Thursday, June 28, 2012

First Impressions of Xtend

I've been interested in Xtend for a while. It's an improvement over Java, without being a big jump like Scala.

I finally got around to playing with it a little. I took one of my Java classes from jSuneido and rewrote it in Xtend. But I got stuck with some weird errors around array access. I did some searching on the web and found that Xtend doesn't support array indexing e.g. array[i]. I was surprised. You can pass arrays and do for loops over them, but not index them.

I imagine part of the reason for this is that square brackets are used for closures, although in different contexts, so it seems like you could still parse it ok.

Then I saw you can write array.get(i) - more verbose, but more consistent with other containers. (Personally, I'd rather be able to use x[i] or x(i) like Scala for all the containers.)

Then I discovered that the way it handles .get(i) is to wrap the array in another class. This is similar to how Scala does implicit conversions to add methods to existing classes.  There's nothing wrong with this approach, but it means allocating a new object for every array access. Considering arrays are usually used for low level efficiency, this doesn't seem ideal. Granted, in some cases the JIT compiler may be able to eliminate the allocation, and if this technique gets common, it's more likely it will be a target for optimization.

It seems like a better approach would be to recognize array.get(i) and compile it to array access byte codes, without any wrapping. Maybe that's hard to do in their compiler.

Another alternative is to leave array code in Java. Which is fine, but doesn't leave a good first impression.

One of the justifications given for not supporting array indexing is that you should use the higher level containers instead. I think there are times when the extra efficiency of bare arrays is justified, but for the most part I agree. And, in fact, the class that I converted could just as easily use ArrayList's rather than arrays, with likely not too much performance penalty.

Even after I fixed this I still had some weird errors left. It turned out that Xtend doesn't have increment and decrement operators (++ and --) Fair enough, Scala doesn't have them either. But I was more surprised to find that it didn't handle += either, although it is listed as an operator in the documentation. For a language that claims "Less Noise", it seems odd to have to write "i = i + 1" instead of "i++".

Once I got past these issues, I was pretty happy with the results. Even without using any of Xtend's more powerful features (like closures and extension methods) the code was cleaner and less verbose, although not radically so.

One of Xtends claims is "top notch" Eclipse IDE support. Unfortunately, that doesn't mean much in the way of refactoring.

I'm still left with doubts about the future of Xtend. It doesn't have the momentum or backing of Scala. With a "smaller" project like this, it could easily stagnate, mutate, or die.

Saturday, June 23, 2012

Immudb in Production

We took the plunge and converted our (Axon's) in-house accounting and CRM system over to the new version of jSuneido with the immudb append-only storage engine. We have about 50 users so it's a decent test, although we're not as heavy users as some of our customers.

So far so good, it's been running for a couple of days with no glitches.

We'll give it a little longer and then look at converting actual customers.

Thursday, June 14, 2012

Optimizing Java Memory Usage

One of the things I've been meaning to work on for a while is the memory usage for temporary indexes in Suneido's database.

I started by measuring the current memory usage. I used two test cases, first a single table with 400,000 records sorting by a non-indexed field, and second the same table but joined with another one. I used Visual VM to watch the heap and force garbage collections to get an idea of memory usage. The first case used about 200mb and the second about 400mb. For the simple case, that's about 500 bytes per record. I was pretty sure I could improve this usage.

My assumption was that a lot of the usage was Java's per-object overhead.

For each record, the temporary index code created a new sort key record, plus an Object[] array that stored references to each of the source data records (more than one in the case of joins). These sort key arrays are then added to a MergeTree.

My first step was to switch from allocating a separate small array for each sort key, to putting them into a big array and referencing them by offset.

I could have used a standard ArrayList, but it works by doubling the size of it's internal array each time it needs to grow. At very large sizes, this is not the best strategy. For example, to grow from 100mb to 200mb you need to allocate a new 200mb array, copy the old 100mb array into it, and then the old array has to be garbage collected. On top of this, if the data is long lived, it will probably end up being moved to older generations by the garbage collector, where it won't get garbage collected as easily.

Instead, I wrote an ArraysList which uses a bunch of medium sized arrays rather than one big array. To grow it simply allocates another medium size array, without any copying or garbage collecting. This is not necessarily better in every situation, but for my usage it is.

The next problem was that now I was storing an int offset (into the ArraysList) instead of an array. My MergeTree is generic, which means to use if for int's they'd have to be boxed into Integers. Then I'd be back to too much per-object overhead. So I had to make a specialized IntMergeTree. It's too bad that Java makes such a big distinction between primitives and references, and that generics only work with references. (Scala tries to reduce the distinction, and it even lets you specialize a generic class for a primitive type, but it still ends up boxing and unboxing most of the time.)

In the common case where you're just sorting a single table, there's no need for the array at all, and instead of storing the ArraysList offset, you can just store the integer "address" of the record.

The next step was to store the sort key record in larger blocks of memory rather than individually, and again reference them by offset rather than a Java reference. For this I could use the MemStorage class I had already written to create in-memory databases for tests. It inherits from the same Storage base class as my MmapFile memory mapped file access. These are based on ByteBuffers. Each sort key record used at least three memory objects (the record, its ByteBuffer, and the ByteBuffer's array) so this was a significant saving. (This doesn't require much serialization because the sort key records are already in a serialized form.)

The end result was that the memory usage was less than 1/4 of what it was before. My first case now took about 40mb instead of 200mb, and my second case took about 90mb instead of 400mb. Not bad for a couple of days work.

I wouldn't normally recommend this kind of messing around - the overhead is seldom a big issue. But in the case of a server, with large numbers of users competing for memory, I think it was worth it.