Xen devel - Jun 2005 - RE: [PATCH] Add DOM0_GETDOMAININFOLIST op for bulkretrieval of domain info

> The attached patch adds a new dom0_op, 
> DOM0_GETDOMAININFOLIST.  This operation allows retrieval of 
> the domain info structures for all domains in one hypercall.
> 
> Using a small test program, on a system with 97 domains with 
> non-contiguous domain IDs, I found that with this hypercall I 
> could retrieve the full array of info structures 12840 times 
> per second, an improvement from 2380 times per second with 
> the DOM0_GETDOMAININFO op.
Can''t you just reuse the existing xc_getdomain_info function and change
the hypercall depending on the number of domains being requested?

Ian

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On 25 Jun 2005, at 02:41, Ian Pratt wrote:
>> Using a small test program, on a system with 97 domains with
>> non-contiguous domain IDs, I found that with this hypercall I
>> could retrieve the full array of info structures 12840 times
>> per second, an improvement from 2380 times per second with
>> the DOM0_GETDOMAININFO op.
>
> Can''t you just reuse the existing xc_getdomain_info function and
change
> the hypercall depending on the number of domains being requested?
Why not just add nr_doms argument to existing GETDOMAININFO op?
Maybe make nr_doms==0 same as nr_doms==1 for backward source 
compatibility.

  -- Keir


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On Sat, 2005-06-25 at 02:41 +0100, Ian Pratt wrote:
> > The attached patch adds a new dom0_op, 
> > DOM0_GETDOMAININFOLIST.  This operation allows retrieval of 
> > the domain info structures for all domains in one hypercall.
> > 
> > Using a small test program, on a system with 97 domains with 
> > non-contiguous domain IDs, I found that with this hypercall I 
> > could retrieve the full array of info structures 12840 times 
> > per second, an improvement from 2380 times per second with 
> > the DOM0_GETDOMAININFO op.
> 
> Can''t you just reuse the existing xc_getdomain_info function and
change
> the hypercall depending on the number of domains being requested?
xc_domain_getinfo?  Ideally yes.  However, xc_domain_getinfo performs a
translation from the xc_domaininfo_t structure (a typedef of
dom0_getdomaininfo_t) returned by DOM0_GETDOMAININFO to the array of
xc_dominfo_t structures passed by the caller.  So in order to use the
bulk call from xc_domain_getinfo, it would need to allocate its own
array of xc_domaininfo_t structures, make the hypercall, and then
perform the translation on the array of structures.  If
xc_domain_getinfo took a caller-provided array of xc_domaininfo_t
structures, I would definitely have just modified it to use the new
hypercall rather than making a new function.

What is the purpose of that translation from one structure to the other?
The only differences seem to be reordered/renamed fields, and splitting
the flags field out into individual bitfields.  If that is the only
purpose, then why not just have xc_domaininfo_t include a union of the
flags field and a structure containing a bitfield?

- Josh Triplett



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On Sat, 2005-06-25 at 10:17 +0100, Keir Fraser wrote:
> On 25 Jun 2005, at 02:41, Ian Pratt wrote:
> 
> >> Using a small test program, on a system with 97 domains with
> >> non-contiguous domain IDs, I found that with this hypercall I
> >> could retrieve the full array of info structures 12840 times
> >> per second, an improvement from 2380 times per second with
> >> the DOM0_GETDOMAININFO op.
> >
> > Can''t you just reuse the existing xc_getdomain_info function
and change
> > the hypercall depending on the number of domains being requested?
> 
> Why not just add nr_doms argument to existing GETDOMAININFO op?
> Maybe make nr_doms==0 same as nr_doms==1 for backward source 
> compatibility.
DOM0_GETDOMAININFO only includes one dom0_getdomaininfo_t structure, not
a pointer to an array of such structures (and the length of that array),
so the operation would have no place to store the data.

- Josh Triplett



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On Mon, 2005-06-27 at 11:33 -0700, Josh Triplett wrote:
> On Sat, 2005-06-25 at 02:41 +0100, Ian Pratt wrote:
> > > The attached patch adds a new dom0_op, 
> > > DOM0_GETDOMAININFOLIST.  This operation allows retrieval of 
> > > the domain info structures for all domains in one hypercall.
> > > 
> > > Using a small test program, on a system with 97 domains with 
> > > non-contiguous domain IDs, I found that with this hypercall I 
> > > could retrieve the full array of info structures 12840 times 
> > > per second, an improvement from 2380 times per second with 
> > > the DOM0_GETDOMAININFO op.
> > 
> > Can''t you just reuse the existing xc_getdomain_info function
and change
> > the hypercall depending on the number of domains being requested?
> 
> xc_domain_getinfo?  Ideally yes.  However, xc_domain_getinfo performs a
> translation from the xc_domaininfo_t structure (a typedef of
> dom0_getdomaininfo_t) returned by DOM0_GETDOMAININFO to the array of
> xc_dominfo_t structures passed by the caller.  So in order to use the
> bulk call from xc_domain_getinfo, it would need to allocate its own
> array of xc_domaininfo_t structures, make the hypercall, and then
> perform the translation on the array of structures.  If
> xc_domain_getinfo took a caller-provided array of xc_domaininfo_t
> structures, I would definitely have just modified it to use the new
> hypercall rather than making a new function.
> 
> What is the purpose of that translation from one structure to the other?
> The only differences seem to be reordered/renamed fields, and splitting
> the flags field out into individual bitfields.  If that is the only
> purpose, then why not just have xc_domaininfo_t include a union of the
> flags field and a structure containing a bitfield?
Looking at the Python bindings, I see that they make use of this
split-out structure to more easily fill the equivalent Python data
structure.  Would it help to provide a patch which would change the
python bindings to call xc_domain_getinfolist and deal with
xc_domaininfo_t (aka dom0_getdomaininfo_t) directly (possibly modifying
dom0_getdomaininfo_t to contain something like a union of the flags and
a split-out bitfield structure)?  With that change, xc_domain_getinfo
could then become a backward-compatibility function; it could also
optionally be changed to fetch domain information in chunks using
DOM0_GETINFOLIST.

Also, is it acceptable for libxc to make use of unnamed unions and
unnamed structs as structure members?

- Josh Triplett



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