I am investigating for how many time it takes for a particular operation to complete. The operation is like the following:
Parallel.ForEach(items, item => SaveScheme(item));
The SaveScheme method works with a database: executes some queries and works with the information. The amount of elements in items collection can be big enough.
When I run this operation, it takes about 20-40 seconds to complete. But when I run it with a profiling turned on, it takes only 3 seconds!
I didn't find any information about this problem. My only guess is that with profiling Parallel.ForEach creates more threads than without it, but I don't know for sure, and even if it's true, I don't know what to do with it.
So, why is that happens and how can I archieve this performance when I run the application without profiling?
UPD. Parallel has nothing to do with this: I tested with simple foreach instead and the operation still completes in 3 seconds!
I found the answer:
The reason is because when you run your application within Visual
Studio, the debugger is attached to it. When you run it using the
profiler, the debugger is not attached.
If you try running the .exe by itself, or running the program through
the IDE with "Debug > Start Without Debugging" (or just press Ctrl+F5)
the application should run as fast as it does with the profiler.
https://stackoverflow.com/a/6629040/1563172
I didn't find it earlier because I thought that the reason is concurrency.
I suspect what you are actually profiling is only the 'not parrarel' part of your application.
I would suggest to take a look here : http://msdn.microsoft.com/en-us/library/gg663532.aspx
Could it be that in profiling you have slowed down (and decreased the concurrency) of the actual database work generation that you have effectively throttled the bottleneck itself - yes the database (or ....).
This result suggests that your application is perhaps trying to be too concurrent: your threads are in fact getting in eachothers' way, or the overhead of creating your threads is greater than any performance gain. This is evidenced by the fact that your non-concurrent version runs faster!
The use of the profiler will affect the performance of your application; perhaps it is slowing down your code enough that you do see some benefit from multiple threads.
Without more detail of the code behind your method, this seems the most likely answer.
Since you are using threading in your program, the Windows timer resolution can also be a reason.
Default windows timer resolution is 15.6ms
When you run your application with the profiler, this is reduced to 1ms causing your application to run faster. Checkout this answer
