How does one flush undo history?

[Bill Lee]

I’m not sure – there is no explicit Flush Undo History, but maybe if you set the User Preferences>Levels of Undo [n] where n is the number of undos that are kept, you will limit the use of your precious RAM being tied up in the Undo stack. You can also quit FCP and restart it – that will flush the undo history buffer.

Every time to do something, it has to keep track of this in the Undo History buffers – the more that is done, the more state information must be stored away in case you want to roll backwards in time to a previous state. So, if you have 99 levels of undo, and you are making massive changes each action, then a huge amount of this state information has to be kept around in RAM. If you are short of RAM, then this amount of state information may cause FCP to run out of room in RAM. It then has to free up space to allow you to continue working (or finish the command you have asked it to do): Closing non-essential project frees up space, closing non-essential sequences frees up space, and truncating the undo history will also free up space. Tweaking the System Settings>Memory & Cache can free up more memory from the System RAM to use in FCP, and reducing the RAM available for stills and thumbnails will free up more space to be used for other things inside FCP.

Should you do this? Well, time is money, and the more time you spend managing this space, the less time you are being productive or creative. RAM is (relatively) cheap, and unless you have some limitation such as “All my RAM slots are full”, then I would buy more RAM. Setting FCP to use up all your available RAM might seem a good thing, but what happens if you want to open Photoshop or Motion to tweak a still or a clip? No spare RAM: must quit FCP then run the app, then restart FCP. Ugly – try not to set the FCP RAM use to 100% because that will give the system some room to expand if it has to.

You could set your Levels of Undo downwards, ultimately to a minimum of 1 undo, but that is also no fun at all. Every click of the mouse in changing parameters in the Motion menu counts as an action, so it is easy to use up many undos when you are adjusting these values. Being able to experiment is part of the creative process so tying yourself down with limited undos will restrict this. More RAM is the best solution, as well as managing levels of undo to reasonable levels is my suggestion.

Bill Lee