Efficient memory modification tracking with hierarchical dirty indicators

A fault tolerant computer system comprising:

• a dirty memory, a plurality of processing sets that include main memory, and logic for managing reinstatement of an equivalent memory state in the main memory of said memory sets following a lock step error, the logic being operable to perform at least one cycle of copying any block of memory that has been dirtied from a first processing set to each other processing set, each cycle including searching said dirty memory for dirty indicators, said dirty memory being hierarchically configured and comprising a first level including groups of dirty indicators, each of which is associated with a respective block of memory, and at least one higher level including dirty group indicators, each of which is associated with a respective group of said dirty indicators.

A method of managing reinstatement of an equivalent memory state in the main memory of a plurality of processing sets of a fault tolerant computer following a lock step error, the method including the performance of at least one cycle of copying any block of memory that has been dirtied from a first processing set to each other processing set, each cycle including:

• identifying any block of memory that has been dirtied from a dirty memory organised hierarchically with a lower level that includes groups of dirty indicators, each dirty indicator being settable to a given state indicative that a block of memory associated therewith has been dirtied, and at least one higher level that includes dirty group indicators settable to a predetermined state indicative that a group of the lower level associated therewith has at least one dirty indicator in a state indicative that a block of memory associated therewith has been dirtied;
• and
• copying a block identified as having been dirtied.