PC GAMESS/Firefly-related discussion club

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Alex Granovsky
gran@classic.chem.msu.su

Recent Intel's Nehalem core processors (e.g., Core i7/Xeon 5500/Xeon 3500)
reimplement hyper-threading technology, which does result in some performance improvements
(see results of our tests in the "Performance" section).

Unfortunately, as of moment of writing this note (April 2009),
both Windows and Linux are not capable to identify correctly
the particular logical processors belonging to each specific CPU
core. In practice this confuses tasks/threads scheduler and
results in performance degradation of parallel or multithreaded jobs
while running with HTT turned on.

This was found for Windows Vista/Windows Server 2008 and most likely
affects older versions of Windows as well. As of now, all modern
Linux distributions with their default kernels are also affected.


The reason of this is most likely the changed semantics of some CPUID functions.

This problem, however, does not affect Firefly running with httfix
option turned on (see http://classic.chem.msu.su/gran/gamess/smp.html
for details on this option).


This option is turned on under Windows by default; however, to use it
under Linux, one needs to upgrade to Firefly version 7.1.E or above
and turn it on manually.


In addition, one may need to provide correct -ncores and -nthreads command line arguments
(see http://classic.chem.msu.su/gran/gamess/comm_line.html for details),
e.g. if running Firefly in environment with CPUID instruction virtualized.
For example, MS Hyper-V (and perhaps some other modern virtualization solutions)
virtualizes CPUID instruction thus limiting the amount of information available to Firefly.
This affects Firefly running both in the virtual machines and on the host OS itself.

Alternatively, one may disable HTT at all. As to Firefly running
using correct options described here, this will result in some
performance degradation. However, this may improve performance of
other programs one may need to use.

Best regards,
Alex Granovsky