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Persistent Hash means that the in-memory hash table can be saved to a disk file and reloaded later for continuing the analysis.
The goal is to be able to interrupt a long position analysis anytime and save the hash table to a disk file. At a later date you can reload the hash file in memory and continue the analysis at the point where it was interrupted.
Correspondence chess players could profit from this feature by keeping one hash disk file per ongoing chess game. For that purpose the following procedure can be used.
To save a Hash file to disk:
To load a Hash file from disk:
File name for saving or loading the hash file with the Save Hash to File or Load Hash from File buttons.
A full file name is required, for example C:\Chess\Hash001.dat.
By default Houdini will use the hash.dat file in the "My Documents" folder of the current user.
Save the current hash table to a disk file specified by the Hash File option.
Use the Save Hash to File button after ending the analysis of the position. Some GUIs (e.g. Shredder, Fritz) wait for sending the button command to the engine until you click OK in the engine options window.
The size of the file will be identical to the size of the hash memory, so this operation could take a while.
This feature can be used to interrupt and restart a deep analysis at any time.
Load a previously saved hash file from disk.
Use the Load Hash from File button after loading the game or position, but before starting the analysis. Some GUIs (e.g. Fritz, Shredder) wait for sending the button command to the engine until you click OK in the engine options window.
The size of the hash memory will automatically be set to the size of the saved file.
Please make sure to check the Never Clear Hash option, as otherwise your loaded Hash could be cleared by a subsequent ucinewgame or Clear Hash command.