default, this option is enabled. ZGC will start a proactive GC cycle if doing so is expected to have minimal impact on the running application. This is useful if the application is mostly idle or allocates very few objects, but you still want to keep the heap size down and allow reference processing to happen even when there are a lot of free space on the heap.

More details about ZGC config. options can be found:

• ZGC Home Page.
• Oracle Documentation