Excessive memory usage due to padding about concurrentlinkedhashmap HOT 5 CLOSED

I suspect that the real memory hog is the padding rather than read buffer 
count. That addition was motivated for large, high volume caches which is the 
typical use-case I tend to be exposed to. When we wrote Guava's cache, that 
became the default general purpose application cache so I shifted CLHM for the 
remaining users who needed more raw performance that Guava's.

I think the padding may not be offering a significant enough win that it could 
be removed to provide balance for low-volume caches. Could you simply replace 
the PaddedAtomicReference usages with an AtomicReference and check memory usage 
against that local SNAPSHOT? If that works, I'd prefer it over a system 
property.

The best long-term approach would be to make the number of read buffers grow 
when contention is detected. See Java 8's Striped64 for numeric sums as an 
example. I plan on implementing that approach soon in my Java 8 rewrite of 
Guava's cache [1]. I hadn't intended to implement it here too, but will if 
necessary.

[1] https://github.com/ben-manes/caffeine/

Agreed that a dynamic solution is the best approach where it grows as 
necessary.  Unfortunately I don't have access to the system where this problem 
occurred, just the heap dump from it.  The scenario was that a team upgraded 
the server from an 8 CPU machine to a 32 CPU machine and started having memory 
issues which we traced to these maps.  The max heap setting for the Java 
process was set at 896 MB and was not altered for the more powerful server.  
Looking at the heap dump, there were 777 ConcurrentLinkedHashMaps which had 
3,183,369 PaddedAtomicReferences, which calculates out to about 430 MB.  If we 
switched to AtomicReference it would save about 364 MB.  Long story short, yes, 
switching to AtomicReference would fix our scenario.

Discussing with other engineers here, PaddedAtomicReference makes sense with 
high contention scenarios with plenty of memory.  If we understand it 
correctly, it is based somewhat on assumptions about the underlying 
architecture and JVM implementation.  In our situation we have low contention 
and not so much memory so it doesn't work as well.  Ideally there would be an 
option or options to choose the behavior most applicable to an application.

An engineer here also mentioned that you can probably remove one or more longs 
from the PaddedAtomicReference since the JVM will use a certain number of bytes 
for each object.  That would slightly optimize memory usage of 
PaddedAtomicReference while retaining its intent, but wouldn't fix our scenario.

Can we rename this ticket since the System property solution is not ideal?  Or 
should this ticket be closed and a new one written?

I think removing the padding entirely is the best option. A lot of work already 
went into avoiding buffer contention and it wastes too much memory to be worth 
it. The Java 8 version will continue to improve that area and is already much 
more more mindful on memory usage. It will have a number of improvements that 
it should be a long-term replacement for CLHM, which was my initial goal with 
Guava's but didn't pan out performance-wise.

I'll try to get a release out tonight.

Release v1.4.2 that removes the padding. This may take up to 2 hours to sync 
with Maven Central.