sched/fair: refine maximum periods for decay_load

Message ID 1470378840-15206-1-git-send-email-leo.yan@linaro.org
State New
Headers show

Commit Message

Leo Yan Aug. 5, 2016, 6:34 a.m.
In current code, decay_load() will consider to set load value to zero
after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.

Kernel can support maximum number of processes and threads to 2^29 and
set task with highest priority with nice=-20 (weight = 88761). So in
worst case, one CPU may have maximum load value is:

    max_load = 2^29 * 88761 < 2^46

In theory after pass 46 periods we can ensure load value will be decayed
to zero. So this patch is to change maximum periods from 64 to 48.

Signed-off-by: Leo Yan <leo.yan@linaro.org>

---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
1.9.1

Comments

Vincent Guittot Aug. 5, 2016, 7:56 a.m. | #1
Hi Leo,

On 5 August 2016 at 08:34, Leo Yan <leo.yan@linaro.org> wrote:
>

> In current code, decay_load() will consider to set load value to zero

> after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.

>

> Kernel can support maximum number of processes and threads to 2^29 and

> set task with highest priority with nice=-20 (weight = 88761). So in

> worst case, one CPU may have maximum load value is:

>

>     max_load = 2^29 * 88761 < 2^46

>

> In theory after pass 46 periods we can ensure load value will be decayed

> to zero. So this patch is to change maximum periods from 64 to 48.

>

> Signed-off-by: Leo Yan <leo.yan@linaro.org>

> ---

>  kernel/sched/fair.c | 2 +-

>  1 file changed, 1 insertion(+), 1 deletion(-)

>

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c

> index e342159..55cb134 100644

> --- a/kernel/sched/fair.c

> +++ b/kernel/sched/fair.c

> @@ -2622,7 +2622,7 @@ static __always_inline u64 decay_load(u64 val, u64 n)

>

>         if (!n)

>                 return val;

> -       else if (unlikely(n > LOAD_AVG_PERIOD * 63))

> +       else if (unlikely(n > LOAD_AVG_PERIOD * 47))


In the equation above, you use 46 then you mentioned : "change maximum
periods from 64 to 48." and finally you use 47.

>                 return 0;

>

>         /* after bounds checking we can collapse to 32-bit */

> --

> 1.9.1

>
Leo Yan Aug. 5, 2016, 8:17 a.m. | #2
Hi Vincent,

Thanks for review.

On Fri, Aug 05, 2016 at 09:56:46AM +0200, Vincent Guittot wrote:
> Hi Leo,

> 

> On 5 August 2016 at 08:34, Leo Yan <leo.yan@linaro.org> wrote:

> >

> > In current code, decay_load() will consider to set load value to zero

> > after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.

> >

> > Kernel can support maximum number of processes and threads to 2^29 and

> > set task with highest priority with nice=-20 (weight = 88761). So in

> > worst case, one CPU may have maximum load value is:

> >

> >     max_load = 2^29 * 88761 < 2^46

> >

> > In theory after pass 46 periods we can ensure load value will be decayed

> > to zero. So this patch is to change maximum periods from 64 to 48.

> >

> > Signed-off-by: Leo Yan <leo.yan@linaro.org>

> > ---

> >  kernel/sched/fair.c | 2 +-

> >  1 file changed, 1 insertion(+), 1 deletion(-)

> >

> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c

> > index e342159..55cb134 100644

> > --- a/kernel/sched/fair.c

> > +++ b/kernel/sched/fair.c

> > @@ -2622,7 +2622,7 @@ static __always_inline u64 decay_load(u64 val, u64 n)

> >

> >         if (!n)

> >                 return val;

> > -       else if (unlikely(n > LOAD_AVG_PERIOD * 63))

> > +       else if (unlikely(n > LOAD_AVG_PERIOD * 47))

> 

> In the equation above, you use 46 then you mentioned : "change maximum

> periods from 64 to 48." and finally you use 47.


Sorry introduce confusion. I want to align to 16 so choose maximum
periods to 48, and due to condition is (n > LOAD_AVG_PERIOD * 47) so
it equeals to (n >= LOAD_AVG_PERIOD * 48).

Precisely I think the code should be:
    else if (unlikely(n >= LOAD_AVG_PERIOD * 46))

Do you think this is okay?

> >                 return 0;

> >

> >         /* after bounds checking we can collapse to 32-bit */

> > --

> > 1.9.1

> >
Vincent Guittot Aug. 5, 2016, 8:31 a.m. | #3
On 5 August 2016 at 10:17, Leo Yan <leo.yan@linaro.org> wrote:
> Hi Vincent,

>

> Thanks for review.

>

> On Fri, Aug 05, 2016 at 09:56:46AM +0200, Vincent Guittot wrote:

>> Hi Leo,

>>

>> On 5 August 2016 at 08:34, Leo Yan <leo.yan@linaro.org> wrote:

>> >

>> > In current code, decay_load() will consider to set load value to zero

>> > after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.

>> >

>> > Kernel can support maximum number of processes and threads to 2^29 and

>> > set task with highest priority with nice=-20 (weight = 88761). So in

>> > worst case, one CPU may have maximum load value is:

>> >

>> >     max_load = 2^29 * 88761 < 2^46

>> >

>> > In theory after pass 46 periods we can ensure load value will be decayed

>> > to zero. So this patch is to change maximum periods from 64 to 48.

>> >

>> > Signed-off-by: Leo Yan <leo.yan@linaro.org>

>> > ---

>> >  kernel/sched/fair.c | 2 +-

>> >  1 file changed, 1 insertion(+), 1 deletion(-)

>> >

>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c

>> > index e342159..55cb134 100644

>> > --- a/kernel/sched/fair.c

>> > +++ b/kernel/sched/fair.c

>> > @@ -2622,7 +2622,7 @@ static __always_inline u64 decay_load(u64 val, u64 n)

>> >

>> >         if (!n)

>> >                 return val;

>> > -       else if (unlikely(n > LOAD_AVG_PERIOD * 63))

>> > +       else if (unlikely(n > LOAD_AVG_PERIOD * 47))

>>

>> In the equation above, you use 46 then you mentioned : "change maximum

>> periods from 64 to 48." and finally you use 47.

>

> Sorry introduce confusion. I want to align to 16 so choose maximum

> periods to 48, and due to condition is (n > LOAD_AVG_PERIOD * 47) so

> it equeals to (n >= LOAD_AVG_PERIOD * 48).

>

> Precisely I think the code should be:

>     else if (unlikely(n >= LOAD_AVG_PERIOD * 46))

>

> Do you think this is okay?


yes, this looks okay


>

>> >                 return 0;

>> >

>> >         /* after bounds checking we can collapse to 32-bit */

>> > --

>> > 1.9.1

>> >

Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e342159..55cb134 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2622,7 +2622,7 @@  static __always_inline u64 decay_load(u64 val, u64 n)
 
 	if (!n)
 		return val;
-	else if (unlikely(n > LOAD_AVG_PERIOD * 63))
+	else if (unlikely(n > LOAD_AVG_PERIOD * 47))
 		return 0;
 
 	/* after bounds checking we can collapse to 32-bit */