puterboy has asked for the wisdom of the Perl Monks concerning the following question:

I am using a large hash (millions of entries) as a cache.

The keys are (sparsely spaced) unsigned integers.
The values are 32 hex char string followed by an optional unsigned integer.

Storing the hash normally as $hash{<u_integer>}=<32 hex> takes about 190 bytes per entry (as determined by using Devel::Size).

However, knowing the format it seems like I should be able to pack the 32 hex characters into 16 bytes. Similarly the optional unsigned integer string, should be able to be packed more efficiently than a char string.

Perhaps, I could even save on the key storage, knowing that they are unsigned integers.

I am looking to do a better "packing" not any fancy compression scheme. Note I tried various combinations of pack such as pack("H32l", <32hex>, <uint>) but it got me only about a 25% saving. There must be a better way of packing (assuming I am willing to sacrifice a little speed). I mean if the key is o(4) bytes and the value is o(16-20) bytes, I would think I could do better than taking o(150-200) bytes which is almost a 90% overhead. Or maybe hashes are by necessity that inefficient...

Any suggestions?