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Roman_Gruber
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Joined: 03 Oct 2006
Posts: 3250
Location: Austro Bavaria

PostPosted: Wed Oct 28, 2015 2:08 pm    Post subject: gentoo-sources and the hidden features Reply with quote

It may be smart to enable those "experimental features of the gentoo sources"

The changes which popped up, oh it supports MIVYBRIDGE. Than i checked the description, and the only thing is that it uses the march=ivybridge settings which i already use.

For the BFQ scheduler, I think it may be wise, because the CFQ scheduler does not provide decent capabilities when i run the powersave governour and do the compile jobs. The box is a bit unresponsive, though it is an i7.

Code:
qlist -Iv gentoo-sources
sys-kernel/gentoo-sources-4.2.5
ASUS-G75VW linux # grep MIVYBRIDGE .config
CONFIG_MIVYBRIDGE=y
ASUS-G75VW linux # equery u gentoo-sources
[ Legend : U - final flag setting for installation]
[        : I - package is installed with flag     ]
[ Colors : set, unset                             ]
 * Found these USE flags for sys-kernel/gentoo-sources-4.2.5:
 U I
 - - build        : !!internal use only!! DO NOT SET THIS FLAG YOURSELF!, used for creating build images and the first half of bootstrapping
                    [make stage1]
 + + experimental : Apply experimental patches; for more information, see "https://wiki.gentoo.org/wiki/Project:Kernel/Experimental".
 - - kdbus        : Apply the kdbus patch. This also requires the "experimental" use flag.
 + + symlink      : Force kernel ebuilds to automatically update the /usr/src/linux symlink


I really wonder why it is called experimental and hidden in that useflag.
Without the experimental use-flag I only had before the option of core2duo which is kinda outdated and 7-8 years old architecture. I already wondered why there was no support for newer processors in the past.

I do not get the point to hide such features from the user, as it may also causes performance impacts.

For those who want to speed up the time to build a kernel may look into the --jobs feature of make.
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chithanh
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Joined: 05 Aug 2006
Posts: 2103
Location: Berlin, Germany

PostPosted: Fri Oct 30, 2015 3:36 pm    Post subject: Reply with quote

This is because the kernel does not support these architecture flags (it ignores make.conf and uses its own flags). Therefore these options should remain hidden from unsuspecting users accidentally stumbling over them and enabling them without knowing what they are doing.

Besides performance, the -march flags can also have a stability impact. If something breaks, you get to keep the pieces.
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Roman_Gruber
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Joined: 03 Oct 2006
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Location: Austro Bavaria

PostPosted: Fri Oct 30, 2015 4:11 pm    Post subject: Reply with quote

It looks like 4-8 percent speed increase building packages here with those changes. Hard to really prove and benchmark it. Could be also that bfq scheduler.

Yes you can say that for anything, regarding stability ...

A bit weird that i see those conftest messages only sometimes when I build with gcc 5.2 and newest glibc. I would expect to see those more often for example, or on every rebuilt of the same package.
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davidm
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Joined: 26 Apr 2009
Posts: 503
Location: US

PostPosted: Fri Oct 30, 2015 4:24 pm    Post subject: Reply with quote

About BFQ I tried it out for a while on spinning rust and it did seem more responsive but the catch was at the time it would freeze when doing a btrfs balance/scrub (I forget which one exactly) so I switched back to CFQ. Sadly now many months later CFQ is having problems with balance operations too so maybe I may as well just switch back.

Anyway I saw one bug report on it with BFQ and balance operations and my impression from the response is that BFQ is not as well tested as CFQ if nothing else because far fewer people are using it. I can see then why it is experimental.
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gjs278
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Joined: 04 Jan 2013
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PostPosted: Sat Oct 31, 2015 5:25 pm    Post subject: Reply with quote

thanks, got support for my processor by turning this on. never knew this one, I switch between ck and gentoo-sources and always wondered why ck had support for this but not gentoo-sources
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