[Interest] Slow compiling on Mac

[Thiago Macieira]

On quinta-feira, 30 de agosto de 2012 15.52.49, Konstantin Tokarev wrote:
> 1. Compilation is not entirely CPU-bound process.
> 2. OS scheduler is not perfect.

The best performance ratio I can get on my dual core x dual thread machine is 
375% CPU utilisation, so a -j4 would be recommended.

However, I always build with -j5. My benchmarks show that from -j4 to -j6, 
there's minimal difference. if I add more, it starts to slow down again; if I 
use fewer, it idles.

The rationale for using more than your number of CPUs is that there's a lot of 
I/O involved. Reading from disk is unbearably slow compared to other 
operations.

I have an SSD, which means it's not that slow, so I don't gain much going from 
-j4 to -j5. But on a mechanical hard drive, that is more pronounced.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
     Intel Sweden AB - Registration Number: 556189-6027
     Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 190 bytes
Desc: This is a digitally signed message part.
URL: <http://lists.qt-project.org/pipermail/interest/attachments/20120830/c6ff0e2f/attachment.sig>