Mark,
On 4/22/25 3:38 AM, Mark Thomas wrote:
On 21/04/2025 17:49, Christopher Schultz wrote:
Ramesh,
On 4/21/25 1:06 AM, Ramesh B R wrote:
How to decide on heap memory size?
is it 25% of total memory? or 50% total memory?
What is the ideal value (in %) for heap memory ?
Only you can answer that question about your own application environment.
The percentage of total memory is completely irrelevant if you ask me.
Presumably, you have a server so that you can serve requests for the
application. If you have other services on the same server, then you
will have to balance the memory requirements of each service against
the others.
If it's a single-service server then I would dedicate ALL of the
available memory to that service. That doesn't mean 100% (or 90% or
80%) dedicated to heap memory, as there is a lot of memory usage in
Java that is not accounted for by the heap.
I would start from the other end: what does your application actually
need under typical (and worst-case) load scenarios. If your
application doesn't need a lot of heap space, then I wouldn't bother
paying for it, whether you are buying physical RAM chips or renting
them from a cloud provider.
If you have other requirements, such as minimal application pauses, or
transaction times where GC could impact your ability to meet certain
goals, then your heap space may be of critical concern. Mot modern GCs
benefit from more memory. Remember that the "cost" of the garbage
collector is related to the number of things that need to stay in
memory rather than the number of things that need to be collected. So
running GC on a large heap is no more expensive than it is running on
a small heap, assuming the same load scenarios.
Big +1 to all of the above.
A long time ago I attended some talks of Java GC, memory usage and
performance. A very quick summary is:
- Memory footprint, throughput and latency. Pick any two at the expense
of the third.
- Look at your application's steady state memory usage. It should be a
sawtooth. Take the minimum value of the sawtooth and multiply it by 5.
Allocate that much memory to the Java heap. That should give GC the room
it needs to operate efficiently.
That exact multiplier may not apply to more modern GC algorithms.
Every application is different. You'll need to do some testing to find
out what works best for your application.
Ooh, one more thing... assuming this is a long-lived service on
dedicated (possibly virtual) hardware: set your minimum and maximum heap
sizes on startup to the save value. There is zero advantage to having
the JVM constantly re-sizing the heap if you expect to use it all
eventually.
-chris
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