[Scons-dev] [Scons-users] Default number of jobs

[Kenny, Jason L]

I would back that up as well. The disk and IO bus can easily get maxed out. 


Jason

-----Original Message-----
From: scons-dev-bounces at scons.org [mailto:scons-dev-bounces at scons.org] On Behalf Of roberto de vecchi
Sent: Monday, July 30, 2012 9:36 AM
To: 'SCons users mailing list'
Cc: 'SCons developer list'
Subject: Re: [Scons-dev] [Scons-users] Default number of jobs

I completely agree with Marc comments. In my experience disk speed is the limiting factor for speeding up builds so increasing the number of parallel job may in some cases only saturate the disk I/O capability.

Roberto



> -----Original Message-----

> From: scons-users-bounces at scons.org 

> [mailto:scons-users-bounces at scons.org]

> On Behalf Of Marc Branchaud

> Sent: Monday, July 30, 2012 16:26

> To: SCons users mailing list

> Cc: SCons developer list

> Subject: Re: [Scons-users] [Scons-dev] Default number of jobs

> 

> Please don't change the default number of jobs.

> 

> For one thing, not all builds benefit from parallelism.  Ours 

> currently doesn't -- the disk cache gets overwhelmed, even with just a 

> small -j value on a 16-core 32GB machine.  What's more, the optimal 

> level of parallelism (best number of jobs/core) differs from project to project.

>   Finding the sweet spot is generally a matter of trial, tuning, and 

> error.

> 

> More importantly, it can be quite difficult to diagnose problems in 

> parallelized builds, especially since output from simultaneous 

> commands is jumbled together.  Parallel failures of simultaneous 

> commands are especially confusing.

> 

> When developing a build, it's best if your tools don't assume they can 

> do surprising things.  Parallelism is one such surprising thing.  

> Using an explicit flag to turn it on is a good way for the user to 

> tell the tool that she's ready for some possible weirdness.

> 

> 		M.

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