LTspice needs high power laptop

[wraper]

For example my Acer V5 573G from 2014 is 2.1 cm thick, has i5 4210u (15W TDP), GTX850 (45W TDP), 90W PSU. It's completely cool and silent when doing general work, watching videos. When gaming, it becomes a bit warm in the middle and slightly hot at exhausts on the back. Fans are still not that loud. Why so? It has 2 heatsisinks larger than burning hot DV6000 had and 2 fans instead of 1 (like 99% of laptops at that time). Today 2 heatsinks/fans is a norm. Even in ultrathin laptops.

[montemcguire]

I use LTspice on a laptop, and it is disturbing to make it go into high speed cooling fan mode. One thing you can do in LTspice is to limit the number of parallel threads that it will spawn. In the SPICE tab of the control panel, there is a "max threads" parameter that you can set. On a quad core Intel, the default will be set to eight, to allow you to use all 8 hypercores available from the Intel chip. If you're doing short simulations, then it's fine to leave it set to 8, but for simulations that take a relatively long time to converge or execute, I find it's helpful to scale "max threads" down to a lower value like 2. Modern Intel CPUs do clock boosting, and can also do core hopping, and this makes them very fast with single threaded applications. If you tell LTspice to only launch 2 threads, these can still hop among cores using a boosted clock while still avoiding thermal throttling, without burning up the chip. I haven't directly measured it, but reducing "max threads" from 8 to 2 seems to increase execution times a lot less than the factor of 4 that you'd expect, and makes long simulations much more practical, since the machine won't go into thermal throttling.

Laptops thermally throttle, and I find it's best to avoid that region to get better overall performance. Of course, depending upon the tricks your CPU can do, your mileage may vary.

[montemcguire]

I certainly cannot imagine that the LTspice developers bother trying to squeeze every clock cycle for every core out of their code. I suspect the core (sorry) code is old and doesn't make use of the more recent multiple processing APIs. If you use Resource Monitor then you'll see that it doesn't exactly go out of its way to make good use of the cores anyway.

Well, you've imagined wrong. LTspice IV is multithreaded, and there's a control panel parameter to set the number of threads used. Further, a good bit of the code is actually stripped of several addressing indirection layers upon execution to increase its speed, just to avoid one more memory indirection layer, basically keeping things closer to the CPU and floating point unit more often. This is true for LTspice IV on MacOS - not sure about the modern version on Windows. It can soak a CPU pretty easily. It's fun to speculate, but sometimes, reality gets in the way. ;-)

[JohnnyMalaria]

Where did I say it wasn't?

My point is that it doesn't make efficient use of the available cores.

It isn't a criticism of the software, it's just a fact and it speaks to the odd claim that it somehow requires a powerful power supply.

[SiliconWizard]

Well, you've imagined wrong. LTspice IV is multithreaded, and there's a control panel parameter to set the number of threads used. Further, a good bit of the code is actually stripped of several addressing indirection layers upon execution to increase its speed, just to avoid one more memory indirection layer, basically keeping things closer to the CPU and floating point unit more often. This is true for LTspice IV on MacOS - not sure about the modern version on Windows. It can soak a CPU pretty easily. It's fun to speculate, but sometimes, reality gets in the way. ;-)

Yep, LTSPice IV and XVII both have a multithreaded solver on Windows as well.

The actual "efficiency" of the solver's multitheading is of course not optimal. On average it uses approx. 30% to 40% of CPU on my Core i7 (6 cores/12 logical cores), nb of threads set to 12, whereas a single-thread app can use only ~8%, so it's already a very significant improvement over a single-core solver.