I have a CPU with four cores, but the specification shows it to have four cores and eight threads. How is that possible? Can I actually run eight threads in parallel?
Depending on the CPU type, each core can have two virtual CPUs (or threads as you put it). The effect is achieved using Hyperthreading.
This one can deceive you. Intel's HT technology does indeed allow the operating system to schedule two threads for each physical core, due to a virtual duplication of the core's resources.
"Technically" you are able to run eight threads. Notice the quotes. The main purpose of this technology was to not let the CPU resources get wasted (e.g. if your instruction pipeline has a width of four instructions, make sure it's almost always getting instructions, since one thread can rarely achieve this).
However, if your system does not have enough resources to accommodate the computations done by your threads, you will not actually have any benefit, or worse, your performance will degrade. For example, say your CPU has only four floating-point units while all your eight threads are doing floating point calculations. In this case you cannot have parallelism. Another case is when all your threads are doing memory-intensive computations. The bus from CPU to main memory will be saturated and eight threads will definitely not be able to execute their code in parallel as you expect. For more about the pitfalls of HT check this article: http://software.intel.com/en-us/articles/performance-insights-to-intel-hyper-threading-technology/
Also, make sure you understand the various levels of threading in the system. I hate to reference myself but here goes: multithreading on dual core machine?
I guess that you have a hyperthreaded machine with two processors per core.
Yes it can run 8 threads concurrently.
Maybe you should take a look at hyperthreading:
http://en.wikipedia.org/wiki/Hyper-threading
It's called a superscalar CPU, where the pipeline is duplicated in each core, allowing it to dispatch multiple instructions in parallel. Note that Intel calls it HyperThreading but it's essentially the same.
Related
When I run lscpu on my host, it shows
CPU(s): 8
Thread(s) per core: 2
Core(s) per socket: 4
My host has 4 physical CPUs, but 8 logical CPUs due to 2 threads per core. ok, "2 threads per core" means one core can execute 2 threads simultaneously so as if we have doubled the CPU capacity? So this is parallel concept?
While we have another concept that "one process can have multiple threads", I believe this means one process can handle multiple threads concurrently by switching context, but not necessarily in parallel. In most cases one CPU can execute one thread at a time, right?
I'd like to confirm my understanding above is correct. Thanks
Ref for concurrent and parallel difference: What is the difference between concurrency and parallelism?
This concept is called Simultaneous multithreading (SMT). It is implemented in many processor, from x86-64 (both AMD and Intel) to POWER. The idea is to execute 2 threads concurrently. Some operation can be parallel regarding the specific target architecture.
one core can execute 2 threads simultaneously so as if we have doubled the CPU capacity?
No. Hardware threads (also called logical cores) are not equivalent to cores (ie. in opposition to physical cores). Some processor units are statically allocated for the hardware threads while some units are dynamically allocated for the hardware thread meaning the threads share the available resources.
The initial idea was to execute something useful when a core was stalling on some operations like memory reads. With 2 hardware threads, a core can execute the instructions of another thread if the current one is waiting on memory, for example due to a cache miss. Memory-bound parallel codes that are limited by the latency of the RAM like naive transpositions or linked-list traversals can benefit from this mechanism.
The SMT implementation has significantly improved over time. Especially in x86-64 processor recently. Nowadays, hardware threads of modern processor can execute computing instructions truly in parallel. For example, an Intel Skylake processor can execute up to 4 arithmetic instructions at a time per cycle, thanks to 4 ALUs. 1 thread can execute 4 instructions per cycle only if the instructions are independent (during the target cycles). This is not always possible as some loops are inherently sequential and do not contain enough independent instruction for each loop (eg. cumulative sum). With a 2-way SMT enabled, 2 software threads can be scheduled on the same core and the core can execute 2 instructions of each thread completely in parallel in a given cycle. It can even load balance the number of instruction regarding the needs of each thread in real time (eg. 1 vs 3 instructions per cycle). In the end, latency-bound codes can be up to 2 times faster on a 2-way SMT processor like Skylake. That being said, it does not speed up codes that can already fully use all the available processor computing units. For example, a parallel matrix multiplication using an optimized BLAS library will nearly always be slower with 2 software threads running per core than with only 1 software thread per core. The execution can be slower because hardware thread share some resources like caches and they can conflict each other with 2 threads per core running simultaneously. Put it shortly, efficient codes should not benefit from it, but people tends to write inefficient code and it is not rare for compilers to fail to generate efficient codes saturating computing units of a core (they often need some help).
While we have another concept that "one process can have multiple threads", I believe this means one process can handle multiple threads concurrently by switching context, but not necessarily in parallel.
I would like to set the record straight: software threads and hardware threads are two very different things despite the name.
A software thread is a logical OS unit that can be scheduled on a hardware thread. A hardware thread can be seen as a physical part of a processor core (it is actually a naive simplistic view). A software thread is a part of an OS process. The OS is responsible for the scheduling of the ready software threads. Processes are not scheduled, software threads are (at least on a modern OS). 2 software threads of 2 different processes can run in parallel on a processor with multiple cores (or even on some 2-way SMT cores).
In most cases one CPU can execute one thread at a time, right?
The term "CPU" is not clear here: it can mean different things regarding the context.
If "one CPU" means a modern microprocessor chip that is typically a multicore one nowadays, then definitively no. Software threads can truly run in parallel on different cores for examples.
If "one CPU" means a core (like often in high-performance computing), then it depends: a 1-way SMT core can execute only 1 thread at a time while a 2-way SMT core can execute 2 thread at a time.
On old microprocessor chip with 1 core and no SMT, it was true that one thread was running at a time and context switches was used to execute thread concurrently from the user point-of-view but not in parallel. This time is long gone (since nearly 2 decades) except maybe on some embedded microprocessor chips.
Is this...parallel?
Maybe.
Hyperthreading is Intel's trademark* for processor cores that have two complete sets of context registers. A hyperthreaded CPU can concurrently execute code on behalf of two threads without any intervention by the operating system (i.e., with no need for context switching.)
The extent to which those two concurrent executions actually are parallel executions varies from CPU model to model, and it depends on what the two threads actually are doing. For example (I'm just making this part up because it's been a few decades since I've needed to worry about any particular CPU architecture) if some "hyperthreaded" CPU has two integer ALUs per core, then the two threads might both be able to perform integer operations in parallel, but if it has only one FPU per core, then they would have to take turns using it.
Some Hyperthreaded CPU models have more duplicate execution units than others have, and so can parallelize more parts of the execution.
* AMD calls their similar capability, "2-way simultaneous multithreading."
Modern CPU specifications seem to always mention twice the number of threads for each core. If it is a 4-core processor, the number of threads mentioned is 8. If it is a 6-core processor, the number of threads is 12.
At first, this felt confusing since, as far as I am aware, a single physical core can execute only a single thread at a time. Furthermore, any number of threads can be executed "simultaneously" with context switching. So, why even mention the number of threads in specifications?
Intel claims that this is possible in their processors due to Hyper-threading, which is their implementation of Simultaneous multithreading. I believe AMD has their own version.
Intel's explanation is that they expose each physical core as two "logical" cores. I am wondering how that improves performance. Why is it more efficient to execute two threads on two logical cores, rather than execute two threads on a single physical core with the help of context switching? The two logical cores are backed by a single physical core anyway. So, how is Simultaneous multithreading making a difference?
There seems to be some hardware implementations that make it more efficient. My vague understanding, after going through the wiki of Simultaneous multithreading, is that instructions from multiple threads are actually being executed at the same time. Apparently, that is the key difference. But, I do not understand why exposing one physical core as two logical cores is necessary. Is it to make the operating system serve more threads at the same time to the CPU? The operating system would think are twice the number of cores, and therefore serve twice the number of threads to the CPU. The OS will control the context switching, while the physical cores use their hardware capability to simultaneously execute the two threads served to each of them via their respective logical cores. Is that what happens?
Follow-up
A follow-up question would be, why not just specify them as logical cores in specifications? Why call them threads? For example:
Intel® Core™ i7-11600H Processor
# of Physical Cores 6
# of Logical Cores 12
I have no background in Computer Science, but I have read some articles about multiprocessing and multi-threading, and would like to know if this is correct.
SCENARIO 1:HYPERTHREADING DISABLED
Lets say I have 2 cores, 3 threads 'running' (competing?) per core, as shown in the picture (HYPER-THREADING DISABLED). Then I take a snapshot at some moment, and I observe, for example, that:
Core 1 is running Thread 3.
Core 2 is running Thread 5.
Are these declarations (and the picture) correct?
A) There are 6 threads running in concurrency.
B) There are 2 threads (3 and 5) (and processes) running in parallel.
SCENARIO 2:HYPERTHREADING ENABLED
Lets say I have MULTI-THREADING ENABLED this time.
Are these declarations (and the picture) correct?
C) There are 12 threads running in concurrency.
D) There are 4 threads (3,5,7,12) (and processes) running in 'almost' parallel, in the vcpu?.
E) There are 2 threads (5,7) running 'strictlÿ́' in parallel?
A process is an instance of a program running on a computer. The OS uses processes to maximize utilization, support multi-tasking, protection, etc.
Processes are scheduled by the OS - time sharing the CPU. All processes have resources like memory pages, open files, and information that defines the state of a process - program counter, registers, stacks.
In CS, concurrency is the ability of different parts or units of a program, algorithm or problem to be executed out-of-order or in a partial order, without affecting the final outcome.
A "traditional process" is when a process is an OS abstraction to present what is needed to run a single program. There is NO concurrency within a "traditional process" with a single thread of execution.
However, a "modern process" is one with multiple threads of execution. A thread is simply a sequential execution stream within a process. There is no protection between threads since they share the process resources.
Multithreading is when a single program is made up of a number of different concurrent activities (threads of execution).
There are a few concepts that need to be distinguished:
Multiprocessing is whenwe have Multiple CPUs.
Multiprogramming when the CPU executes multiple jobs or processes
Multithreading is when the CPU executes multiple mhreads per Process
So what does it mean to run two threads concurrently?
The scheduler is free to run threads in any order and interleaving a FIFO or Random. It can choose to run each thread to completion or time-slice in big chunks or small chunks.
A concurrent system supports more than one task by allowing all tasks to make progress. A parallel system can perform more than one task simultaneously. It is possible though, to have concurrency without parallelism.
Uniprocessor systems provide the illusion of parallelism by rapidly switching between processes (well, actually, the CPU schedulers provide the illusion). Such processes were running concurrently, but not in parallel.
Hyperthreading is Intel’s name for simultaneous multithreading. It basically means that one CPU core can work on two problems at the same time. It doesn’t mean that the CPU can do twice as much work. Just that it can ensure all its capacity is used by dealing with multiple simpler problems at once.
To your OS, each real silicon CPU core looks like two, so it feeds each one work as if they were separate. Because so much of what a CPU does is not enough to work it to the maximum, hyperthreading makes sure you’re getting your money’s worth from that chip.
There are a couple of things that are wrong (or unrealistic) about your diagrams:
A typical desktop or laptop has one processor chipset on its motherboard. With Intel and similar, the chipset consists of a CPU chip together with a "northbridge" chip and a "southbridge" chip.
On a server class machine, the motherboard may actually have multiple CPU chips.
A typical modern CPU chip will have more than one core; e.g. 2 or 4 on low-end chips, and up to 28 (for Intel) or 64 (for AMD) on high-end chips.
Hyperthreading and VCPUs are different things.
Hyperthreading is Intel proprietary technology1 which allows one physical to at as two logical cores running two independent instructions streams in parallel. Essentially, the physical core has two sets of registers; i.e. 2 program counters, 2 stack pointers and so on. The instructions for both instruction streams share instruction execution pipelines, on-chip memory caches and so on. The net result is that for some instruction mixes (non-memory intensive) you get significantly better performance than if the instruction pipelines are dedicated to a single instruction stream. The operating system sees each hyperthread as if it was a dedicated core, albeit a bit slower.
VCPU or virtual CPU terminology used in cloud computing context. On a typical cloud computing server, the customer gets a virtual server that behaves like a regular single or multi-core computer. In reality, there will typically be many of these virtual servers on a compute node. Some special software called a hypervisor mediates access to the hardware devices (network interfaces, disks, etc) and allocates CPU resources according to demand. A VCPU is a virtual server's view of a core, and is mapped to a physical core by the hypervisor. (The accounting trick is that VCPUs are typically over committed; i.e. the sum of VCPUs is greater than the number of physical cores. This is fine ... unless the virtual servers all get busy at the same time.)
In your diagram, you are using the term VCPU where the correct term would be hyperthread.
Your diagram shows each core (or hyperthread) associated with a distinct group of threads. In reality, the mapping from cores to threads is more fluid. If a core is idle, the operating system is free to schedule any (runnable) thread to run on it. (Some operating systems allow you to tie a given thread to a specific core for performance reasons. It is rarely necessary to do this.)
Your observations about the first diagram are correct.
Your observations about the second diagram are slightly incorrect. As stated above the hyperthreads on a core share the execution pipelines. This means that they are effectively executing at the same time. There is no "almost parallel". As I said, above, it is simplest to think of a hyperthread as a core "that runs a bit slower".
1 - Intel was not the first computer to com up with this idea. For example, CDC mainframes used this idea in the 1960's to get 10 PPUs from a single core and 10 sets of registers. This was before the days of pipelined architectures.
Can someone shed some light on it?
An i7 processor can run 8 threads but I am pretty sure we can create more than 8 threads in a JAVA or C++ program(not sure though). I have an i5 processor and while studying concurrency I have created 10 threads for assignments. I am just trying to understand how Core rating of CPU is related to threads.
The thread you are refering to is called a software thread; and you can create as many software threads as you need, as long as your operating system allows it. Each software thread, or code snippet, can run concurrently from the others.
For each core, there is at least one hardware thread to which the operating system can assign a software thread. If you have 8 cores, for example, then you have a hardware thread pool of capacity 8. You can map tens or hundreds of software threads to this 8-slot pool, where only 8 threads are actually running on hardware at the same time, i.e. in parallel.
Software threads are like people sharing the same computer. Each one can use this computer up to some time, not necessarily have his task completed, then give it up to another.
Hardware threads are like people having a computer for each of them. All of them can proceed with their tasks at the same time.
Note: For i7, there are two hardware threads (so called hyper-threading) in each core. So you can have up to 16 threads running in parallel.
There are already a couple of good answers talking about the hardware side of things, but there isn't much talk about the software side of things.
The essential fact that I believe you're missing is that not all threads have to be executing all the time. When you have thousands of threads on an 8 core machine, only a few of them are actually running at any given time. The others are sitting around doing nothing until some processor time becomes free. This has huge advantages because threads might be waiting on other resources, too. For example, if I have one thread trying to read a file from disk, then there's no reason for it to be taking up CPU time while it's waiting for the hard disk data to load into RAM. Another example is when the thread is waiting for a response from some other machine (such as a web request over the internet). When you have more threads than your processor can handle at once, the operating system and/or runtime (It depends on the OS and runtime implementation.) are responsible for deciding which threads should get available processor time. This kind of set up lets you maximize your machine's productivity because CPU cycles are doing something useful almost all the time.
A "thread" is a software abstraction which defines a single, self-consistent path of execution through the program: in most modern systems, the number of threads is basically only limited by memory. However, only a comparatively small number of threads can be run simultaneously by the CPU. Broadly speaking, the "core count" is how many threads the CPU can run truly in parallel: if there are more threads that want to run than there are cores available, the operating system will use time-slicing of some sort to let all the threads get some time to execute.
There are a whole bunch of terms which are thrown around when it comes to "cores:"
Processor count: the number of physical CPU chips on a system's motherboard. This used to be the only number that mattered, until CPUs with multiple cores became available.
Logical core count: the number of threads that the system hardware can run in parallel
Physical core count: the number of copies of the CPU execution hardware that the system has -- this is not always equal to the logical core count, due to features like SMT ("simultaneous multithreading") which use a single piece of hardware to run multiple threads in parallel
Module count: Recent (Bulldozer-derived) AMD processors have used an architecture which is a hybrid between SMT and the standard one-logical-core-per-physical-core model. On these CPUs, there is a separate copy of the integer execution hardware for each logical core, but two logical cores share a floating-point unit and the fetch-and-decode frontend; AMD calls the unit containing two logical cores a module.
It should also be briefly mentioned that GPUs, graphics cards, have enormous core counts and run huge numbers (thousands) of threads in parallel. The trade-off is that GPU cores have very little memory and often a substantially restricted programming model.
Threads are handled by the OS's scheduler. The number of cores in a CPU determine how many threads it can run at the same time.
Note that threads are constantly switched in and out by the scheduler to give the "illusion" that everything is running at the same time.
More here, if you're interested.
no, no, no... Your I7 has eight execution threads and can run 8 threads at once.
1000 threads or more can be waiting for processor time.
calling thread.sleep moves a thread off the execution core and back into memory where it waits until woken.
If you're spawning multiple threads (or processes) concurrently, is it better to spawn as many as the number of physical processors or the number of logical processors, assuming the task is CPU-bound? Or is it better to do something in between (say, 3 threads)?
Does the performance depend on the kind of instructions that are getting executed (say, would non-local memory access be much different from cache hits)? If so, in which cases is it better to take advantage of hyperthreading?
Update:
The reason I'm asking is, I remember reading somewhere that if you have as many tasks as the number of virtual processors, tasks on the same physical core can sometimes starve some CPU resources and prevent each other from getting as many resources as needed, possibly decreasing performance. That's why I'm wondering if having as many threads as virtual cores is a good idea.
The performance depends on a huge variety of factors. Most tasks are not strictly CPU bound, since even if all of the data is in memory it is usually not on-board in the processor cache. I have seen examples (like this one) where memory access patterns can dramatically change the performance profile of a given 'parallel' process.
In short, there is no perfect number for all situations.
Chances are pretty good that you will see a performance improvement running 2 threads per core with HyperThreading enabled. Jobs that appear to be entirely CPU bound usually aren't, and HyperThreading can extract a few "extra" cycles out of the occasional interrupt or context switch.
On the other hand, with a core iX processor that has Turbo Boost, you might actually do better running 1 thread per core to encourage the CPU to overclock itself.
At work, we routinely run many-core servers at full CPU doing various kinds of calculation for days at a time. A while back we measured the performance difference with and without HT. We found that on average, with HyperThreading, and running twice as many jobs at once, we could complete the same amount of jobs about 10% faster than than without HyperThreading.
Assume that 2 × cores is a good place to start, but the bottom line is: measure!
I remember info that hyperthreading can give you up to 30% of performance boost. in general you'd better to treat them as 4 different cores. of course in some specific circumstances (e.g. having the same long running task bound to each core) you can divide your processing better taking into account that some cores are just logical ones
more info about hyperthreading itself here
Using Hyperthreading to run two threads on the same core, when both threads have similar memory access patterns but access disjoint data structures, would be very roughly equivalent to running them on two separate cores each with half the cache. If the memory-access patterns are such that half the cache would be sufficient to prevent thrashing, performance may be good. If the memory-access patterns are such that halving the cache induces thrashing, there may be a ten-fold performance hit (implying one would have been much better off without hyperthreading).
On the other hand, there are some situations where hyperthreading may be a huge win. If many threads will all be reading and writing the same shared data using lock-free data structures, and all threads must see a consistent view of the data, trying to run threads on disjoint processor may cause thrashing since only one processor at a time may have read-write access to any given cache line; running such a threads on two cores may take longer than running only one at a time. Such cache arbitration is not required, however, when a piece of data is accessed by multiple threads on a single core. In those cases, hyperthreading can be a huge win.
Unfortunately, I don't know any way to give the scheduler any "hints" to suggest that some threads should share a core when possible, while others should run separately when possible.
HT allows a boost of approximately 10-30% for mostly cpu-bound tasks that use the extra virtual cores. Although these tasks may seem CPU-bound, unless they are custom made assembly, they will usually suffer from IO waits between RAM and local cache. This allows one thread running on a physical HT-enabled core to work while the other thread is waiting for IO. This does come with a disadvantage though, as two threads share the same cache/bus, which will result in less resources each which may cause both threads to pause while waiting for IO.
In the last case, running a single thread will decrease the maximum simultaneous theoretical processing power(by 10-30%) in favor of running a single thread without the slowdown of cache thrashing which may be very significant in some applications.
Choosing which cores to use is just as important as choosing how many threads to run. If each thread is CPU-bound for roughly the same duration it is best to set the affinity such that threads using mostly different resources find themselves on different physical cores and threads using common resources be grouped to the same physical cores(different virtual core) so that common resources can be used from the same cache without extra IO wait.
Since each program has different CPU-usage characteristics and cache thrashing may or may not be a major slowdown(it usually is) it is impossible to determine what the ideal number of threads should be without profiling first. One last thing to note is that the OS/Kernel will also require some CPU and cache space. It is usually ideal to keep a single (physical)core set aside for the OS if real-time latency is required on CPU-bound threads so as to avoid sharing cache/cpu resources. If threads are often waiting for IO and cache thrashing is not an issue, or if running a real-time OS specifically designed for the application, you can skip this last step.
http://en.wikipedia.org/wiki/Thrashing_(computer_science)
http://en.wikipedia.org/wiki/Processor_affinity
All of the other answers already give lots of excellent info. But, one more point to consider is that the SIMD unit is shared between logical cores on the same die. So, if you are running threads with SSE code, do you run them on all 4 logical cores, or just spawn 2 threads (assuming you have two chips)? For this odd case, best to profile with your app.