It's true for IBM POWER, where all of a core's threads have to be in
kernel mode or user mode and cannot intermix.
It's not true for x86. I also don't believe there is a stall during the
mode transition, otherwise a thread that continuously issues null system
calls could stall its sibling. That should be easy to test.
On 02/07/2022 06.54, Peter Veentjer wrote:
Hi,
I'm reading the following book "Developing High-Frequency Trading
Systems". My goal is not to write any high-frequency trading systems,
but to get some insights into the domain and learn as much as possible
from the applied techniques. It is a Packt book and they are not known
for their quality. But it is written by 3 engineers with a combined
HFT experience of almost 50 years.
On page 62 of the book, they make the following claim. If you are
using a hyper-threading and there is an interrupt or a system call on
one logical core, then the hyper-sibling will stall as well because
both need access to the kernel (mode switch).
I don't believe this is correct. Each logical core has its own
architectural state; so its own set of architectural registers and its
own APIC including an interrupt descriptor table. The current
privilege level is stored in the first 2 bits of the CS register and
since every logical core has its own copy of that, the hyper siblings
should be able to run independently no matter if there is a mode switch.
Of course, disabling hyper-threading will lead to improved performance
of a single core, because it doesn't need to share any resources like
rob, line fill buffers, store buffers, load buffets, execution units,
reservation stations, caches, etc. So that is a valid reason to
disable hyper-threading.
I'm by no means an X86 expert and certainly not a high-frequency
trading expert. So perhaps I'm missing something.
Regards,
Peter.
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