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Re: wake_wide mechanism clarification

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  Re: wake_wide mechanism clarification Joel Fernandes <joelaf@google.com> - 2017-07-29 10:10 +0200
    Re: wake_wide mechanism clarification Joel Fernandes <joelaf@google.com> - 2017-07-29 10:20 +0200
    Re: wake_wide mechanism clarification Mike Galbraith <umgwanakikbuti@gmail.com> - 2017-07-29 17:10 +0200
      Re: wake_wide mechanism clarification Joel Fernandes <joelaf@google.com> - 2017-07-29 22:20 +0200
        Re: wake_wide mechanism clarification Joel Fernandes <joelaf@google.com> - 2017-07-30 00:30 +0200
          Re: wake_wide mechanism clarification Joel Fernandes <joelaf@google.com> - 2017-07-30 00:50 +0200
            Re: wake_wide mechanism clarification Josef Bacik <josef@toxicpanda.com> - 2017-07-31 14:30 +0200

#1699256 — Re: wake_wide mechanism clarification

Hi Mike,

I have take spent some time understanding the email thread and
previous discussions. Unfortunately the second condition we are
checking for in the wake_wide still didn't make sense to me (mentioned
below) :-(

On Fri, Jun 30, 2017 at 10:02 AM, Mike Galbraith
<umgwanakikbuti@gmail.com> wrote:
> On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote:
>> On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote:
>>
>> > That makes sense that we multiply slave's flips by a factor because
>> > its low, but I still didn't get why the factor is chosen to be
>> > llc_size instead of something else for the multiplication with slave
>> > (slave * factor).
>
>> Yeah I don't know why llc_size was chosen...
>
> static void update_top_cache_domain(int cpu)
> {
>         struct sched_domain_shared *sds = NULL;
>         struct sched_domain *sd;
>         int id = cpu;
>         int size = 1;
>
>         sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
>         if (sd) {
>                 id = cpumask_first(sched_domain_span(sd));
>                 size = cpumask_weight(sched_domain_span(sd));
>                 sds = sd->shared;
>         }
>
>         rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
>         per_cpu(sd_llc_size, cpu) = size;
>
> The goal of wake wide was to approximate when pulling would be a futile
> consolidation effort and counterproductive to scaling.  'course with
> ever increasing socket size, any 1:N waker is ever more likely to run
> out of CPU for its one and only self (slamming into scaling wall)
> before it needing to turn its minions loose to conquer the world.

Actually the original question was why do we have the second condition
as "master < slave * factor", instead of "master < factor". that's
what didn't make sense to me. Why don't we return 0 from wake_wide if
master < factor ?

Infact, as the factor is set to the llc_size, I think the condition
that makes sense to me is:

if ((master + slave) < llc_size)
  return 0;

In other words, if the master flips and the slave flips are totally
higher than the llc_size, then we are most likely waking up too many
tasks as affine and should then switch to wide to prevent overloading.

Digging further into the original patch from Michael Wang (I also CC'd
him), this was the code (before you had changed it to master/slave):

wakee->nr_wakee_switch > factor &&
waker->nr_wakee_switch > (factor * wakee->nr_wakee_switch)

To explain the second condition above, Michael Wang said the following in [1]

"Furthermore, if waker also has a high 'nr_wakee_switch', imply that multiple
tasks rely on it, then waker's higher latency will damage all of them, pull
wakee seems to be a bad deal."

Again I didn't follow why the second condition couldn't just be:
waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
wakee->nr_wakee_switch) > factor, based on the above explanation from
Micheal Wang that I quoted.
and why he's instead doing the whole multiplication thing there that I
was talking about earlier: "factor * wakee->nr_wakee_switch".

Rephrasing my question in another way, why are we talking the ratio of
master/slave instead of the sum when comparing if its > factor? I am
surely missing something here.

Just taking an example:

Say we have llc_size = 3, we have 3 masters M1, M2 and M3. M1 has 8
slaves, M2 has 4 slaves and M3 has 4 slaves. Only 1 slave is common
between all 3 masters. Also to make it a bit more interesting, let s8
wake up some random task T0. A diagram to show the master/slave
relation ships could look like:

                                    +-----+
                                    |     |
+------------------------+   +------+ M2  |
|                        |   |      |     |
|        M1              |   |      +--+--+----
|                        |   |      |  |  |   |
|                        |   |      |  |  |   s15
+--+--+--+--+--+--+---+--+---+      v  v  v
   |  |  |  |  |  |   |      |      s9 s10 s11
   v  v  v  v  v  v   v      v
   s1 s2 s3 s4 s5 s6  s7     s8 ---> T0
                             ^
                             |
                           +-+---+
                           |     |
                           | M3  |
                           |     |
                           +--+--+-----
                           |  |  |    |
                           v  v  v    v
                          s12 s13 s14 s16


Lets consider the case of M1 waking up s8. As per the above diagram,
M1 has 8 flips and s8 has 4 flips.

With llc_size = 3, the condition

(slave < factor) would return FALSE, so then we would turn to the
(master < slave * factor) condition. This would be TRUE (8 < 4 * 3),
so wake_wide would return 0 and would cause s8 to be woken up as
affine with relation to M1's core.

So basically, it seems the heuristic is saying (with help of the
second condition - master < slave * factor). that Its a good idea for
s8 to be affine-woken-up with respect to M1's core. Why is it a good
idea to do that? It seems to me M1 has already several tasks its
waking as affine so causing s8 to be woken up affine could be harmful
and it may be a better choice to wake it up elsewhere.

Thanks for your help!

-Joel

[1] https://lkml.org/lkml/2013/7/4/20


>
> Something else to consider: network interrupt waking multiple workers
> at high frequency.  If the waking CPU is idle, do you really want to
> place a worker directly in front of a tattoo artist, or is it better
> off nearly anywhere but there?
>
> If the box is virtual, with no topology exposed (or real but ancient)
> to let select_idle_sibling() come to the rescue, two workers can even
> get tattooed simultaneously (see sync wakeup).
>
>         -Mike

[toc] | [next] | [standalone]

#1699261

+Michael Wang on his current email address (old one bounced). (my
reply was to Mike Galbraith but I also meant to CC Michael Wang for
the discussion). Thanks

On Sat, Jul 29, 2017 at 1:01 AM, Joel Fernandes <joelaf@google.com> wrote:
> Hi Mike,
>
> I have take spent some time understanding the email thread and
> previous discussions. Unfortunately the second condition we are
> checking for in the wake_wide still didn't make sense to me (mentioned
> below) :-(
>
> On Fri, Jun 30, 2017 at 10:02 AM, Mike Galbraith
> <umgwanakikbuti@gmail.com> wrote:
>> On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote:
>>> On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote:
>>>
>>> > That makes sense that we multiply slave's flips by a factor because
>>> > its low, but I still didn't get why the factor is chosen to be
>>> > llc_size instead of something else for the multiplication with slave
>>> > (slave * factor).
>>
>>> Yeah I don't know why llc_size was chosen...
>>
>> static void update_top_cache_domain(int cpu)
>> {
>>         struct sched_domain_shared *sds = NULL;
>>         struct sched_domain *sd;
>>         int id = cpu;
>>         int size = 1;
>>
>>         sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
>>         if (sd) {
>>                 id = cpumask_first(sched_domain_span(sd));
>>                 size = cpumask_weight(sched_domain_span(sd));
>>                 sds = sd->shared;
>>         }
>>
>>         rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
>>         per_cpu(sd_llc_size, cpu) = size;
>>
>> The goal of wake wide was to approximate when pulling would be a futile
>> consolidation effort and counterproductive to scaling.  'course with
>> ever increasing socket size, any 1:N waker is ever more likely to run
>> out of CPU for its one and only self (slamming into scaling wall)
>> before it needing to turn its minions loose to conquer the world.
>
> Actually the original question was why do we have the second condition
> as "master < slave * factor", instead of "master < factor". that's
> what didn't make sense to me. Why don't we return 0 from wake_wide if
> master < factor ?
>
> Infact, as the factor is set to the llc_size, I think the condition
> that makes sense to me is:
>
> if ((master + slave) < llc_size)
>   return 0;
>
> In other words, if the master flips and the slave flips are totally
> higher than the llc_size, then we are most likely waking up too many
> tasks as affine and should then switch to wide to prevent overloading.
>
> Digging further into the original patch from Michael Wang (I also CC'd
> him), this was the code (before you had changed it to master/slave):
>
> wakee->nr_wakee_switch > factor &&
> waker->nr_wakee_switch > (factor * wakee->nr_wakee_switch)
>
> To explain the second condition above, Michael Wang said the following in [1]
>
> "Furthermore, if waker also has a high 'nr_wakee_switch', imply that multiple
> tasks rely on it, then waker's higher latency will damage all of them, pull
> wakee seems to be a bad deal."
>
> Again I didn't follow why the second condition couldn't just be:
> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
> wakee->nr_wakee_switch) > factor, based on the above explanation from
> Micheal Wang that I quoted.
> and why he's instead doing the whole multiplication thing there that I
> was talking about earlier: "factor * wakee->nr_wakee_switch".
>
> Rephrasing my question in another way, why are we talking the ratio of
> master/slave instead of the sum when comparing if its > factor? I am
> surely missing something here.
>
> Just taking an example:
>
> Say we have llc_size = 3, we have 3 masters M1, M2 and M3. M1 has 8
> slaves, M2 has 4 slaves and M3 has 4 slaves. Only 1 slave is common
> between all 3 masters. Also to make it a bit more interesting, let s8
> wake up some random task T0. A diagram to show the master/slave
> relation ships could look like:
>
>                                     +-----+
>                                     |     |
> +------------------------+   +------+ M2  |
> |                        |   |      |     |
> |        M1              |   |      +--+--+----
> |                        |   |      |  |  |   |
> |                        |   |      |  |  |   s15
> +--+--+--+--+--+--+---+--+---+      v  v  v
>    |  |  |  |  |  |   |      |      s9 s10 s11
>    v  v  v  v  v  v   v      v
>    s1 s2 s3 s4 s5 s6  s7     s8 ---> T0
>                              ^
>                              |
>                            +-+---+
>                            |     |
>                            | M3  |
>                            |     |
>                            +--+--+-----
>                            |  |  |    |
>                            v  v  v    v
>                           s12 s13 s14 s16
>
>
> Lets consider the case of M1 waking up s8. As per the above diagram,
> M1 has 8 flips and s8 has 4 flips.
>
> With llc_size = 3, the condition
>
> (slave < factor) would return FALSE, so then we would turn to the
> (master < slave * factor) condition. This would be TRUE (8 < 4 * 3),
> so wake_wide would return 0 and would cause s8 to be woken up as
> affine with relation to M1's core.
>
> So basically, it seems the heuristic is saying (with help of the
> second condition - master < slave * factor). that Its a good idea for
> s8 to be affine-woken-up with respect to M1's core. Why is it a good
> idea to do that? It seems to me M1 has already several tasks its
> waking as affine so causing s8 to be woken up affine could be harmful
> and it may be a better choice to wake it up elsewhere.
>
> Thanks for your help!
>
> -Joel
>
> [1] https://lkml.org/lkml/2013/7/4/20
>
>
>>
>> Something else to consider: network interrupt waking multiple workers
>> at high frequency.  If the waking CPU is idle, do you really want to
>> place a worker directly in front of a tattoo artist, or is it better
>> off nearly anywhere but there?
>>
>> If the box is virtual, with no topology exposed (or real but ancient)
>> to let select_idle_sibling() come to the rescue, two workers can even
>> get tattooed simultaneously (see sync wakeup).
>>
>>         -Mike

[toc] | [prev] | [next] | [standalone]

#1699331

On Sat, 2017-07-29 at 01:01 -0700, Joel Fernandes wrote:
> Hi Mike,
> 
> I have take spent some time understanding the email thread and
> previous discussions. Unfortunately the second condition we are
> checking for in the wake_wide still didn't make sense to me (mentioned
> below) :-(
> 
> On Fri, Jun 30, 2017 at 10:02 AM, Mike Galbraith
> <umgwanakikbuti@gmail.com> wrote:
> > On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote:
> >> On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote:
> >>
> >> > That makes sense that we multiply slave's flips by a factor because
> >> > its low, but I still didn't get why the factor is chosen to be
> >> > llc_size instead of something else for the multiplication with slave
> >> > (slave * factor).
> >
> >> Yeah I don't know why llc_size was chosen...
> >
> > static void update_top_cache_domain(int cpu)
> > {
> >         struct sched_domain_shared *sds = NULL;
> >         struct sched_domain *sd;
> >         int id = cpu;
> >         int size = 1;
> >
> >         sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
> >         if (sd) {
> >                 id = cpumask_first(sched_domain_span(sd));
> >                 size = cpumask_weight(sched_domain_span(sd));
> >                 sds = sd->shared;
> >         }
> >
> >         rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
> >         per_cpu(sd_llc_size, cpu) = size;
> >
> > The goal of wake wide was to approximate when pulling would be a futile
> > consolidation effort and counterproductive to scaling.  'course with
> > ever increasing socket size, any 1:N waker is ever more likely to run
> > out of CPU for its one and only self (slamming into scaling wall)
> > before it needing to turn its minions loose to conquer the world.
> 
> Actually the original question was why do we have the second condition
> as "master < slave * factor", instead of "master < factor". that's
> what didn't make sense to me. Why don't we return 0 from wake_wide if
> master < factor ?
> 
> Infact, as the factor is set to the llc_size, I think the condition
> that makes sense to me is:
> 
> if ((master + slave) < llc_size)
>   return 0;

That says to me turn affine wakeups off for nearly everything.

> In other words, if the master flips and the slave flips are totally
> higher than the llc_size, then we are most likely waking up too many
> tasks as affine and should then switch to wide to prevent overloading.

The heuristic is mostly about the ratio of flip counts.

> Digging further into the original patch from Michael Wang (I also CC'd
> him), this was the code (before you had changed it to master/slave):
> 
> wakee->nr_wakee_switch > factor &&
> waker->nr_wakee_switch > (factor * wakee->nr_wakee_switch)

Yeah, it was originally unidirectional.

> To explain the second condition above, Michael Wang said the following in [1]
> 
> "Furthermore, if waker also has a high 'nr_wakee_switch', imply that multiple
> tasks rely on it, then waker's higher latency will damage all of them, pull
> wakee seems to be a bad deal."

Yes, "Furthermore". To detect 1:N, Michael chose llc_size as his N.  Is
the one flipping partners at least N/s, and the other about N times as
often?  If so, the two may be part of a too big to wisely pull 1:N.

If you have a better idea, by all means, pull it out.  Nobody is
attached to wake_wide(), in fact, I suspect Peter hates it.  I'm not
fond of it either, it having obvious holes.  The only thing it has
going for it is simplicity.  Bend it up, replace it, fire away.
  
> Again I didn't follow why the second condition couldn't just be:
> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
> wakee->nr_wakee_switch) > factor, based on the above explanation from
> Micheal Wang that I quoted.
> and why he's instead doing the whole multiplication thing there that I
> was talking about earlier: "factor * wakee->nr_wakee_switch".
> 
> Rephrasing my question in another way, why are we talking the ratio of
> master/slave instead of the sum when comparing if its > factor? I am
> surely missing something here.

Because the heuristic tries to not demolish 1:1 buddies.  Big partner
flip delta means the pair are unlikely to be a communicating pair,
perhaps at high frequency where misses hurt like hell.

> Just taking an example:
> 
> Say we have llc_size = 3, we have 3 masters M1, M2 and M3. M1 has 8
> slaves, M2 has 4 slaves and M3 has 4 slaves. Only 1 slave is common
> between all 3 masters. Also to make it a bit more interesting, let s8
> wake up some random task T0. A diagram to show the master/slave
> relation ships could look like:

I don't need the artwork, as soon as you describe a squid orgie in a
bucket with no adult supervision, I can visualize what happens: the
load balancer tries to make pedantically perfect load numbers, caring
not one whit about any of the relationships in either text or graphic
description, stirs bucket vigorously, making squid soup.

IMHO, placement optimization of that is a job for a human, the
scheduler ain't that smart. (quick like bunny -> smart like bunny)

> 
>                                     +-----+
>                                     |     |
> +------------------------+   +------+ M2  |
> |                        |   |      |     |
> |        M1              |   |      +--+--+----
> |                        |   |      |  |  |   |
> |                        |   |      |  |  |   s15
> +--+--+--+--+--+--+---+--+---+      v  v  v
>    |  |  |  |  |  |   |      |      s9 s10 s11
>    v  v  v  v  v  v   v      v
>    s1 s2 s3 s4 s5 s6  s7     s8 ---> T0
>                              ^
>                              |
>                            +-+---+
>                            |     |
>                            | M3  |
>                            |     |
>                            +--+--+-----
>                            |  |  |    |
>                            v  v  v    v
>                           s12 s13 s14 s16
> 
> 
> Lets consider the case of M1 waking up s8. As per the above diagram,
> M1 has 8 flips and s8 has 4 flips.
> 
> With llc_size = 3, the condition
> 
> (slave < factor) would return FALSE, so then we would turn to the
> (master < slave * factor) condition. This would be TRUE (8 < 4 * 3),
> so wake_wide would return 0 and would cause s8 to be woken up as
> affine with relation to M1's core.
> 
> So basically, it seems the heuristic is saying (with help of the
> second condition - master < slave * factor). that Its a good idea for
> s8 to be affine-woken-up with respect to M1's core. Why is it a good
> idea to do that? It seems to me M1 has already several tasks its
> waking as affine so causing s8 to be woken up affine could be harmful
> and it may be a better choice to wake it up elsewhere.

No matter what you do with s8, you'll be wrong from some POV.. unless
of course s8 is a high frequency waker, then migration is a good bet.

	-Mike

[toc] | [prev] | [next] | [standalone]

#1699364

Hi Mike,

On Sat, Jul 29, 2017 at 8:07 AM, Mike Galbraith
<umgwanakikbuti@gmail.com> wrote:
> On Sat, 2017-07-29 at 01:01 -0700, Joel Fernandes wrote:
>> Hi Mike,
>>
>> I have take spent some time understanding the email thread and
>> previous discussions. Unfortunately the second condition we are
>> checking for in the wake_wide still didn't make sense to me (mentioned
>> below) :-(
>>
>> On Fri, Jun 30, 2017 at 10:02 AM, Mike Galbraith
>> <umgwanakikbuti@gmail.com> wrote:
>> > On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote:
>> >> On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote:
>> >>
>> >> > That makes sense that we multiply slave's flips by a factor because
>> >> > its low, but I still didn't get why the factor is chosen to be
>> >> > llc_size instead of something else for the multiplication with slave
>> >> > (slave * factor).
>> >
>> >> Yeah I don't know why llc_size was chosen...
<snip>
>> >
>> > The goal of wake wide was to approximate when pulling would be a futile
>> > consolidation effort and counterproductive to scaling.  'course with
>> > ever increasing socket size, any 1:N waker is ever more likely to run
>> > out of CPU for its one and only self (slamming into scaling wall)
>> > before it needing to turn its minions loose to conquer the world.
>>
>> Actually the original question was why do we have the second condition
>> as "master < slave * factor", instead of "master < factor". that's
>> what didn't make sense to me. Why don't we return 0 from wake_wide if
>> master < factor ?
>>
>> Infact, as the factor is set to the llc_size, I think the condition
>> that makes sense to me is:
>>
>> if ((master + slave) < llc_size)
>>   return 0;
>
> That says to me turn affine wakeups off for nearly everything.

Ok, I see that now. thanks

>> To explain the second condition above, Michael Wang said the following in [1]
>>
>> "Furthermore, if waker also has a high 'nr_wakee_switch', imply that multiple
>> tasks rely on it, then waker's higher latency will damage all of them, pull
>> wakee seems to be a bad deal."
>
> Yes, "Furthermore". To detect 1:N, Michael chose llc_size as his N.  Is
> the one flipping partners at least N/s, and the other about N times as
> often?  If so, the two may be part of a too big to wisely pull 1:N.
>
> If you have a better idea, by all means, pull it out.  Nobody is

Sure yeah, first I'm trying to understand the heuristic itself which
I'm glad to be making progress with thanks to yours and others' help!

> attached to wake_wide(), in fact, I suspect Peter hates it.  I'm not
> fond of it either, it having obvious holes.  The only thing it has
> going for it is simplicity.  Bend it up, replace it, fire away.
>

Ok, it makes much more sense to me now. Also for the N:N case,
wouldn't the excessive wake-affine increase the latency and a
spreading might be better? Say if slave and master flips are much
greater than factor (llc_size), then slave > factor && master < slave
* factor, would probably return true a lot (and we would return 0
causing an affine wakeup). That's probably a bad thing right as it
could overload the waker's CPU quickly? I guess the heuristic tries to
maximize cache-hits more than reduce latency?

>> Again I didn't follow why the second condition couldn't just be:
>> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
>> wakee->nr_wakee_switch) > factor, based on the above explanation from
>> Micheal Wang that I quoted.
>> and why he's instead doing the whole multiplication thing there that I
>> was talking about earlier: "factor * wakee->nr_wakee_switch".
>>
>> Rephrasing my question in another way, why are we talking the ratio of
>> master/slave instead of the sum when comparing if its > factor? I am
>> surely missing something here.
>
> Because the heuristic tries to not demolish 1:1 buddies.  Big partner
> flip delta means the pair are unlikely to be a communicating pair,
> perhaps at high frequency where misses hurt like hell.

But it does seem to me to demolish the N:N communicating pairs from a
latency/load balancing standpoint. For he case of N readers and N
writers, the ratio (master/slave) comes down to 1:1 and we wake
affine. Hopefully I didn't miss something too obvious about that.

thanks,

-Joel

[toc] | [prev] | [next] | [standalone]

#1699385

Hi Mike,

On Sat, Jul 29, 2017 at 1:19 PM, Joel Fernandes <joelaf@google.com> wrote:
<snip>
>
>>> To explain the second condition above, Michael Wang said the following in [1]
>>>
>>> "Furthermore, if waker also has a high 'nr_wakee_switch', imply that multiple
>>> tasks rely on it, then waker's higher latency will damage all of them, pull
>>> wakee seems to be a bad deal."
>>
>> Yes, "Furthermore". To detect 1:N, Michael chose llc_size as his N.  Is
>> the one flipping partners at least N/s, and the other about N times as
>> often?  If so, the two may be part of a too big to wisely pull 1:N.
>>
>> If you have a better idea, by all means, pull it out.  Nobody is
>
> Sure yeah, first I'm trying to understand the heuristic itself which
> I'm glad to be making progress with thanks to yours and others' help!
>
>> attached to wake_wide(), in fact, I suspect Peter hates it.  I'm not
>> fond of it either, it having obvious holes.  The only thing it has
>> going for it is simplicity.  Bend it up, replace it, fire away.
>>
>
> Ok, it makes much more sense to me now. Also for the N:N case,
> wouldn't the excessive wake-affine increase the latency and a
> spreading might be better? Say if slave and master flips are much
> greater than factor (llc_size), then slave > factor && master < slave
> * factor, would probably return true a lot (and we would return 0
> causing an affine wakeup). That's probably a bad thing right as it
> could overload the waker's CPU quickly? I guess the heuristic tries to
> maximize cache-hits more than reduce latency?
>
>>> Again I didn't follow why the second condition couldn't just be:
>>> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
>>> wakee->nr_wakee_switch) > factor, based on the above explanation from
>>> Micheal Wang that I quoted.
>>> and why he's instead doing the whole multiplication thing there that I
>>> was talking about earlier: "factor * wakee->nr_wakee_switch".
>>>
>>> Rephrasing my question in another way, why are we talking the ratio of
>>> master/slave instead of the sum when comparing if its > factor? I am
>>> surely missing something here.
>>
>> Because the heuristic tries to not demolish 1:1 buddies.  Big partner
>> flip delta means the pair are unlikely to be a communicating pair,
>> perhaps at high frequency where misses hurt like hell.
>
> But it does seem to me to demolish the N:N communicating pairs from a
> latency/load balancing standpoint. For he case of N readers and N
> writers, the ratio (master/slave) comes down to 1:1 and we wake
> affine. Hopefully I didn't miss something too obvious about that.

I think wake_affine() should correctly handle the case (of
overloading) I bring up here where wake_wide() is too conservative and
does affine a lot, (I don't have any data for this though, this just
from code reading), so I take this comment back for this reason.

thanks,

-Joel

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#1699392

On Sat, Jul 29, 2017 at 3:28 PM, Joel Fernandes <joelaf@google.com> wrote:
<snip>
>>>> Again I didn't follow why the second condition couldn't just be:
>>>> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
>>>> wakee->nr_wakee_switch) > factor, based on the above explanation from
>>>> Micheal Wang that I quoted.
>>>> and why he's instead doing the whole multiplication thing there that I
>>>> was talking about earlier: "factor * wakee->nr_wakee_switch".
>>>>
>>>> Rephrasing my question in another way, why are we talking the ratio of
>>>> master/slave instead of the sum when comparing if its > factor? I am
>>>> surely missing something here.
>>>
>>> Because the heuristic tries to not demolish 1:1 buddies.  Big partner
>>> flip delta means the pair are unlikely to be a communicating pair,
>>> perhaps at high frequency where misses hurt like hell.
>>
>> But it does seem to me to demolish the N:N communicating pairs from a
>> latency/load balancing standpoint. For he case of N readers and N
>> writers, the ratio (master/slave) comes down to 1:1 and we wake
>> affine. Hopefully I didn't miss something too obvious about that.
>
> I think wake_affine() should correctly handle the case (of
> overloading) I bring up here where wake_wide() is too conservative and
> does affine a lot, (I don't have any data for this though, this just
> from code reading), so I take this comment back for this reason.

aargh, nope :( it still runs select_idle_sibling although on the
previous CPU even if want_affine is 0 (and doesn't do the wider
wakeup..), so the comment still applies.. its easy to get lost into
the code with so many if statements :-\  sorry about the noise :)

thanks,

-Joel

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#1699990

On Sat, Jul 29, 2017 at 03:41:56PM -0700, Joel Fernandes wrote:
> On Sat, Jul 29, 2017 at 3:28 PM, Joel Fernandes <joelaf@google.com> wrote:
> <snip>
> >>>> Again I didn't follow why the second condition couldn't just be:
> >>>> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
> >>>> wakee->nr_wakee_switch) > factor, based on the above explanation from
> >>>> Micheal Wang that I quoted.
> >>>> and why he's instead doing the whole multiplication thing there that I
> >>>> was talking about earlier: "factor * wakee->nr_wakee_switch".
> >>>>
> >>>> Rephrasing my question in another way, why are we talking the ratio of
> >>>> master/slave instead of the sum when comparing if its > factor? I am
> >>>> surely missing something here.
> >>>
> >>> Because the heuristic tries to not demolish 1:1 buddies.  Big partner
> >>> flip delta means the pair are unlikely to be a communicating pair,
> >>> perhaps at high frequency where misses hurt like hell.
> >>
> >> But it does seem to me to demolish the N:N communicating pairs from a
> >> latency/load balancing standpoint. For he case of N readers and N
> >> writers, the ratio (master/slave) comes down to 1:1 and we wake
> >> affine. Hopefully I didn't miss something too obvious about that.
> >
> > I think wake_affine() should correctly handle the case (of
> > overloading) I bring up here where wake_wide() is too conservative and
> > does affine a lot, (I don't have any data for this though, this just
> > from code reading), so I take this comment back for this reason.
> 
> aargh, nope :( it still runs select_idle_sibling although on the
> previous CPU even if want_affine is 0 (and doesn't do the wider
> wakeup..), so the comment still applies.. its easy to get lost into
> the code with so many if statements :-\  sorry about the noise :)
> 

I've been working in this area recently because of a cpu imbalance problem.
Wake_wide() definitely makes it so we're waking affine way too often, but I
think messing with wake_waide to solve that problem is the wrong solution.  This
is just a heuristic to see if we should wake affine, the simpler the better.  I
solved the problem of waking affine too often like this

https://marc.info/?l=linux-kernel&m=150003849602535&w=2

So why do you care about wake_wide() anyway?  Are you observing some problem
that you suspect is affected by the affine wakeup stuff?  Or are you just trying
to understand what is going on for fun?  Cause if you are just doing this for
fun you are a very strange person, thanks,

Josef

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