Advice on hash sizes please

I am now writing some database extraction utilities at the company I
work for, using Perl on Windows 2000/XP. There are several static tables
I download via ODBC that I perform lookups into... An ideal application
for a hash.

After the 5th table, I started to worry about how much memory this was
using, so I checked the combined size of these hashes, and it came to
about 64KB. This is without factoring-in the hash overhead. The app runs
well right now, but I'm concerned about breakage going forward.

Since this is used in-house, I can guarantee that the app will always
run on a machine with at least 2 GB of RAM and a 4 GB swap file. The
user might also be running a couple of Office apps, but that would be
all.

Granted that the answer to this question includes *many* variables I
have not addressed, is there still some sort of guideline I can use to
determine how much memory tied up in hashes is "too much"? Also, if I
reinitialize an existing hash to (), will that return the memory to
Windows?

Barry Brevik
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RE: Advice on hash sizes please

Barry Brevik <> wrote:
> I am now writing some database extraction utilities at the company I
> work for, using Perl on Windows 2000/XP. There are several static
> tables I download via ODBC that I perform lookups into... An ideal
> application for a hash. =

> =

> After the 5th table, I started to worry about how much memory this
> was using, so I checked the combined size of these hashes, and it
> came to about 64KB. This is without factoring-in the hash overhead.
> The app runs well right now, but I'm concerned about breakage going
> forward. =

> =

> Since this is used in-house, I can guarantee that the app will always
> run on a machine with at least 2 GB of RAM and a 4 GB swap file. The
> user might also be running a couple of Office apps, but that would be
> all. =

> =

> Granted that the answer to this question includes *many* variables I
> have not addressed, is there still some sort of guideline I can use
> to determine how much memory tied up in hashes is "too much"?

If you want to get a better picture of how much memory your hashes are
using, then something like Devel::Size might help, but I think you will
need considerably more than 64kb to give you any problems.

Of course if you are worried about the size of your hashes, it is
generally recommended to store them on disk, using Tie and the DB
modules, but you pay a penalty in access times, ...

.... or alternatively store your hashes in a database!

> Also,
> if I reinitialize an existing hash to (), will that return the memory
> to Windows? =


Perl may well return some freed memory to the OS in some circumstances,
but don't rely on it.

HTH

-- =

Brian Raven =


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