Re: [boost] [1.58.0] Release candidates available

On 07. april 2015 12:05, Vladimir Prus wrote:

On 04/07/2015 11:40 AM, Rob Stewart wrote:

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On April 7, 2015 4:22:05 AM EDT, Olaf van der Spek wrote:

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On Tue, Apr 7, 2015 at 10:00 AM, Andrey Semashev wrote:

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Could b2 default to -jX where X is the number of cores?

This would be unexpected

Why?

It would be unlike other build tools, like make. It would be an breaking interface change.

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and detrimental if there is not enough RAM.

How much RAM is needed?

It scales almost linearly, I imagine. Each compiler instance requires memory to build a translation unit.

If Boost used to build fine on a limited machine, it might slow to a crawl due to paging with the added parallelism. Such users would have to use -j1 to avoid that problem.

Those with memory enough and more CPUs need to opt in to benefit, of course, but the build doesn't swamp such machines by default. However, if the option could be set in user.jam, then those that wish to opt in can do so once and then forget about it.

Rob, it used to be possible to do:

import option ; option.set jobs : 4 ;

in user-config.jam, but that appears to have regressed. Is that what you're after?

This would be a nice config setting. Also, in principle, bjam could check available free RAM before invoking a new parallel task. I think -j 4 does not mean it _has_ to run 4 tasks in parallel, rather it means up to 4 tasks in paralell. An implicit -j certainly should throttle on system resources, so why not available RAM as well as available cores. http://stackoverflow.com/questions/150355/programmatically-find-the-number-o... http://stackoverflow.com/questions/2513505/how-to-get-available-memory-c-g -- Bjørn