Hyper-Threading - please explain?

[Aleksbgbg]

All I understand about hyperthreading is that it's threading that is hyper.

 

No not really, I understand that it doubles the cores in the PC's eyes but it doesn't really, and I think that 8 core CPU (no hyper-threading) > 4 core CPU (hyper-threading) if they are identical apart from the cores.

[gilm0re3]

[AMICLG]

-EDIT- Ninja'd.

[Starelementpoke]

All I understand about hyperthreading is that it's threading that is hyper.

 

No not really, I understand that it doubles the cores in the PC's eyes but it doesn't really, and I think that 8 core CPU (no hyper-threading) > 4 core CPU (hyper-threading) if they are identical apart from the cores.

I look at it like a sandwich. Say you have 2 slices of bread, with a bit of ham, a bit of beef, and mustard. Instead of making one big sandwich(A single core), you can divide the resources to make 2 lesser sandwiches(A core with hyper threading).

[Godlygamer23]

All I understand about hyperthreading is that it's threading that is hyper.

 

No not really, I understand that it doubles the cores in the PC's eyes but it doesn't really, and I think that 8 core CPU (no hyper-threading) > 4 core CPU (hyper-threading) if they are identical apart from the cores.

Hyper-threading is a way to create additional tasks to be scheduled which can significantly improve performance. In the case of comparing an 8 core CPU with no hyper-threading vs a CPU with 4 cores and hyper-threading, if the CPUs have the same architecture, the 8 core CPU will perform better as physical cores are better than virtual cores.

 

Additionally in the some applications, hyper-threading can reduce performance or make the application completely unusable. In testing single threaded performance, hyper-threading will lower performance versus having no hyper-threading and performing the same test.

 

Moved to CPUs, Motherboards, and Memory.

Edited November 27, 2015 by Godlygamer23

[Darel321]

well.. its like your hands... you have 1 brain to control 2 hands.. you can do 2 different things at the same time with 2 hands.. the problem is that you can think only of 1 thing at a time.. soo.. you get the idea??

[BrightCandle]

When it works its worth about +20% performance in total, that is it. The reason it exists is because modern Intel processors have 7 ports for doing work, they already look for parallelism in the code being run and schedule it into these 7 different arithmetic units (although they all do somewhat different things). Most programs however don't have workloads that use all 7 or the level of parallelism extracted at the code level to fully utilise all of the ports the CPU has internally, so hyperthreading just adds an additional instruction decoder front end onto the CPU to allow a second thread to use these unused ports. It has to maintain the program counter and its own set of registers but most of the core is shared with hyperthreading. Its a cheap way to get better utilisation of the processors underlying arithmetic units since there are now quite a lot of them in parallel.

 

But in order for it to work well you need both multiple threads of execution and those threads need to have different workloads, ie some working on floating point and others on integer in order for to actually improve performance. If you have code with a lot of branches that is poorly predicted (like a database query for example) then it can also allow better usage while the other front end is stalled awaiting a memory read (processors now run 100x faster than memory so predicting what you need is vital). You could just take away that it allows one core to run 2 threads in parallel and its worth at best 20% if none of that makes sense, or go with Linus' awful food analogy but I felt someone at least should give a basic explanation of what it really does.