Mike Pall (LuaJIT author, maintainer) makes the point that the case presented isn't close to real applications: http://www.freelists.org/post/luajit/LuaJIT-benchmark,6
And first and foremost, according to Pall, "it's just measuring the overhead of branch mispredictions and cache misses for an atypical use case of hash tables."
If I could rerun this afternoon, what test case should I use? What is a good hash test? Mike says
[Typical hash table accesses use a) string keys with b) a high temporal correlation and c) they usually have a high hit-rate or a high miss-rate, but it's rarely 50/50.]
Surely there is a simple, simple routine that does all this. I want to see this benchmark done right. If it's so simple, why hasn't anyone written it down? The author updated:
If have some extra time I'll test some variables later on I will test different hit rates and key types. If you have a reference to a source where statistics around hash table use cases is investigated I'll use that as a basis.
When microbenchmarking Java it is a good idea to use a framework to make sure you get an accurate result despite the compiler trying to eliminate your code. In their case it looks like they are getting real numbers since Java and C are the same order of magnitude, but blog posts like this are where people learn benchmark hygiene.
I recommend JMH which I believe is what was developed @ Oracle to handle this task.
I also found the discussion of various map implementation tradeoffs and caveats useful. That Google group is a great resource.
One thing you have to keep in mind when benchmarking maps is locality and cache affinity and how things will work outside the benchmark when things don't fit in cache or have been evicted by your application.
In my case map lookups showed up in profiles relating to various lookup tables that define logic and policy. There can be a surprising number spread out as well as some hit hard in tight loops.
The biggest issue with this benchmark is the unrealistic data model and the access pattern.
From my admittedly limited knowledge a better benchmark would involve:
- String keys because who uses int keys? It would be important to create a new set of query keys for retrieving values that would contain a mix of newly created strings as well as older ones to account for any hashcode memoization.
- Non-primitive values. This would address the boxing overhead someone mentioned and would be much closer to real life usage.
- Many more runs. Since there are performance implications of data access patterns due to spatial locality using a larger number of runs this would help even those out.
Another question is, why? The ultimate choice of Java vs C++ won't depend on how fast the standard hash tables are.
The Java benchmark probably shows mostly boxing overhead, which probably isn't the bottleneck in most real applications. It is, therefore, more interesting to test with strings.
As another comment suggests, large maps are likely to be accessed concurrently, and so a comparison of concurrent hash maps would also be interesting.
that's because in java you can't distinguish between no object found, and null with the default hashmap implementation. in this "benchmark" it would probably not matter though.
> A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.
Assuming you care about the distinction between null and no such key (for many applications you can avoid storing null, IMO), you can actually perform the containsKey() after the get() for only keys which get() returns null. I think that see all of the perf overhead of a single lookup if most values are non-null.
Plus you're just testing the standard library implementations. If the hash lookup was the major focus for your program, you would be using a specialized hash map implementation.
EDIT: Here is the kind of performance differences you get between hash implementations to demonstrate why you wouldn't just use the standard implementations if it's a performance critical part: http://b010.blogspot.com/2009/05/speed-comparison-of-1-javas...
Shows Colt int map running 3.5x faster and with 65% of the ram usage as the standard hashmap.
And for C++, http://bannalia.blogspot.com/2014/01/a-better-hash-table-gcc... shows significant differences between 3 different implementations of the same API (and which one is fastest varies between operations, as they're optimized for different things).
> Plus you're just testing the standard library implementations. If the hash lookup was the major focus for your program, you would be using a specialized hash map implementation.
If there's a better general-purpose hash table, the standard library hash should be using it.
(Special-purpose, sure, use a custom data structure.)
In the case of Java, there is a lot of inefficiency in the standard hash table in the form of Entry<T, K> objects for making it a bit nicer to use. There is also a lot of extra state stored for some of the extra features offered. Everything has a cost. If you just need a straight Int->Int or Int->Object map, there are a lot of ways to make it far more efficient.
The standard library doesn't contain these, because in 99% of apps, the actual performance of the hash table is a very minor part and a small bit of overhead for added convenience is worth it. However in a program where the hashmap is the key point, you'd obviously choose to optimize it to fit your goals.
Now you are never going to satisfy everyone with your choice of the contenders :) If you allow C libraries it would interesting to see where Judy falls in that spectrum, it has quite a reputation for being fast, although the code itself is quite impenetrable.
I'm curious what the takeaway from this is. Specifically, I'm curious how well these results translate to the use of a hash table.
If your application is even 80% giant hash table lookup, I would presume it is doing so in a fairly highly multithreaded environment. Caching systems, maybe?
Otherwise, I would think the times where this benchmark matters are few and far between. If you find you are looking up a ton of items in a hash table in such a way that that shows up in a profile, you are probably best off moving to a sorted list and just iterating through the items as you need. Right?
