SInce Nehalem, the ITLB is partitioned 50/50 statically under HT, DTLB and shared TLB are partioned on demand, not necessarily 50/50.
With the improved bandwidth of L1, there may be situations where HT performance is improved.
You should be able (as root) to unset the linux /proc entries of the sibling logicals so as to effectively shut off HT, as you could on earlier processors.

Regardingsome parts ofquestion #3:

1. I think Microsoft enforces itfor all versions ofWindowswith Single-CPU license, right? I don't know if they call some API from BIOS or not.

2.Win32 API allows to enforce execution of allOS processes on one CPU.

3. Do you want to enforce you application to work all the time on one CPU? ( because of some synchronization problems, right? )

This is a piece of code that demostrates it in general for Windows:

...
#define RUN_ON_CPU_010x01
#define RUN_ON_CPU_020x02
...

...
bRc = ( RTBOOL )::GetProcessAffinityMask( hProcess, &dwProcessMask, &dwSystemMask );
if( bRc == RTFALSE )
break;
...

...
dwThreadAM = RUN_ON_CPU_01;// Main Thread willRunon 1st CPU
dwThreadAMPrev = ::SetThreadAffinityMask( hThread, dwThreadAM );
::Sleep( 0 );
...

PS: A callSleep( 0 ) is needed and I read about it in somedocuments.

...The HALT instruction is a privileged instruction, so it can only be executed by the operating system. I want to find a way for 64-bit Windows 7 to execute a HALT instruction on one virtual core of each physical core. This would be equivalent to disabling hyperthreading.Does anyone know how to do that?..

What about an idea of aVirtual Driver with some set of IOCTL commands? Like:

IOCTL_HYPERTHREADING_DISABLE

and thencalla Win32 function DeviceIControl( hDevice, IOCTL_HYPERTHREADING_DISABLE, ... )...

Best regards,
Sergey

Sergey,

At your program start use GetProcessAffinityMask to get the logical processors available to your application.
Then bitwise AND the lpProcessAffinityMask with binary 0101010101010101010101010101010101010101010101010101010101010101
Then use SetProcessAffinityMask with the result.

Do this before you create additional threads.
This will restrict the threads of your process tothe low numbered HT siblings
This will not preventthe system or other applications from using those logical processors

You can also use SetThreadAffinityMask to the above mask (result ofAND above) on selected program threads, leaving the others (e.g. i/o thread or monitoring thread) to float.

Third option, for the alternate bitmask'dprocessors, create high priority threads that sit in an infinite loop

while(true)
_mm_pause();

I'd tryother options before inserting the _mm_pause() hack.

Jim Dempsey

Thank you for your response. I'll trythe 3rd option. It looks very simple!
Best regards,
Sergey

The load, store, and fill buffers (the latter being the most significant) are partitioned according to demand between the hyperthreads. Each logical has its own set of register files, with no sharing of any kind.

Mike,

If you boot with HT enabled then any static partitioning (if any) will be set until reboot with HT disabled. HLT-ing a hardware thread will not disable HT (assuming you HLT from ring 0). Note, your system (unless embedded) will undoubtably have interrupts enabled, therefore and interrupt will resume from HLT* (*some of the newer processors have a way of not doing this for power saving, but also have a way to force continue, as to how this works I cannot say). The PAUSE (_mm_pause()) can be issued from any ring level. The stall loop (earlier thread) will at least mitigate some of the cach evictions, while not increasing L1 cache capacity (assuming it is not shared with sibling(s) in HT).

Before you ponder too long as to how to do this, it would be easire to run some experiments. I cannot imagine it would take you more than an hour to setup the runtime environment(s) and run tests.

a) with HT enabled, all threads used for "application"
b) with HT disabled, all threads used for "application"
c) with HT enabled, with process affinity set to even bits (of process bits), half threads used for "application"
d) with HT enabled
withthread affinity set to even bits (of process bits), half threads used for "application"
with other thread affinity set to odd bits (of process bits), half threads used for _mm_pause() loop

On SandyBridge you will be hard pressed to find a negative effect with HT enabled. If Kathy Farrel gets around to releasing the second half of "Have your cake and eat it too" in the Parallel Programming Blogs, you will see how I addressed this issue. The article addresses the SSE/AVX sharing issues with HT on/off as well as L1 instruction cache read, L1 data cach read, L1 data cache read/write, L1 data cache write. These variations combining SSE/AVX tasks with integer tasks.

The point of the blog was to illustrate that any potential gain with turning HT off is only experienced in a few applications. It took me 3-4 weeks to construct (choose) such an example. On SandyBridge, with Turbo Off, there was no difference (in SSE/AVX), with Turbo On there was ~1% improvement of the SSE/AVX code with HT off. However, the article goes on to show that by turning HT off, should your program have a combination of integer (non-SSE/AVX) and SSE/AVX, you cal loose between 20% to 40% of performance. Your application experience will vary from this.

To summarize:

The only benefit of turning HT off .AND. turning Trubo Boost off is in running scalability benchmarks as well as diagnosing performance issues with your code.

With HT on and Turbo Boost on, you will have a preponderance of net performance improvement, although your scalability charts will indicate less scalability.

Do you want to make your charts look good or do you want to make your system more productive?

Jim Dempsey

I searched the web with a phrase "Disabling Hyper-Threading in software" and found a couple of links. Take a look, please:

http://solidlystated.com/software/how-to-disable-a-cpu-core

http://www.virtualdj.com/homepage/GargantulaKon/blogs/2691/How_To_Permanently_Disable_Hyper-Threading_For_Virtual_DJ.html

http://www.computing.net/answers/hardware/hyper-threading-disabled/35896.html

...HLT-ing a hardware thread will not disable HT...

Here is adescriptionfrom Intel's "Instruction Set Reference. A-M. Vol 2A" ( Nov 2007 version ). Yes,it clearly says that it will be 'halted', but not 'disabled', and could be 'resumed':

Sergey,

Please re-read the description section for HLT.

HLT will stop the hardware thread (not core on HT) of the logical processor, this hardware thread may be resumedby way ofan interrupt.

This is not disabling HT within the processor.
Disabling HT within the processor occurs as aPower-On Configuration Option, configurable as a BIOS option.
The BIOS can be configured to assert or non-assert a signal into the processor at power-on time. This signal is used to disable (usually signal asserted) or enable (usually signal not asserted) HT.
This is not a programmable software option (other than for reprogramming BIOS and rebooting)

Therefore, IIF (if and only if) the processor partitions a resource, such as L1 ICache and/or L1 DCache with HT enabled, or not with non-HT configuration, the partitioning only occurs (or not occurs) at power on time.

Jim Dempsey