CODEWITHSUNDEEP
javamemorygarbage-collectionjvm
Patrick Collins
Patrick Collins

Reputation: 10574

Can I make the heap too big?

I understand that a larger heap means longer GC pauses. I'm okay with that -- my code is doing analysis of some data, and all I care about is minimizing the time spent doing garbage collection, the length of a single pause doesn't make a difference to me.

Can making the heap too large hurt performance? My understanding is that "young" objects get GC'd quickly, but "old" objects can take longer, so my worry is that a large heap will push some short-lived objects into the longer-lived space. I do a lot of allocation of strings that get thrown away quickly (on the order of 60 GB over the course of a single run) and so I don't want to increase GC time spent on those.

I'm testing on a machine with 8 gb of RAM, so I've been running my code with -Xms4g -Xmx4g, and as of my last profiled run, I spent about 20% of my runtime doing garbage collection. I found that increasing the heap to 5 gb helped reduce it. The production server will have 32 gb of RAM, and much higher memory requirements.

Can I safely run it with -Xms31g -Xmx31g, or might that end up hurting performance?

Upvotes: 3

Views: 3076

Answers (1)

Peter Lawrey
Peter Lawrey

Reputation: 533442

Can making the heap too large hurt performance?

When you go over 31 GB you can lose CompressedOops which can mean you have to jump to 48 GB just to get more usable memory. I try to keep under 31 GB if I can.

My understanding is that "young" objects get GC'd quickly, but "old" objects can take longer, so my worry is that a large heap will push some short-lived objects into the longer-lived space.

For this reason I tend to have large young generations, e.g. up to 24 GB.

Can I safely run it with -Xms31g -Xmx31g, or might that end up hurting performance?

On a 32 GB machine this would be very bad. By the time you include the off heap the JVM uses, the OS, the disk cache, you are likely to find that a heap over 24-28 GB will hurt performance. I would start with 24 GB and see how that goes, you might find you can reduce it will little effect if 5 GB runs ok now.

You might find moving your data off heap will help GC times. I have run systems with 1 GB heap and 800 GB off heap, but it depends on your applications requirements.

I spent about 20% of my runtime doing garbage collection

I suggest you reduce your allocation rate. Using a memory profiler you can reduce your allocation rate to below 300 MB/s, but less than 30 MB/s is better. For an extreme system you might want less than 1 GB/hour as this would allow you to run all day without a minor collection.

Upvotes: 8

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