credit2 data structures

credit2 data structures

[Jan Beulich]

George,

while hunting down direct assignments of cpumask_t variables (which I'm
trying to eliminate so that hypervisor binaries built for many CPUs won't
have undue memory overhead on "normal" [smaller] systems), I stumbled
across memory references that at the first glance looked buggy to me
due to their huge immediates. As it turns out, they're not buggy but a
result of credit2's struct csched_private - particularly having a NR_CPUS
sized array of struct csched_runqueue_data, which in turn has three
cpumask_t-s, totaling to a structure size of about 6.5Mb when building
for 4095 CPUs (the current upper limit in Xen).

Apart from the possibility of allocating the arrays (and maybe also the
cpumask_t-s) separately (for which I can come up with a patch on top
of what I' currently putting together) - is it really necessary to have
all these, the more that there can be multiple instances of the structure
with CPU pools?

Jan

Re: credit2 data structures

[George Dunlap]

On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich <JBeulich@suse.com> wrote:
> Apart from the possibility of allocating the arrays (and maybe also the
> cpumask_t-s) separately (for which I can come up with a patch on top
> of what I' currently putting together) - is it really necessary to have
> all these, the more that there can be multiple instances of the structure
> with CPU pools?

I'm not quite sure what it is that you're asking.  Do you mean, are
all of the things in each runqueue structure necessary?  Specifically,
I guess, the cpumask_t structures (because the rest of the structure
isn't significantly larger than the per-cpu structure for credit1)?

At first blush, all of those cpu masks are necessary.  The assignment
of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
 In theory, "idle" and "tickled" only need bits for the cpus actually
assigned to this runqueue (which should be 2-8 under normal
circumstances).  But then we would need some kind of mechanism to
translate "mask just for these cpus" to "mask of all cpus" in order to
use the normal cpumask mechanisms, which seems like a lot of extra
complexity just to save a few bytes.  Surely a system with 4096
logical cpus can afford 6 megabytes of memory for scheduling?

For one thing, the number of runqueues in credit2 is actually meant to
be smaller than the number of logical cpus -- it's meant to be one per
L2 cache, which should have between 2 and 8 logical cpus, depending on
the architecture.  I just put NR_CPUS because it was easier to get
working.  Making that an array of pointers, which is allocated on an
as-needed basis, should reduce that requirement a great deal.

 -George

Re: credit2 data structures

[Keir Fraser]

On 13/10/2011 11:11, "George Dunlap" <George.Dunlap@eu.citrix.com> wrote:

> For one thing, the number of runqueues in credit2 is actually meant to
> be smaller than the number of logical cpus -- it's meant to be one per
> L2 cache, which should have between 2 and 8 logical cpus, depending on
> the architecture.  I just put NR_CPUS because it was easier to get
> working.  Making that an array of pointers, which is allocated on an
> as-needed basis, should reduce that requirement a great deal.

That would suffice. If we can put per-cpu stuff in the per_cpu() data area
then even better. The fact that credit2 burns a couple of kB per CPU isn't a
problem at all, as long as it does it only for active CPUs.

 -- Keir

Re: credit2 data structures

[Jan Beulich]

>>> On 13.10.11 at 12:57, Keir Fraser <keir@xen.org> wrote:
> On 13/10/2011 11:11, "George Dunlap" <George.Dunlap@eu.citrix.com> wrote:
> 
>> For one thing, the number of runqueues in credit2 is actually meant to
>> be smaller than the number of logical cpus -- it's meant to be one per
>> L2 cache, which should have between 2 and 8 logical cpus, depending on
>> the architecture.  I just put NR_CPUS because it was easier to get
>> working.  Making that an array of pointers, which is allocated on an
>> as-needed basis, should reduce that requirement a great deal.
> 
> That would suffice. If we can put per-cpu stuff in the per_cpu() data area
> then even better.

That might not be possible, as there can be more than one instance of
that scheduler.

> The fact that credit2 burns a couple of kB per CPU isn't a
> problem at all, as long as it does it only for active CPUs.

Fully agree.

Jan

Re: credit2 data structures

[Jan Beulich]

>>> On 13.10.11 at 12:11, George Dunlap <George.Dunlap@eu.citrix.com> wrote:
> On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich <JBeulich@suse.com> wrote:
>> Apart from the possibility of allocating the arrays (and maybe also the
>> cpumask_t-s) separately (for which I can come up with a patch on top
>> of what I' currently putting together) - is it really necessary to have
>> all these, the more that there can be multiple instances of the structure
>> with CPU pools?
> 
> I'm not quite sure what it is that you're asking.  Do you mean, are
> all of the things in each runqueue structure necessary?  Specifically,
> I guess, the cpumask_t structures (because the rest of the structure
> isn't significantly larger than the per-cpu structure for credit1)?

No, it's really the NR_CPUS-sized array of struct csched_runqueue_data.
Credit1 otoh has *no* NR_CPUS sized arrays at all.

> At first blush, all of those cpu masks are necessary.  The assignment
> of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
>  In theory, "idle" and "tickled" only need bits for the cpus actually
> assigned to this runqueue (which should be 2-8 under normal
> circumstances).  But then we would need some kind of mechanism to
> translate "mask just for these cpus" to "mask of all cpus" in order to
> use the normal cpumask mechanisms, which seems like a lot of extra
> complexity just to save a few bytes.  Surely a system with 4096
> logical cpus can afford 6 megabytes of memory for scheduling?

I'm not concerned about the total amount if run on a system that
large. I'm more concerned about this being a single chunk (possibly
allocated post-boot, where we're really aiming at having no
allocations larger than a page at all) and this size being allocated
even when running on a much smaller system (i.e. depending only
on compile time parameters).

> For one thing, the number of runqueues in credit2 is actually meant to
> be smaller than the number of logical cpus -- it's meant to be one per
> L2 cache, which should have between 2 and 8 logical cpus, depending on
> the architecture.  I just put NR_CPUS because it was easier to get
> working.  Making that an array of pointers, which is allocated on an
> as-needed basis, should reduce that requirement a great deal.

That would help, but would probably not suffice (since a NR_CPUS
sized array of pointers is still going to be larger than a page). We
may need to introduce dynamic per-CPU data allocation for this...

Jan

Re: credit2 data structures

[Juergen Gross]

On 10/13/2011 02:24 PM, Jan Beulich wrote:
>>>> On 13.10.11 at 12:11, George Dunlap<George.Dunlap@eu.citrix.com>  wrote:
>> On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich<JBeulich@suse.com>  wrote:
>>> Apart from the possibility of allocating the arrays (and maybe also the
>>> cpumask_t-s) separately (for which I can come up with a patch on top
>>> of what I' currently putting together) - is it really necessary to have
>>> all these, the more that there can be multiple instances of the structure
>>> with CPU pools?
>> I'm not quite sure what it is that you're asking.  Do you mean, are
>> all of the things in each runqueue structure necessary?  Specifically,
>> I guess, the cpumask_t structures (because the rest of the structure
>> isn't significantly larger than the per-cpu structure for credit1)?
> No, it's really the NR_CPUS-sized array of struct csched_runqueue_data.
> Credit1 otoh has *no* NR_CPUS sized arrays at all.
>
>> At first blush, all of those cpu masks are necessary.  The assignment
>> of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
>>   In theory, "idle" and "tickled" only need bits for the cpus actually
>> assigned to this runqueue (which should be 2-8 under normal
>> circumstances).  But then we would need some kind of mechanism to
>> translate "mask just for these cpus" to "mask of all cpus" in order to
>> use the normal cpumask mechanisms, which seems like a lot of extra
>> complexity just to save a few bytes.  Surely a system with 4096
>> logical cpus can afford 6 megabytes of memory for scheduling?
> I'm not concerned about the total amount if run on a system that
> large. I'm more concerned about this being a single chunk (possibly
> allocated post-boot, where we're really aiming at having no
> allocations larger than a page at all) and this size being allocated
> even when running on a much smaller system (i.e. depending only
> on compile time parameters).
>
>> For one thing, the number of runqueues in credit2 is actually meant to
>> be smaller than the number of logical cpus -- it's meant to be one per
>> L2 cache, which should have between 2 and 8 logical cpus, depending on
>> the architecture.  I just put NR_CPUS because it was easier to get
>> working.  Making that an array of pointers, which is allocated on an
>> as-needed basis, should reduce that requirement a great deal.
> That would help, but would probably not suffice (since a NR_CPUS
> sized array of pointers is still going to be larger than a page). We
> may need to introduce dynamic per-CPU data allocation for this...

Couldn't the run-queue data be dynamically allocated and the pcpu-data of
credit2 contain a pointer to it?


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
PDG ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@ts.fujitsu.com
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

Re: credit2 data structures

[Jan Beulich]

>>> On 13.10.11 at 14:54, Juergen Gross <juergen.gross@ts.fujitsu.com> wrote:
> On 10/13/2011 02:24 PM, Jan Beulich wrote:
>>>>> On 13.10.11 at 12:11, George Dunlap<George.Dunlap@eu.citrix.com>  wrote:
>>> On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich<JBeulich@suse.com>  wrote:
>>>> Apart from the possibility of allocating the arrays (and maybe also the
>>>> cpumask_t-s) separately (for which I can come up with a patch on top
>>>> of what I' currently putting together) - is it really necessary to have
>>>> all these, the more that there can be multiple instances of the structure
>>>> with CPU pools?
>>> I'm not quite sure what it is that you're asking.  Do you mean, are
>>> all of the things in each runqueue structure necessary?  Specifically,
>>> I guess, the cpumask_t structures (because the rest of the structure
>>> isn't significantly larger than the per-cpu structure for credit1)?
>> No, it's really the NR_CPUS-sized array of struct csched_runqueue_data.
>> Credit1 otoh has *no* NR_CPUS sized arrays at all.
>>
>>> At first blush, all of those cpu masks are necessary.  The assignment
>>> of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
>>>   In theory, "idle" and "tickled" only need bits for the cpus actually
>>> assigned to this runqueue (which should be 2-8 under normal
>>> circumstances).  But then we would need some kind of mechanism to
>>> translate "mask just for these cpus" to "mask of all cpus" in order to
>>> use the normal cpumask mechanisms, which seems like a lot of extra
>>> complexity just to save a few bytes.  Surely a system with 4096
>>> logical cpus can afford 6 megabytes of memory for scheduling?
>> I'm not concerned about the total amount if run on a system that
>> large. I'm more concerned about this being a single chunk (possibly
>> allocated post-boot, where we're really aiming at having no
>> allocations larger than a page at all) and this size being allocated
>> even when running on a much smaller system (i.e. depending only
>> on compile time parameters).
>>
>>> For one thing, the number of runqueues in credit2 is actually meant to
>>> be smaller than the number of logical cpus -- it's meant to be one per
>>> L2 cache, which should have between 2 and 8 logical cpus, depending on
>>> the architecture.  I just put NR_CPUS because it was easier to get
>>> working.  Making that an array of pointers, which is allocated on an
>>> as-needed basis, should reduce that requirement a great deal.
>> That would help, but would probably not suffice (since a NR_CPUS
>> sized array of pointers is still going to be larger than a page). We
>> may need to introduce dynamic per-CPU data allocation for this...
> 
> Couldn't the run-queue data be dynamically allocated and the pcpu-data of
> credit2 contain a pointer to it?

Not if the per-CPU data is also per scheduler instance (which I can't
easily tell whether it is).

Jan

Re: credit2 data structures

[Juergen Gross]

On 10/13/2011 04:17 PM, Jan Beulich wrote:
>>>> On 13.10.11 at 14:54, Juergen Gross<juergen.gross@ts.fujitsu.com>  wrote:
>> On 10/13/2011 02:24 PM, Jan Beulich wrote:
>>>>>> On 13.10.11 at 12:11, George Dunlap<George.Dunlap@eu.citrix.com>   wrote:
>>>> On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich<JBeulich@suse.com>   wrote:
>>>>> Apart from the possibility of allocating the arrays (and maybe also the
>>>>> cpumask_t-s) separately (for which I can come up with a patch on top
>>>>> of what I' currently putting together) - is it really necessary to have
>>>>> all these, the more that there can be multiple instances of the structure
>>>>> with CPU pools?
>>>> I'm not quite sure what it is that you're asking.  Do you mean, are
>>>> all of the things in each runqueue structure necessary?  Specifically,
>>>> I guess, the cpumask_t structures (because the rest of the structure
>>>> isn't significantly larger than the per-cpu structure for credit1)?
>>> No, it's really the NR_CPUS-sized array of struct csched_runqueue_data.
>>> Credit1 otoh has *no* NR_CPUS sized arrays at all.
>>>
>>>> At first blush, all of those cpu masks are necessary.  The assignment
>>>> of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
>>>>    In theory, "idle" and "tickled" only need bits for the cpus actually
>>>> assigned to this runqueue (which should be 2-8 under normal
>>>> circumstances).  But then we would need some kind of mechanism to
>>>> translate "mask just for these cpus" to "mask of all cpus" in order to
>>>> use the normal cpumask mechanisms, which seems like a lot of extra
>>>> complexity just to save a few bytes.  Surely a system with 4096
>>>> logical cpus can afford 6 megabytes of memory for scheduling?
>>> I'm not concerned about the total amount if run on a system that
>>> large. I'm more concerned about this being a single chunk (possibly
>>> allocated post-boot, where we're really aiming at having no
>>> allocations larger than a page at all) and this size being allocated
>>> even when running on a much smaller system (i.e. depending only
>>> on compile time parameters).
>>>
>>>> For one thing, the number of runqueues in credit2 is actually meant to
>>>> be smaller than the number of logical cpus -- it's meant to be one per
>>>> L2 cache, which should have between 2 and 8 logical cpus, depending on
>>>> the architecture.  I just put NR_CPUS because it was easier to get
>>>> working.  Making that an array of pointers, which is allocated on an
>>>> as-needed basis, should reduce that requirement a great deal.
>>> That would help, but would probably not suffice (since a NR_CPUS
>>> sized array of pointers is still going to be larger than a page). We
>>> may need to introduce dynamic per-CPU data allocation for this...
>> Couldn't the run-queue data be dynamically allocated and the pcpu-data of
>> credit2 contain a pointer to it?
> Not if the per-CPU data is also per scheduler instance (which I can't
> easily tell whether it is).

Each cpu has only one dynamically allocated scheduler pcpu-data area which is
anchored in the per_cpu area of that cpu.


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
PDG ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@ts.fujitsu.com
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html