This article gives a rough indication when a dual Xeon system gives better performance than a single i7 system.
- Last Updated on Wednesday, 04 November 2015 21:07
With a regular frequency, questions arise about the use of Xeon E5 CPU's instead of a more consumer oriented i7 CPU. Questions that relate to issues like longevity of the CPU, stability, build quality, professionalism, etcetera.
First of all, longevity of CPU's is mainly determined by the temperatures under which they operate. The higher the temperature of the CPU, the shorter the expected life-span. With all things equal (temps, load, etc.) there is no difference at all in the expected life-span between Xeon's and i7 CPU's, as demonstrated by the same time of warranty. Stability of the CPU is determined by the services and processes running on a system. Again, there is no difference between Xeon's and i7 when they operate with identical systems and processes loaded. Build quality and professionality of the system depends on the builder of the system and has nothing to do with the CPU itself.
With those remarks out of the way, let's concentrate on the reason why one would opt for a single or dual CPU configuration.
The first and maybe most important observation is that a dual CPU configuration is NOT twice as powerful as a single CPU. Generally, a dual CPU configuration is around 50 - 60% faster than a single CPU with the same number of cores and the same clock speed. The relative loss of performance is caused by the overhead of using two CPU's, two memory controllers, shared resources like PCIe busses, etcetera.
In a perfect world the choice would be easy, but the world is not perfect. If all applications were perfectly threaded, things would be easy, but that is not the case. Even Adobe CC software, among them After Effects and Premiere Pro, is not perfectly threaded. On the contrary, it does not even support more than 64 logical cores. Some popular plugins like RedGiant are notorious for their lousy threading. These software limitations cause threading problems and as a consequence muddy the water.
To complicate matters even more, editing styles and workflows differ vastly among users of CC. Some use no plug-ins, so they depend on the threading capabilities of the Adobe software, others use ill-threaded plug-ins to a large extent and therefore require different hardware to achieve optimal performance from their system. Source material varies widely, from standard definition and relatively easy HD to 6K material or even higher resolutions. That also has consequences for the hardware of choice in a particular case. For some the emphasis is on AE, for others it is PR and that also impacts the choice of optimal hardware. In other words, there is no clear cut solution that applies to all users of CC. The following considerations are somewhat generic, but may be helpful to determine when a choice of a single CPU or a dual CPU system is logical.
Simply comparing a single i7 CPU, based on number of cores, clock speed, turbo speed and realistic overclock speed to several dual Xeon CPU configurations with their respective number of cores, clock speed, turbo speed (remember, Xeons can not be overclocked) and the overhead of dual CPU systems, will lead to the following comparison:
Clockspeed and number of cores determine the 'thruput' of a CPU. When handling a single one-thread application, the CPU can use it's Turbospeed, in other cases the number of logical cores and the standard clockspeed determine determine how many threads can be handled. In this chart the maximum number of threads is 72 because in theory the 18 core E5-2699v3 can handle 36 threads per CPU, which translates to 72 threads maximum, if we include hyperthreading.
Noteworthy is the limitation of Adobe software that puts the maximum number of threads on 64 and thus the maximum number of cores that can be used at 16 physical cores per CPU. If the CPU has more than 16 physical cores but less than 64 logical cores, only ONE CPU can be used. The second CPU will not work at all. This is shown by the dashed line for the E5-2699v3 Ltd.
As expected, a single i7-5690X, overclocked to 4.5 GHz outperforms dual Xeon 6 and 8 core CPU's because of the much lower clockspeed of the E5 processors and the burden of the overhead in dual processor systems.
If we take cost into consideration, a factor for most of us, we can compare the expense of the CPU or CPU's, and the memory needed to run such a system. In the case of 6 cores, 32 GB memory is sufficient, but in the case of two 16 cores, 128 GB memory is required. Taking only CPU and memory into consideration and forgetting about other components, we arrive at the following BFTB, 'Bang for the Buck' summary, based on October 2015 prices:
Clearly a single i7 CPU gives a much better BFTB than a dual Xeon system. The E5-2699v3 18 core CPU is severely handicapped by Adobe software and its lack of support for processor groups. Maybe Adobe will follow the example of various professional software companies that do offer support for processor groups and do use all the potential of these powerful CPU's.
However, we don't live in a perfect world. Not every application is perfectly multi-threaded. AE is a prime example of an application that does not use multi-threading very good, at least up to and including CC2015. Even with PR there are a number of eccefts that don't thread very good, some even are plain single-threaded. All these factors make for a less than perfect world, where some work is perfectly multi-threaded at 100%, but where some other work is simply single-threaded or badly threaded at 0% or a number significantly less than 100%.
That changes the impact of multi-core CPU's and drastically changes the BFTB, 'Bang for the Buck' of various CPU's.
Look what happens when the typical work is no longer 100% multi-threaded, but also comprises single-threaded parts, as it does in the real world out there.
CPU performance in terms of throughput falls significantly when the proportion of bad or single-threaded work increases. As a consequence the differences between single and dual CPU configurations drop and show to an even larger extent the attractiveness of the single i7 systems. When we look at the consequences of different proportions of multi-threaded versus single-threaded work from the perspective of 'Bang for the Buck' the following picture emerges:
Now it becomes even clearer that in a less than perfect world the BFTB advantage of the i7 single CPU over a dual E5 Xeon system is absolutely far bigger than some manufacturers try to make us believe. The overclocked i7-59xx CPU's leave all the competition in the dust.
Keep in mind that the practical advantage of a single i7 over dual Xeons is even more significant when using fast DDR4 memory modules. Using DDR4 modules faster than 2133 improves the effective speed of an i7 system to even better levels than shown in the graphs above.