Download User guide for GADGET-2

User’s guide for GADGET-2
NumFilesWrittenInParallel?
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This parameter determines how many files may be written (or read) simultaneously by the code when generating (reading) snapshot files, or
when writing (reading) of restart files. Because each processor writes
its own restart file, it is in principle possible to do this fully in parallel,
with each processor writing ‘his’ file at the same time. However, for big
runs, say with 512 processors, a very large number of simultaneous I/O requests for large files may cause severe problems for the operating system,
since the number of available disk drives on the file server will likely be
smaller than the number of processors. As a result, the I/O system may
get congested because these requests cannot all be served efficiently at
the same time. In order to avoid such a congestion and to reach an optimum overall throughput of the I/O subsystem, it is often better not to
write too many files simultaneously. This can be achieved with the parameter NumFilesWrittenInParallel, which restricts the number of
files GADGET-2 processes concurrently during restart or snapshot file reading/writing. Note that NumFilesWrittenInParallel must be equal
or smaller than the number of processors used.
5.9 System of units
UnitVelocity in cm per s
1e5
This sets the internal velocity unit in cm/sec. The above choice
is convenient – it sets the velocity unit to km/sec.
Note that
the specification of UnitLength in cm, UnitMass in g, and
UnitVelocity in cm per s also determines the internal unit of time.
The definitions made above imply that in internal units the Hubble constant
has a numerical value independent of h and hence HubbleParam. For
the numerical examples above, the Hubble constant has always the value
0.1 in internal units, independent of h, and the Hubble time is always 10 in
internal units, with one internal time unit corresponding to 9.8 × 10 8 yr/h.
However, you are free to choose a different system of units if you like.
Note also that this implies that for purely gravitational dynamics, the code
will not need to know the value of h at all. HubbleParam is kept in
the parameterfile only because additional physics in the hydrodynamical
sector (which is not contained in the public distribution of GADGET-2) may
require it.
UnitLength in cm
3.085678e21
This sets the internal length unit in cm/h, where H0
=
100 h km s−1 Mpc−1 . The above choice is convenient – it sets the
length unit to 1.0 kpc/h.
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