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    <p>The "optimal" configuration when you have two threads running at full tilt is to have a core for each. If they aren't moved around i e each thread stays on "its" core you'll have better performance than if they are moved back and forth between the cores. So essentially a 2+2 thread solution will require 4 cores to run optimally.</p> <p>In addition since two cores are running the same code it is vital that they (in your case) aren't moved from "their" core. This is because the operating environment for both cores is more or less the same which makes switching between them less cumbersome (at the cache level) than if you need to load everything onto a different core.</p> <p>Then you have the issue of memory system saturation. A "normal" single-threaded program will usually use up most if not all of the available memory bandwidth. Its speed will usually be determined by the rate at which the memory system provides it with data. There are exceptions such as when you're in a division instruction during which no memory activity occurs or when you're in a tight loop which doesn't require data reads or writes. In most other cases the memory system will be working its butt off to shove memory into the program and a lot of the time not as fast as the program can make use of it.</p> <p>A program which doesn't take this into account will run slower multi-threaded than single because both threads will start colliding when they need memory access and this slows things down a lot. This for compiled lanbguages such as C or C++. With Java there are a lot of memory accesses going on behind the scenes (caused by the engine) over which the programmer has little control. So the Java engine and its workings will use up a lot of the cache memory and bandwidth which will mean that your shared memory will be competing with the engine's needs and be in and out of the cache more or less constantly.</p> <p>My two cents.</p>
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    1. POmultithreading in two processes communicating over shared memory gives a huge slowdown and then core-mapping recovers it
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    1. USOlof Forshell
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    1. COI think you are very right about the operating enviornment of the cores. when there is no core-mapping the os is scheduling the threads of both the processes on both the cores which obviously harms in terms of code caching or what you say as the operating environment. As soon as they are mapped, each core runs a separate process and no cache misses occur as the environemnt on each core remains the same. Code caching is important in my case as both the processes in my case are JIT compiled.
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    2. COthank you for the lovely answer. this is why I love SO :)
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