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

David,

Sorry to hear that didn't work.  We may actually be able to work on
this again after Tuesday next week, since that's when our AMS paper
is due.

In the meantime, if it would be any help at all, I could send the C
source that duplicates your example with chunking, so you could verify
the speedup.

You probably know this already, but just to make sure, writing the
256x256x64 variables in 32x64x64 sections is slow because when the
data is stored contiguously, writing each section involves 64x64=4096
calls to a low-level write function, each call writing only 32 values.
Also each write is preceded by a seek call to start writing at the
right location, and this seek call flushes I/O buffers.

It's my unverified theory that the occasional erratic slowdown by two
orders of magnitude is due to all these small writes getting queued up
until there's no more space to queue more pending writes, so the
process gets blocked and swapped out until the pending I/O pipe gets
drained, and when the process is swapped in again, all its buffers
have to get reestablished.

One way to prove something like this is going on would be to write a
program that just used low-level I/O (not netCDF) to write data in the
same pattern of 4096 writes of 32 4-byte values after a seek, to see
if the same erratic slowdowns are observed.  I may do this at some
point to see if my intuition is correct.

--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