Why would you need to know about each processor in particular?

Question

I'm curious to understand what could be the motivation behind the fine-grained detail of each virtual processor that the Windows 8 task manager seems to be focusing on.

Here's a screenshot (from here):

                

I know this setup could only exist in a non-standard, costly, important server environment (1TB RAM!), but what is the use of a heatmap? Or, setting processor affinity:

        

What I'm asking is, under what circumstances a developer would care if specific processor X is being used more than processor Y (instead of just knowing that a single non-multithreaded process is maxing out a core, which would be better shown as a process heatmap, instead of a processor heatmap), or care whether a process will use this or that processor (which I can't expect a human to guess better than an auto-balancing algorithm)?

Answer 1

In most cases, it doesn't matter, and the heatmap does nothing more than look cool.

Big servers, though, are different. Some processors have a "NUMA", or Non-Uniform Memory Access, architecture. In these cases, some processor cores are able to access some chunks of memory faster than other cores. In these cases, adjusting the process affinity to keep the process on the cores with faster memory access might prove useful. Also, if a processor has per-core caches (as many do), there might be a performance cost if a thread were to jump from one core to another. The Windows scheduler should do a good job avoiding switches like these, but I could imagine in some strange workloads you might need to force it.

These settings could also be useful if you want to limit the number of cores an application is using (say to keep some other cores free for another dedicated task.) It might also be useful if you're running a stress test and you are trying to determine if you have a bad CPU core. It also could work around BIOS/firmware bugs such as the bugs related to high-performance timers that plagued many multi-core CPUs from a few years back.

Answer 2

I can't give you a good use case for this heat map (except that it looks super awesome), but I can tell you a sad story about how we used CPU affinity to fix something. We were automating some older version of MS Office to do some batch processing of Word documents and Word was occasionally crashing. After a while of troubleshooting and desperation, we tried setting Word process' affinity to just one CPU to reduce concurrency and hence reduce the likelihood of race conditions. It worked. Word stopped crashing.

Answer 3

One possible scenario would be a server that is running multiple VMs where each client is paying to have access to their VM.

The administrator may set the processor affinities so that each VM has guaranteed access to X number of cores (and would charge the client appropriately).

Now, suppose that the administrator notices that the cores assigned to ABC Company Inc.'s VMs are registering highly on the heatmap. This would be a perfect opportunity to upsell ABC Company Inc and get them to pay for more cores.

Both the administrator and ABC Company Inc win - the administrator makes more money, and ABC Company Inc experience better performance.

In this way, the heatmap can function as a Decision Support System which helps ABC Company Inc decide whether their needs merit more cores, and helps the administrator to target their advertising better to the their customers that can benefit.