The so-called "Hash Memory" or "Transposition Table" is used by Houdini to store its analysis results.

 

Hash

Amount of hash table memory used by Houdini, in MB.

Default 128, min 4, max 1024 (32-bit) or 4096 (64-bit Standard) or 262144 (64-bit Pro).

The value is rounded down to a power of 2 (4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144 MB).

 

For infinite analysis or long time control matches you should use the largest hash that fits in the physical memory of your system. For example, on a system with 4 GB of memory you can use up to 2048 MB hash size.

For shorter games, for example 3 to 5 minute games, it’s better to use 256 MB or 512 MB as this will provide the best performance. For 16 min games 1024 or 2048 MB hash size should be fine.

 

In the FAQ about Hash Size you'll find a formula to compute the optimal hash size for your hardware and time control.

 

Clear Hash (button)

Button to clear the Hash Memory.

If the Never Clear Hash option is enabled, this button doesn't do anything.

 

Never Clear Hash (checkbox)

This option prevents the Hash Memory from being cleared between successive games or positions belonging to different games.

Check this option also if you want to Load the Hash from disk file, otherwise your loaded Hash could be cleared by a subsequent ucinewgame or Clear Hash command.