Thursday, August 20, 2009

GAMESS, memory and parallel

The screencast above is a repeat from the last post, where I computed the frequencies of a molecule at the RHF/3-21G level of theory (second screencast) and discovered that the amount of memory that GAMESS requests (1,000,000 words) was not enough. While GAMESS tells me how much it needs in the output file, it does so after a lot of computation, which is wasted because I have to start over.

In this post I discuss the basics of GAMESS memory requirements. The simplest case is when you are not running in parallel, i.e. the number of processors in GAMESSQ is set to 1, so I discuss this case first.

1. Memory is specified in $system, and the default is 1 mega-word (i.e. 1,000,000 words):

$system mwords=1 $end

mword can only be integers (1, 2, 3, ..). (In the screencast above I used the older "memory" keyword but mwords is much more convenient.)

2. A word is 8 bytes, so the default memory request (1 mega-word) is a very modest 8 MB of RAM.

3. This is the maximum amount of memory GAMESS is allowed to use. Depending on the type of calculation GAMESS might use less. The option is there to avoid filling up the memory on the computer entirely, which will crash or freeze the computer.

4. Here is the simplest way to deal with memory: My current laptop computer has 2 GB of RAM, and I use it for other things while GAMESS is running, so I am willing to give GAMESS a maximum of roughly 1 GB of RAM. This translates to

1 GB ≈ 1,000 MB ≈ 125 mwords

5. Adding

$system mwords=125 $end
to all input files will allow me to run most GAMESS jobs that I would want to run on a laptop without ever getting a "not enough memory" error (of course if you have less memory you need to adjust mwords accordingly).

That's basically it. What follows is some more details that most casual users of GAMESS won't have to worry about.

6. Most common GAMESS runs will never get close to using 1 GB of RAM, meaning the memory is free to be used by other programs. The most common types of runs that potentially could use that much memory are frequency calculations using RHF and any kind of MP2 calculation. Slightly less common ones are TDDFT and NMR calculations. In GAMESS, DFT frequency calculations are done numerically, and do not require a lot of memory.

7. If you are in doubt whether you have reqeusted enough memory to perform a calculation it is possible to use GAMESS to check using

$contrl exetyp=check $end

This keyword will simulate an actual GAMESS run by skipping the most time consuming steps so it is very fast.

I show two examples (an RHF/3-21G frequency and MP2/6-31G(d) single calculations on 3TSa) in this screencast.

Parallel runs

8. Many desktop computers (and almost all larger computers) have more than one CPU (also known as cores). For example, my current laptop has one processor with 2 cores. Thus, I could make my GAMESS calculations go faster by specifying 2 processors when submitting with GAMESSQ (processor = core in GAMESS-speak). This means that two separate but related GAMESS calculations are running simultaneously, and this affects the memory request:

9. Most types of runs use replicated memory. This means that if mwords=1 but I ask for 2 processors, GAMESS will use a maximum of 2 mega-words. Thus, if I routinely use 2 processors when running on my laptop and want to impose a limit of 1 GB RAM, I should use

$system mwords=63 $end

instead of 125.

10. The most common exception to this is MP2 calculations, where GAMESS uses both replicated (mwords) and distributed memory (memddi). Distributed memory is memory shared by the cores. If I specify memddi=100 and ask for 2 processors then 100 mega-words of RAM is distributed among the 2 processors (the simplest case is that each core gets 50 mega-words, but GAMESS figures that out for you).

So running on 2 processors with

$system mwords=15 memddi=100 $end
requests a total of (15 + 15 + 100 =) 130 mwords. You can use exetyp=check to figure out the optimum values of memory and memddi, as I show in this screencast.

The screencast shows how you can use 1 processor and

$contrl exetyp=check $end
$system memddi=100 parall=.t. $end

to check the memddi requirements. You need to make memddi large for this to work, but GAMESS will not use this memory during the check-run. Note that parall only "does something" for check runs: true parallel runs are specified by choosing more than 1 processors when you submit GAMESS.

You can get a complete list of runs that require memddi in Chapter 2 of the GAMESS documentation under the entry for $system. Also, Chapter 5 has a more in-depth description of how memory is handled.
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