Intel Hyper-Threading Technology Technical User’s Guide contains some hints about why
Intel did not try for more than two threads per core in its consumer CPUs,
which it did do in some server CPUs.
When explaining Hyper-Threading Technology, it says :
Each logical processor
- Has its own architecture state
- Executes its own code stream concurrently
- Can be interrupted and halted independently
The two logical processors share the same
- Execution engine and the caches
- Firmware and system bus interface
The important part is that the two logical processors share the same
Execution engine, meaning that the units that make up the core are not
duplicated. Once, for example, the arithmetic unit is used by one thread,
it cannot be used by the other thread.
This prevents total parallelism, so does not allow two threads to execute
in parallel instructions of the same type - one has to wait for the other
Intel has quantified the average performance gain by threads as follows:
A processor with Hyper-Threading Technology may provide a performance
gain of 30 percent when executing multi-threaded operating system and
application code over that of a comparable Intel architecture
processor without Hyper-Threading Technology.
The statistical gain of two threads versus one is therefore only in the order
of 30%, which is very far from the 100% that one would expect if two threads
on the same core could do double the work of one.
I would therefore estimate that if Intel would have of enabled, say,
three threads on the core, the statistical gain would be much lower,
maybe on the order of 10% or less.
Given the fact that some hardware needs to be duplicated per each thread,
namely the architecture state and interrupt logic,
the gain is probably not worth the cost that this additional hardware would add
to the price of the core.
For effective Hyper-Threading, Intel would have had to increase the number
of units of the same type inside each core.
It has done just that in the
which has 4 ports for load/stores, 4 for integer, and 2 for branch, so
even two threads running identical integer workloads probably wouldn't introduce
much contention. However, Intel has still kept to the model of two
hyper-threads per core, I would guess probably in order to economize on the
hardware needed to allow more hyper-threads, or maybe even also because
modern operating systems cannot actually use efficiently such an architecture.