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Old 08-04-2012, 11:53 AM
Cyril Brulebois
 
Default Byte queue limits in Linux for wheezy / linux kernel ABI bumps

Hi Ben,

Ben Hutchings <ben@decadent.org.uk> (03/08/2012):
> Byte queue limits (BQL) are an important step in reducing 'bufferbloat'
> and network latency on Linux. The feature was introduced in Linux 3.3
> and has had a few bug fixes since then; I think it can be considered
> stable now. It requires specific support in drivers and is then
> unconditionally enabled for them. It provides some tuning parameters,
> but should not normally require these to be changed.

thanks for the background.

> I have prepared a backport of this to Linux 3.2 that I would like to
> include in wheezy, but I ran out of time to do that before the freeze.
> Since BQL is a new feature, it would not normally be allowed to make such
> a change now. However, it may also be considered an important fix for
> performance problems (high network latency). Therefore I am requesting a
> freeze exception in category 5, "pre-approved fixes". I believe it can
> be easily backed-out in case it causes regressions.

That looks good to me.

> BQL requires an ABI change, but so do some of the fixes in Linux 3.2.25,
> so the next upload will have an ABI bump in any case.


[ Hijacking the subject a bit, adding -boot@ ]

That's one of the things I wanted to spend a little time on, while getting
back to the block-udeb-ified packages.

Now that d-i wheezy beta1 is finally out, I'll try and figure out what
exactly breaks in d-i when the linux kernelgets its ABI bumped. Apparently
netboot and mini.iso are concerned, but it would be nice to have that
checked, and documented.

Maybe we could just have a way to prevent some udebs from being decrufted?

Now, for other images, maybe we should just perform a debian-installer
upload, tweaking the linux ABI bits, each time we let a new linux ABI into
testing. Getting that debian-installer into testing would ensure weekly
builds work properly, even if no {alpha,beta,rc} are planned when that
migration happens.


Mraw,
KiBi.
 
Old 08-04-2012, 12:06 PM
Julien Cristau
 
Default Byte queue limits in Linux for wheezy / linux kernel ABI bumps

On Sat, Aug 4, 2012 at 13:53:17 +0200, Cyril Brulebois wrote:

> Now that d-i wheezy beta1 is finally out, I'll try and figure out what
> exactly breaks in d-i when the linux kernelgets its ABI bumped. Apparently
> netboot and mini.iso are concerned, but it would be nice to have that
> checked, and documented.
>
AFAIK, all images that download udebs from the archive can break with
any kernel migration to testing, because they'd get modules newer than
the kernel (without an ABI bump).

In case of ABI bumps, where the old udebs go away, these same images
won't be able to download the modules, so they'll break.

[Please somebody correct the above if this is nonsense]

Cheers,
Julien
 
Old 08-04-2012, 09:06 PM
Ben Hutchings
 
Default Byte queue limits in Linux for wheezy / linux kernel ABI bumps

On Sat, 2012-08-04 at 14:06 +0200, Julien Cristau wrote:
> On Sat, Aug 4, 2012 at 13:53:17 +0200, Cyril Brulebois wrote:
>
> > Now that d-i wheezy beta1 is finally out, I'll try and figure out what
> > exactly breaks in d-i when the linux kernelgets its ABI bumped. Apparently
> > netboot and mini.iso are concerned, but it would be nice to have that
> > checked, and documented.
> >
> AFAIK, all images that download udebs from the archive can break with
> any kernel migration to testing, because they'd get modules newer than
> the kernel (without an ABI bump).

This often works, but is certainly not guaranteed to (we would need a
lot more ABI bumps if we considered added symbols).

A possible solution would be something like:
1. Keep multiple versions of udebs in the same suite (currently possible
for arch:all, but maybe not supported for arch-dependent packages)
2. Make kernel-wedge add versioned dependencies to the udebs
3. Make anna assume that kernel-image is not upgradeable
4. Make anna try older package versions if dependencies can't be reolved
for the newest version

Steps 1 and 4 may be rather hard.

> In case of ABI bumps, where the old udebs go away, these same images
> won't be able to download the modules, so they'll break.

But that seems to be more easily fixed by holding off decrufting.

Ben.

> [Please somebody correct the above if this is nonsense]

--
Ben Hutchings
Experience is directly proportional to the value of equipment destroyed.
- Carolyn Scheppner
 
Old 08-05-2012, 05:43 PM
Bastian Blank
 
Default Byte queue limits in Linux for wheezy / linux kernel ABI bumps

On Sat, Aug 04, 2012 at 10:06:48PM +0100, Ben Hutchings wrote:
> A possible solution would be something like:
> 1. Keep multiple versions of udebs in the same suite (currently possible
> for arch:all, but maybe not supported for arch-dependent packages)

Breaks the expectations within d-i. This is listed on my TODO.

> 2. Make kernel-wedge add versioned dependencies to the udebs
> 4. Make anna try older package versions if dependencies can't be reolved
> for the newest version

Versions in dependencies will be ignored.

Bastian

--
Intuition, however illogical, is recognized as a command prerogative.
-- Kirk, "Obsession", stardate 3620.7


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Old 08-13-2012, 10:49 AM
Bastian Blank
 
Default Byte queue limits in Linux for wheezy / linux kernel ABI bumps

On Sun, Aug 05, 2012 at 07:43:50PM +0200, Bastian Blank wrote:
> On Sat, Aug 04, 2012 at 10:06:48PM +0100, Ben Hutchings wrote:
> > A possible solution would be something like:
> > 1. Keep multiple versions of udebs in the same suite (currently possible
> > for arch:all, but maybe not supported for arch-dependent packages)
> Breaks the expectations within d-i. This is listed on my TODO.
> > 4. Make anna try older package versions if dependencies can't be reolved
> > for the newest version
> Versions in dependencies will be ignored.

The packages stuff used in d-i is limited. It only allows one version of
a package, which produced problems with cdebootstrap. In addition it
disallows several other constructs: Breaks, Conflicts and ignores all
versions in relations.

I have the problem with multiple version on my list for a larger
rewrite, which will make it need more memory. It can't be fixed in a
backward compatible way.

Bastian

--
The sight of death frightens them [Earthers].
-- Kras the Klingon, "Friday's Child", stardate 3497.2


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