> From: Mark Thomas [mailto:ma...@apache.org] > Subject: Re: Setting the Right Amount of Memory > > So what is the impact say (picking some random numbers of the air) if > you have an app that is quite happy with 256MB of heap space but has > 1024MB allocated?
Probably unmeasurable, assuming the larger heap has not pushed the OS into paging. > I understand from what you have written that for the > young gen there isn't much impact Zero impact, actually. > but what about the old gen when there is a full GC? Since a full GC is a relatively rare event, and having a significant number of unreachable objects in the tenured generation is even rarer, the effect is negligible. > Does the fact that there is (roughly) 700MB of additional > dead objects slow down the full GC There aren't really 700 MB of dead objects when a full GC kicks in. That occurs when a survivor space is full, and there's no more room to copy objects to it from the eden space. At that point, the scan of the tenured generation space occurs, which will normally encounter only a few dead objects. (Note that it's the number of dead objects, not the size, that's of any interest here.) The eden space, where nearly all the dead objects are, does not have to be scanned for empty areas. > and if so, is it significant? Probably not, unless you've got a really strange app. Doing some testing with a fairly trivial program to create lots of short-lived objects and a fixed number of cycled long-term ones shows that a larger heap improves the "throughput" of iterations, but does plateau after a bit. Let's see if a .java attachment makes it through... (if it doesn't and you want a look at it, let me know and I'll e-mail it directly). Sample output from running on my 2 GHz, dual-core, 4 GB laptop: C:\JavaUtil>java -Xms32m -Xmx32m TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:07:56.203 -> 14:08:58.869 1595761.657 objects per second C:\JavaUtil>java -Xms32m -Xmx32m TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:09:06.209 -> 14:10:09.995 1567742.137 objects per second C:\JavaUtil>java -Xms512m -Xmx512m TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:10:27.605 -> 14:11:25.914 1715001.114 objects per second C:\JavaUtil>java -Xms512m -Xmx512m TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:11:35.517 -> 14:12:33.195 1733763.306 objects per second C:\JavaUtil>java -Xms2g -Xmx2g TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:12:57.895 -> 14:13:55.462 1737106.328 objects per second C:\JavaUtil>java -Xms2g -Xmx2g TestGC 100000000 total objects: 100000000, objects to keep: 1000, keep interval: 1000, object size: 1000 14:14:01.601 -> 14:14:58.758 1749566.982 objects per second - Chuck THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY MATERIAL and is thus for use only by the intended recipient. If you received this in error, please contact the sender and delete the e-mail and its attachments from all computers.
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