This has been discussed in the past, and I believe there are some 64 bit executables around somewhere although they aren’t officially supported (and I’m not sure what they are capable of). I’m not saying this is impossible, but Notepad++ wasn’t really designed with this large of file size in mind. As you are aware this can lead to very poor performance. Personally I’d suggest using an editor made specifically for handling large files.
Since a few other people have asked about this, I’ll point out just a few things that would need addressed before people should routinely work with large files using Notepad++.
Auto-complete - You better hope you have this option turned off, or any time you start typing, Notepad++ will run a regex over the entire document to find all possible auto-complete hints. With large documents there will be lag spikes.
Periodic backups - If this is on and you edit a file, I can almost guarantee you will corrupt the file in a matter of no time.
Plugins - First of all they’d have to be recompiled for a 64 bit version. Secondly, plugins would have to take some precautions to not do anything like scanning the entire document, etc. I’ve had to remove some plugins even working with files a few hundred megabytes in size because they caused a very big performance hit.
Memory - A 4GB file can take up alot more than just 4GB. Especially for something like an XML file. Also there are at least a few places in the Notepad++ source code that it can possibly copy the entire file in memory, leading to even more memory usage.
Scintilla - This is what actually handles the styling/editing of the text within Notepad++. I’m not sure how well it would handle files of this size.
Again I’m not saying it is impossible, and even as a “first attempt” Notepad++ could turn off features it knows could lead to poor performance and warn the user. However for Notepad++ to gracefully open/edit large files, would take a good bit of work to make sure everything in the code is aware that it could be dealing with a very large file.
But I guess if you are desperate (and/or adventurous) then why not try out a 64bit version if you can find it. Would be interesting to see how well it works. :)
