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[netCDF #NNS-961385]: nf90_put_var slows down erratically

David,

The nf90_close() function does flush buffers in user space and call close(2) on
the
file descriptor, but that doesn't necessarily mean all the data is written to
disk
yet.

I still think the behavior you are seeing, since it's erratic and affected by
other I/O
that's happening on the system at the same time, is an artifact of the way the
operating
system schedules and buffers writes to disk.  I suspect if you just used binary
writes
instead of netCDF writes that accessed disk space the same way, you would see
similar
delays and inconsistencies.  It's hard to diagnose a performance problem that's
not
exactly repeatable like this one, but I'm skeptical that it's something we can
fix in
netCDF.

I appreciate the effort you've put into creating an example program that
demonstrates the
problem, and after translating your program to C, I've found it useful for
benchmarking
the use of chunked data access in netCDF-4.  The program runs remarkably faster
using netCDF-4
when the chunking parameters are set to store the data in 32x64x64x1 chunks,
with no
occasional delays such as you see in the contiguous storage case with netCDF-3,
because it
writes each 32x64x64 section with a single write() call, rather than with 64x64
write() calls
writing 32 floats each time.

The wall-clock times I saw were 49 minutes on local disk with netCDF-3 versus
17 seconds using
netCDF-4 with chunked storage.  One new call sets the chunk parameters, then
all subsequent
netCDF write and read calls are the same as you already use.

This faster write performance comes at the expense of somewhat slower read
performance when
accessing the data contiguously, since now 8 reads would be required to get a
256 element
row (column in Fortran), instead of one read for all 256 values.  But chunking
the data means
it can be accessed moderately quickly along any dimension, instead of fast
along only one
dimension.

You may not be interested in using netCDF-4 now, since it's only available in
beta release,
but it does support complete backward compatibilty with the netCDF-3 API as
well as format.

I'm not completely done looking at this example, so if I find anything else
about it in the
future, I'll let you know.  For the next week or so, I'll be involved in the
AMS meeting ...

--Russ



Russ Rew                                         UCAR Unidata Program
address@hidden                     http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: NNS-961385
Department: Support netCDF
Priority: Normal
Status: Closed