Hyper-threading Performance Comparison - c++

Question

I have written a project, which uses some basic functions in openssl such as RAND_bytes and des_ecb_encrypt.
My computer has i7-2600(4 cores and 8 logic CPU). When I run my project with 4 threads, it will costs 10 seconds. When I run it with 8 threads, it also costs 10 seconds.
What I mean is that hyper-threading doesn't give me any performance improvement. In Linux, the experiment result is same.
I found here tells me that hyper-threading doesn't give me some improvement in some situations. Also, I found here give me some intuitive results.
However, I have tried to write some simple tests and found some simple examples which will show hyper-threading won't give me apparent improvement. Sadly, I don't find it.
So, my questions is that whether there are some simple tests shows the hyper-threading won't give me any performance improvement.

Answer 1

You may find that hyperthreading helps more on code that is using large amounts of memory, so that the processor is regularly blocked on fetching from memory.
In my experience, it's quite hard to find "simple code" that shows benefits from hyperthreading. It tends to be more complex examples that show the benefit. Still, the benefit will most likely not be 2x that of "no hyperthreading". Count on getting perhaps 20-30% improvement.

Answer 2

Hyper threading takes advantage of the fact that the CPU has many components and when one is used, when there's no hyper threading, the others just sit there idle. You can try writing two types of threads, one doing integer calculations (that will hopefully use the ALU) and one doing floating point arithmetic (that will hopefully use the FPU).
I did not try this myself but it seems that in such a scenario hyper threading should improve the performance.
To show the opposite you can use only one type of the threads (either threads only doing integer operations or threads only doing floating point operations).
It may also be that your test is flawed, but in order to know if that is the case we'll need more information about that test.

Answer 3

I have written a project, which use some basic functions in openssl such as RAND_bytes and des_ecb_encrypt... My computer has i7-2600(4 cores and 8 logic CPU). When I run my project with 4 threads, it will costs 10 seconds. When I run it with 8 threads, it also costs 10 seconds.
When using RDRAND (which RAND_bytes will do in this case), the bus us the limiting factor. You should peak at around 800MB/sec. It does not matter how many threads you have - the bus cannot transfer data fast enough. See Intel rdrand instruction revisited.
If you used AES, then you might see a better speedup over the DES/3DES observations. Your Ivy Bridge has AES-NI and it can achieve almost 1.3 cycle/byte, and that should be about double or triple AES is software. To ensure you are using the AES-NI instructions, you have to use the EVP_* interfaces.
I found here tells me that hyper-threading doesn't give me some improvement in some situations. Also, I found here give me some intuitive results.
I think #selalerer and #Mats Petersson answered your question. The problem does not scale linearly and there's a maximum speedup you will encounter. Intel states its about 30%.
Intel's newest architecture favors of Out-Of-Order execution over Hyper-threading execution because its supposed to be more efficient. Read about the Silvermont processor cores.
But if you want a formal deep dive, then see a book on computer engineering. Here's the book we used when I studied it in college: Computer Organization and Design (its probably a bit dated now).
However, I have tried to write some simple tests and found some simple examples which will show hyper-threading won't give me apparent improvement.
OpenSSL also has a benchmarking app. See the source code in <openssl source>/apps/speed.c.
Also, benchmarking apps have their own personalities. An encryption stress test may not reveal the differences as predominantly as you hope to see them. See, for example, Benchmarking Tools.

Answer 4

Following are details and results of my MP benchmarks for Linux and Windows, that can behave differently. Not much HT but Linux tests include Atom (1 core 2 threads) and Windows has Core i7 results (4+4).
http://www.roylongbottom.org.uk/linux%20multithreading%20benchmarks.htm
http://www.roylongbottom.org.uk/quad%20core%208%20thread.htm
Take your pick, depending what you want to prove whether HT provides better or worse performance. Following are RandMem results on i7 (Linux seems better using this test). For such as i7, you also need to consider Turbo Boost that might be lower with multiple threads.
CPUs MBytes Per Second Using Threads Gain At Threads
/HTs 1 2 4 6 8 2 4 6 8
Serial RD
Core i7 4/8 L1 11458 22661 37039 43717 46374 2.0 3.2 3.8 4.0
930 L2 10380 20832 32853 41711 42839 2.0 3.2 4.0 4.1
#### MHz L3 8828 17743 29610 38414 40330 2.0 3.4 4.4 4.6
Win 764 RAM 4266 8712 17347 24946 25589 2.0 4.1 5.8 6.0
Serial RW
Core i7 4/8 L1 15282 13724 16240 16209 18379 0.9 1.1 1.1 1.2
930 L2 12223 18216 25326 28104 27047 1.5 2.1 2.3 2.2
#### MHz L3 10234 19266 21931 24450 26351 1.9 2.1 2.4 2.6
Win 764 RAM 4533 7656 13876 14543 13390 1.7 3.1 3.2 3.0
Random RD
Core i7 4/8 L1 11266 22548 38174 45592 47141 2.0 3.4 4.0 4.2
930 L2 6233 12463 20059 24986 25667 2.0 3.2 4.0 4.1
#### MHz L3 3499 6915 9211 10002 9531 2.0 2.6 2.9 2.7
Win 764 RAM 459 909 1241 1398 1364 2.0 2.7 3.0 3.0
Random RW
Core i7 4/8 L1 14375 3027 2780 2901 3297 0.2 0.2 0.2 0.2
930 L2 5887 4555 6117 6693 7281 0.8 1.0 1.1 1.2
#### MHz L3 3104 4604 4721 5047 4933 1.5 1.5 1.6 1.6
Win 764 RAM 428 860 899 948 1026 2.0 2.1 2.2 2.4
#### 2.8 GHz running at up to 3.06 GHz via Turbo Boost, dual channel 1066 MHz DDR3 RAM
Then the MP Whetstone benchmark that shows real gains
MWIPS MFLOP MFLOP MFLOP COS EXP FIXPT IF EQUAL
CPU MHz 1 2 3 MOPS MOPS MOPS MOPS MOPS
Core i7 1 Thrd #### 3115 1065 886 738 79.3 39.7 2447 2936 1154
Core i7 Win7 #### 21690 8676 7621 5844 531 291 16643 12027 5034
Quad Core Thread 1 1091 1027 728 66.4 36.5 2050 1501 629
Plus HT Thread 2 1089 1037 742 66.0 36.5 2090 1507 630
Thread 3 1090 946 742 66.8 36.5 2069 1534 631
Thread 4 1092 1037 727 66.6 36.6 2031 1501 630
Thread 5 1042 959 736 66.4 36.5 1912 1483 630
Thread 6 1091 874 723 66.6 36.1 2049 1507 629
Thread 7 1090 867 725 65.6 36.3 2094 1516 631
Thread 8 1091 874 722 66.3 36.3 2350 1476 624
Gain % 696 815 860 792 670 733 680 410 436