Large application/file load-time

Question

I'm sure many have noticed that when you have a large application (i.e. something requiring a few MBs of DLLs) it loads much faster the second time than the first time. The same happens if you read a large file in your application. It's read much faster after the first time.

What affects this? I suppose this is the hard-drive cache, or is the OS adding some memory-caching of its own.

What techniques do you use to speed-up the loading times of large applications and files?

Thanks in advance

Note: the question refers to Windows

Added: What affects the cache size of the OS? In some apps, files are slow-loading again after a minute or so, so the cache fills in a minute?

Answer 1

Two things can affect this. The first is hard-disk caching (done by the disk which has little impact and by the OS which tends to have more impact). The second is that Windows (and other OS') have little reason to unload DLLs when they're finished with them unless the memory is needed for something else. This is because DLLs can easily be shared between processes.

So DLLs have a habit of hanging around even after the applications that were using them disappear. If another application decides the DLL is needed, it's already in memory and just has to be mapped into the processes address space.

I've seen some application pre-load their required DLLs (usually called QuickStart, I think both MS Office and Adobe Reader do this) so that the perceived load times are better.

Answer 2

Windows's memory manager is actually pretty slick -- it services memory requests AND acts as the disk cache. With enough free memory on the system, lots of files that have been recently accessed will reside in memory. Until the physical memory is needed, those DLLs will remain in cache -- all ala the CacheManager.

As far as how to help, look into Delay Loading your DLLs. The advantages of LoadLibrary only when you need it, but automatic so you don't have LoadLibrary/GetProcAddress on all of your code. (Well automatic, as far as just needing to add a linker command switch):

http://msdn.microsoft.com/en-us/library/yx9zd12s.aspx

Or you could pre-load like Office and others do (as mentioned above), but I personally hate that -- slows down the computer at initial boot up.