Tuning Garbage Collection parameters in Java

问题:

I have a server java component which has a huge memory demand at startup which mellows down gradually. So as an example at startup the memory requirement might shoot unto 4g; which after the initial surge is over will go down to 2g. I have configured the component to start with memory of 5g and the component starts well; the used memory surges upto 4g and then comes down close to 2g. The memory consumed by the heap at this point still hovers around 4g and I want to bring this down (essentially keep the free memory down to few hundred mb's rather than 2g. I tried using the MinFreeHeapRatio and MaxFreeHeapRatio by lowering them down from the default values but this resulted in garbage collection not being triggered after the initial run during the initial spike and the used memory stayed at a higher than usual level. Any pointers would greatly help.

回答1:

First, I ask why you are worried about freeing up 2 GB of ram on a server? 2GB of ram is about $100 or less. If this is on a desktop I guess I can understand the worry.

If you really do have a good reason to think about it, this may be tied to the garbage collection algorithm you are using. Some algorithms will release unused memory back to the OS, some will not. There are some graphs and such related to this at http://www.stefankrause.net/wp/?p=14 . You might want to try the G1 collector, as it seems to release memory back to the OS easily.

Edit from comments What if they all choose to max their load at once? Are you ok with some of them paging memory to disk and slowing the server to a crawl? I would run them on a server with enough memory to run ALL applications at max heap, + another 4-6GB for the OS / caching. Servers with 32 or 64 GB are pretty common, and you can get more.

回答2:

You have to remember that the JVM reserves virtual memory on startup and never gives it back to the OS (until the program exits) The most you can hope for is that unused memory is swapped out (this is not a good plan) If you don't want the application to use more than certain amount of memory, you have to restrict it to that amount.

If you have a number of components which use a spike of memory, you should consider re-writing them to use less memory on startup or use multiple components in the same JVM so the extra memory is less significant.