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POPoor performance of reads in pthreads hash table using read-write locks
 

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    <p>I've put together a simple key value store which speaks a subset of the <a href="http://redis.io/" rel="nofollow">Redis</a> protocol. It uses pthreads on Linux to share the hash table; I use pthreads rwlocks to manage access to this table. I've been testing the K-V store using the Redis benchmark tool.</p> <p>With a single client, I can do about 2500 SET operations a second. However, it can only do about 25 GETs per second; I'd expect the other way around, so this surprises me. It scales to some extent, so if I throw 10 clients at it I'll get nearly 9000 SETs per second and around 250 GETs per second.</p> <p>My GET code is pretty simple; I lock the table, find the appropriate hash table location, and check for a matching key in the linked-list there. For a GET, I use <code>pthread_rwlock_rdlock</code> and <code>pthread_rwlock_unlock</code> when I'm done. For SET, I use <code>pthread_rwlock_wrlock</code> and <code>pthread_rwlock_unlock</code>. SET is quite a bit more complex than GET.</p> <p>I also made the code work on Plan 9, using shared-memory processes and their own implementation of read/write locks. There, GETs are almost as fast as SETs, instead of 100x slower. This makes me think my hash table code is probably ok; I use exactly the same hash table code for both OSes, I simply use #defines to select the appropriate lock for each OS (the interface is the same in both cases, lucky!).</p> <p>I'm not very experienced with pthreads. Can anyone help me figure out why my performance is sucking so badly?</p> <p>(Note: this isn't meant to be a high-performing K-V store, it's meant to be a naively written test application/benchmark. It handles requests in about the simplest method possible, by spinning off a new thread for every client)</p>
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    1. COAre you setting `TCP_NODELAY` on the socket?
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    2. COCould you show the relevant bits of the code?
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