To recap, I need a hash table that can support the following:
- 1.5 billion 64-bit keys, uniformly and randomly distributed
- values between 16 bytes and 16 kilobytes, with sizes in a Zipfian distribution
- deployed to one machine, all in-memory
- sustained 200,000 writes per second over the course of many hours
The API should support a non-bulk, mutable, key-value interface with an append command.
The final requirement is that the source be obtainable. After all, this is just as much about finding a viable candidate as understanding how the results are achieved.
My approach to testing the initial viability of candidates was to replicate a subset of the required production load using some of our production logs. The test amounted to writing 212 million records to a bit over 78 million unique keys. Each record’s key is 8 bytes and its value 16 bytes. The value bytes are simply appended to the existing value corresponding to the key. This closely mimics our real write workload for the project.
Note that throughput and latency are the primary concerns here: we seek a consistently high write rate. Memory overhead, at this stage, is not under scrutiny. (This may strike some as odd, given the hard bounds on a single machine’s memory, but honestly the raw data set we’re seeking to store is easily within the bounds of the servers I described in my previous post. As long as nothing absurd is going on, we can afford to trade some memory for speed.)
Given the API requirements, the candidates that immediately came to mind were:
- Berkeley DB
- Kyoto Cabinet
Note that the scope here is restricted to hash table “services”, not hash table libraries. Specifically, I don’t want to manage memory, rehashing, growing, or shrinking. I’ll be covering libraries in the next post.
Under the hood these all use slightly different hashing and collision resolution schemes.
Berkeley DB uses an implementation of Litwin’s Extended Linear Hashing. In particular, it implements linear hashing using a hybrid split control: bucket overflow and load factor independently trigger splits. (Look for ffactor and do_expand in hash_page.c’s __ham_add_el().) Notably, BDB chains memory pages, not object pointers. This is a sensible optimization in a world where main memory is small and disk seeks are costly. However, the cost in code complexity is immense. For an idea of just how much attention to detail is required, download BDB’s source and check out hash_page.c’s __ham_replpair() and __ham_add_el(). It is fascinating to see how much work goes into managing the differences between small and large values. 
Kyoto Cabinet “boringly” uses the C++ stdlib’s std::unordered_map. I had trouble finding implementations other than GCC’s, so I can’t really speak to anything but that. The tr1/hashtable implementation uses chaining, with a prime bucket count and a max-load-factor-based rehashing policy. When the ratio of elements to buckets passes a certain threshold (1, I believe), a full stop-the-world rehash is performed. (_M_rehash() on line 1146) The resizing policy finds the smallest prime greater than twice the current number of buckets, and the table is resized. (_M_need_rehash() on line 455) The prime policy default can be seen here.
Redis implements its own hash table that uses chaining as well with a target load factor of 1. Interestingly, it rehashes the keys incrementally in the background, pushing updates to a new table while checking both the old and new tables for reads. The incremental work is spread over all subsequent reads and writes issued to the table. This is perhaps the easiest of the four implementations to fully understand on the first read.
Similar to BDB, memcached implements linear hashing, but it chains object pointers, not memory pages. It uses what the paper calls “load control[led]” splits, meaning that incremental rehashing occurs when the load factor exceeds a certain value. (In this case, 3/2.) Unlike Redis, it does the rehashing in an another thread in the background, not as a part of the read or write operations. assoc.c very nicely illustrates the gist of linear hashing with controlled splits; check out assoc_find() and assoc_insert(). Beware, assoc_expand() just sets up some state to signal incremental rehashing. The real guts of the rehashing is in assoc_maintenance_thread(). It is notable how much simpler the code for object-pointer chaining is than the page chaining used in BDB.
Ease of Use
Note: I’m talking specifically about ease of use from a developer perspective. I’m not qualified or interested in commenting on their operational merits here.
Without a doubt, the easiest candidate to set up, use, and analyze was Redis. Between the trivial build from source, the simplicity of the Jedis API, and the visibility provided by the INFO command, using Redis was a walk in the park. The redis.conf file has a lot of knobs but most of them can safely be ignored and the inline documentation is ample.
Kyoto Cabinet came in a close second. I forgot to set $JAVA_HOME before installing the Java bindings, which caused me some grief, but once I figured that bit out everything was right as rain. Instantiating and using it were painless if somewhat sparsely documented.
Memcached was actually a pain to use, not because of the daemon itself, but because of the client libraries available in Java. The fact that the append command required a CAS value in some clients and not in others was the main culprit. One qualm with the daemon itself is that an append command only succeeds if used on an existing key.
Finally, BDB was by far the most frustrating candidate. The configuration is arcane and poorly documented. The errors are undescriptive and often cryptic. Setting the proper combination of permissions for a client is exceedingly difficult unless you peruse the documentation with a very keen eye. The distinctions made between what configuration should be done on the EnvironmentConfig versus the DatabaseConfig is unclear and poorly documented. Despite specifying an in-memory hash database, a home directory for BDB is still required, even though it never touches it. One has to manually initialize the memory subsystem. Blah! Maybe I’m just uninitiated, but I don’t think I’ve ever been more frustrated with a piece of software. To boot, only the Heap, Queue, and Recno access methods support append puts, leaving me to manually do a get/append/put in the client. Even if BDB is fast enough, there’s absolutely no chance I’ll use it in production due to these limitations.
I’ll briefly note that memcached was so slow that it didn’t complete the test suite in the two days I left it running. As such, I’ve removed it entirely from this comparison. I was probably doing something wrong vis-a-vis configuration of the client and server. Similarly, a simple un-pipelined Redis connection proved to be incredibly slow, at least an order of magnitude slower than BDB. As such, I reran the original Redis test with a pipelined connection, flushing every 10,000 records. Both versions of the test are included in the source for posterity.
The first plot includes a baseline processing rate (‘xfer’ in the legend) which indicates how quickly the records can be read and prepared. The second simply excludes the baseline, for a clearer view. You can click through for larger versions of the plots.
You can find the source code used to run these comparisons on GitHub.
- Despite the drastically different algorithms used by BDB and KC, their results were roughly equivalent. KC’s performance proved to be slightly smoother, and seems to have reached a stable point at around 170 million records while BDB continued to degrade. A concern is that they were the only two packages that were used through JNI. This may have limited performance, but I am disinclined to investigate further as we use the JVM in production which necessitates this cost when interacting with these services. That said, tr1/hashtable’s underlying algorithm is still quite attractive. It performed smoother despite not having a hint about the number of unique keys while BDB did.
- Though Redis’ throughput proved to be about 50% greater than KC and BDB, the precipitous drops during (what I assume is) resizing are extremely worrisome. (I’m guessing it’s resizing since the distance between drop-offs roughly doubles each time.) The performance drop off just doesn’t jive with the goal of continuously high throughput. Equally worrying is the cost of at least doubling memory use during rehashing. Even though I mentioned this is a secondary concern in this comparison, it is an important operational problem.
- The performance difference between tr1/hashtable (KC) and Redis is marked, given they both use chaining. I suspect this is either a result of pipelining or JNI overhead. The purpose of adding the pipelined version of the Redis test was to emulate a scenario where issuing commands did not carry network and serialization overhead. Perhaps it optimized Redis access unfairly by instead emulating a bulk command API as opposed to just mitigating protocol overhead.