Question

I’m looking for some advice on the best approach to synchronising access to properties of an object in C++. The application has an internal cache of objects which have 10 properties. These objects are to be requested in sets which can then have their properties modified and be re-saved. They can be accessed by 2-4 threads at any given time but access is not intense so my options are:

1. Lock the property accessors for each object using a critical section. This means lots of critical sections – one for each object.

2. Return copies of the objects when requested and have an update function which locks a single critical section to update the object properties when appropriate.

I think option 2 seems the most efficient but I just want to see if I’m missing a hidden 3rd option which would be more appropriate.

Thanks,
J

Answer 1

Firstly, I think you are worrying about the wrong thing. How do you know locking or copying causes bottlenecks in your code? Critical sections are fairly lightweight and don’t cause much overhead, or, at least, not as much as you think. Just use the most lightweight locking primitive available. If you anticipate your system to be run on multi-processor hardware, you can even use a spinlock.

Secondly, do worry about simplicity of your concurrency model, before performance (hint: simpler model is easier to understand, to get right and to optimize). So if you can afford it, take copies of objects, this will ease the pain of dealing with TOCTOU race conditions in case you are doing complex transformations on the object set that depend on a number of previous values.