CODEWITHSUNDEEP
c++cgcccompilation
Aracthor
Aracthor

Reputation: 5907

Why is the first compilation of the day slower than next ones?

I'm using GCC/G++ for almost all my C and C++ projects for years, and there is something I never understood: the first use of one of them that you do after the start of your machine take always one or two seconds more than every next compilation until you shut down your computer.

As it is only once in a days, it is not really troublesome, but I just wonder why. Does it initialize anything and re-use it then?
If yes, does it save something somewhere?
If yes, why does it have to reinitialize it everyday?

Upvotes: 0

Views: 98

Answers (2)

Peter
Peter

Reputation: 36607

The short answer is caching, possibly at multiple levels.

For example, a lot of modern hard drives have a built in cache, so if some data is being read repeatedly (e.g. the gcc executables, libraries, makefiles, source files) the first access will result in data being in the device cache, and subsequent operations will interact with the cache rather than reading from the platters. Operations on the cache are must faster than - for example - positioning drive platters for reading.

A number of operating systems also implement some form of page cache, which does a similar thing (just using system RAM as a cache).

Most modern machines have various levels of cache (in devices, managed by the OS, multiple cache levels in the CPU, etc).

Caching can, depending on configuration, affect both read and write operations. Caches that affect writing (write operations actually write to cache and return, and are committed - such as to a drive - at a later time) are part of the reason operating systems require an explicit shutdown, so they commit all cached writes.

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Notably because of the page cache. If you have enough RAM, most of the data (including your source code files, and object files also) tend to stay there.

If that bothers you, use some SSD disks and/or run while drinking your first coffee before your first compilation a command reading all your source code, e.g. on Linux wc *.cc (and you could even make it a crontab job with @reboot); read also http://linuxatemyram.com/

You might also use the suspend-to-disk facility instead of the shutdown: but I don't think it is worth the pain (and I like /tmp/ being cleaned at boot time)

(I am guessing or hoping you are using Linux with a native filesystem, e.g. Ext4 or BTRFS)

Upvotes: 3

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