为什么堆内存使用图看起来像这样？

Question

I have install glowroot (java application monitoring) to my JVM. When my application idles, I get this kind graph formation of memory heap usage. The pattern seems almost uniform. Could someone please explain and point me to whatever blog post on why does the graph looks like that? I am curious.

Answer 1

The large-scale sawtooth pattern represents the memory utilization between GC cycles. The application is allocating objects steadily (the upsloping line) until the heap gets full enough for the VM to decide to run the GC (the point). Then the GC reclaims a large amount of garbage (the steep drop) and the process starts again.

The short spikes up and down are harder to understand. It is possible that the upward spikes represent anomalous "large" allocations (of short life-time objects) that are triggering a young generation cycle. The downward spikes might represent cached objects being freed in response to "memory pressure".

If you want a better understanding of the spikes, you need to look at the GC log messages, and try to correlate them with the graphs.

Answer 2

It looks like that because (at the least) you're observing it. If your application did absolutely nothing, and there were no threads doing any allocations, you'd get a horizontal line for the heap.

However since you're observing it, there are things being done in the JVM to get that data back to you. That's why you get the ubiquitous sawtooth pattern seen in many, many profiling questions. A few examples below.

java memory leak, visualvm showing wrong data

visualvm monitors memory usage

Answer 3

This is the java garbage collection at work. Each drop is the garbage collector freeing memory by removing unused references. The reason why it grows again is simply because your application keeps creating new references.

You can also have a look at this post for a more in-depth explaination: Why a sawtooth shaped graph?