Re: [Xen-devel] [PATCH 2/2] libxl: xl mem-set should not enforce memory limits

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Fri, Mar 15, 2013 at 08:01:00PM +0000, Ian Campbell wrote:
> On Fri, 2013-03-15 at 17:26 +0000, Konrad Rzeszutek Wilk wrote:
> > On Fri, Mar 15, 2013 at 05:09:51PM +0000, Ian Jackson wrote:
> > > Daniel Kiper writes ("Re: [PATCH 2/2] libxl: xl mem-set should not 
> > > enforce memory limits"):
> > > > I think that xl mem-max should be used to enforce limits. If admin
> > > > would like to enforce "hard" limit it should call xl mem-set and
> > > > xl mem-max in sequence. If we would like to leave old xl mem-set
> > > > behavior we should change comment for this command because now
> > > > it does not mention anythig about limit enforcement. Or we should
> > > > add an option which explicitly disables memory limit enforment
> > > > (this behavior is in line with xm mem-set behavior).
> > > 
> > > I think this conversation is related to the fact that at Oracle you
> > > have a different model of the Xen memory allocation model to everyone
> > > else.
> > 
> > Daniel is trying to fix an bug that Linux kernel is tripping over
> > b/c of this. Look at the converstation and patch that Daniel posted
> > a week ago for the Linux kernel.
> 
> It would be useful to mention (or at least) the rationale for a change
> such as this in the commit message.
> 
> However that's rather moot in this case because the rationale is surely
> wrong. Linux (and indeed balloon drivers generally) are expected to
> behave correctly whether the toolstack chooses to be enforcing or
> non-enforcing regarding the balloon target. So you can't "fix" the
> kernel by simply mandating that all toolstacks are non-enforcing, sorry.
> 
> > > Outside Oracle, guests are supposed to aim for the balloon target and
> > > are not permitted to exceed it (when ballooning up) or to regress
> > > (when ballooning down).
> > 
> > s/Oracle/Xend/. As Xend had this distinction. 'xm mem-set' would only set
> > the target. 'xm mem-max' on the other hand would enforce the limit.
> > 
> > Daniel is just bringing this behavior to 'xl'.
> 
> xl deliberately deviated from xend on this point.

Ah, I did not dig deep enough in the git annotate to see if there was a
story behind it. Perhaps then it would make sense to do two things:

 1). Add a comment in the code explicitly mentioning it.

 2). If we really want to provide an Xend-type behavior add an global
     configuration value that is called "xend-backwarts=1' ? That way
     we can fit all the other stuff that is inconsistent underneath
     that (such as timer_mode changing from number to string, etc).

     And as part of this global value also explain in the docs what
     those inconsistencies are?

> 
> Ian.
> 
> 
> 
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