Re: [Xen-devel] [PATCH] re-order struct domain fields

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.01.15 at 17:25, <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> On Mon, 2015-01-19 at 15:42 +0000, Jan Beulich wrote:
>> ... to reduce padding holes.
>> 
>> I also wonder whether having independent spin locks side by side is
>> really a good thing cache-line-bouncing-wise.
> 
> AIUI the general wisdom is to put each spinlock next to the data it
> protects (I suppose on the assumption that after acquiring the lock you
> will next touch that data).
> 
> Is the problem is that the domain_lock doesn't actually protect anything
> in this struct since it is a more umbrella lock, so there is nothing to
> put next to it?

Kind of. The more relevant aspect of why I didn't move it right away
was that two locks side by side are (on x86) efficient padding-wise.

> You could move either it or the page lock below all the page counts
> stuff.

Yeah, that would apparently be best, albeit I'm not sure it would
guarantee them to be on different cache lines in all cases.

Jan


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