Re: [netcdfgroup] Inquire about cache size and nslots ?

• To: Jennifer Adams <jma@xxxxxxxxxxxxx>,netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: Re: [netcdfgroup] Inquire about cache size and nslots ?
• From: Ed Hartnett <ed@xxxxxxxxxxxxxxxx>
• Date: Tue, 17 Nov 2009 10:25:24 -0700

Jennifer Adams <jma@xxxxxxxxxxxxx> writes:

> Dear Experts, 
> I am working with the 4.1 snapshot of 20091102. I have been using the
> nc_inq_libvers() routine to discover the version of the library that is
> linked with my program (GrADS). Now I am testing various configure
> options, specifically "--with-chunk-cache-size=n" and
> "--with-chunk-cache-nelems=n".  Is there any way to discover these cache
> parameters from within my program -- something like nc_inq_cache()? Proper
> settings for cacheing and chunking are critical for optimizing performance
> when working with compressed netcdf files. It would be good if the users
> could know these settings before setting chunk sizes for creating
> compressed netcdf files. 
> --Jennifer
> --
> Jennifer M. Adams
> IGES/COLA
> 4041 Powder Mill Road, Suite 302
> Calverton, MD 20705
> [1]jma@xxxxxxxxxxxxx
>

Howdy Jennifer!

Thanks for asking a question for which there is an easy answer. Some of
the recent support issues have been real stumpers!

There are functions to both set and learn about the chunk
cache. Changing the cache settings only applies to future opens/creates,
it does not affect already-open files.

These don't tell you what the file was opened/created with, just the
current settings. So if you change them, and want to know what they are
for each file you open/create, then you must remember what they were
when the file was opened/created.

/* Set the cache size, nelems, and preemption policy. */
EXTERNL int
nc_set_chunk_cache(size_t size, size_t nelems, float preemption);

/* Get the cache size, nelems, and preemption policy. */
EXTERNL int
nc_get_chunk_cache(size_t *sizep, size_t *nelemsp, float *preemptionp);

Your question makes me note that these functions are not documented. I
am adding them now, and they will be in the snapshot documentation
tomorrow.

Thanks,

Ed

-- 
Ed Hartnett  -- ed@xxxxxxxxxxxxxxxx

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