Re: OT: Can lot of RAM can slow down a calculation workstation?

[tomas@tuxteam.de]

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On Fri, Jun 26, 2015 at 11:58:40AM +0200, Dan wrote:
> On Fri, Jun 26, 2015 at 11:46 AM,  <tomas@tuxteam.de> wrote:

[...]

> No I do not know that. I am a scientist, and I use the computers as a
> tool to do simulations that I write in C++ with (Threading Building
> Blocks). I have a limited knowledge of the computer architecture. That
> would mean that a calculation that fits in 64GB will run slower with
> 256GB? or that means that when I increase the size the calculation
> will be slower.

Note that I'm deep in hand-waving territory here.

Suppose you could reduce your problem size to one-fourth its current
size, so that it fits in your 64 GB. Let's say it needs now a time
T_0.

Now consider your (real) four-fold problem. Leaving out all effects
of swapping and all that (and only considering the "pure" algorithm),
your run time will be T_1, which most probably is (depending on
the algorithm's properties) bigger than T_0. For a linear algorithm:
T_1 >= 4 * T_0 (you're lucky!), for a quadratic one T_1 >= 16 * T_0
and so on (if you're _very_ lucky, you have a sublinear algorithm,
but given all you've written before I wouldn't bet on that).

Taking into account the effects of RAM, you get for 64 GB and your
problem ("real" size) some time T_1_64 which is most probably
significantly bigger than T_1: T_1_64 >> T_1, due to all the caching
overhead. How much bigger will depend on how cache-friendly the
algorithm is: if it is random-accessing data from all over the
place, the slowdown will be horrible (in the limit, of the order
of magnitude of the relation of the SSD speed to the RAM speed
(yeah, latency, bandwidth. Some combination of both. Pick the worst
of them ;-)

Now to the 256 GB case. Ideally, the thing fits in there: ideally
the time would be T_1_256 =~ T_1, since no swapping overhead,
etc.

What I was saying is that you might quite well get T_1_256 > T_1,
because there are other factors (the CPU has a whole hierarchy
of caches between itself and the RAM, because the RAM is horribly
slow from the POV of the CPU). Those caches might be more
overwhelmed by the bigger addressable memory.

Now how much bigger, that is a tough question. Most probably you
get
    T_1_64 >> T_1_256 > T_1

so the extra RAM will help, but in some cases the slowdown from
the "ideal" T_1 to the "real" T_1_256 might prove disappointing.

Sometimes, partitioning the problem might give you more speed
than throwing RAM at it. Sometimes!

I think you have no choice but to try it out.

Regards
- -- tomás
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