[PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Ying Han]

1. If soft_limit are all set to MAX, it wastes first three periority iterations
without scanning anything.

2. By default every memcg is eligibal for softlimit reclaim, and we can also
set the value to MAX for special memcg which is immune to soft limit reclaim.

This idea is based on discussion with Michal and Johannes from LSF.

Signed-off-by: Ying Han <yinghan@google.com>
---
 kernel/res_counter.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/kernel/res_counter.c b/kernel/res_counter.c
index d508363..8017d01 100644
--- a/kernel/res_counter.c
+++ b/kernel/res_counter.c
@@ -18,7 +18,6 @@ void res_counter_init(struct res_counter *counter, struct res_counter *parent)
 {
 	spin_lock_init(&counter->lock);
 	counter->limit = RESOURCE_MAX;
-	counter->soft_limit = RESOURCE_MAX;
 	counter->parent = parent;
 }
 
-- 
1.7.7.3

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Rik van Riel]

On 04/11/2012 06:00 PM, Ying Han wrote:
> 1. If soft_limit are all set to MAX, it wastes first three periority iterations
> without scanning anything.
>
> 2. By default every memcg is eligibal for softlimit reclaim, and we can also
> set the value to MAX for special memcg which is immune to soft limit reclaim.
>
> This idea is based on discussion with Michal and Johannes from LSF.

Combined with patch 2/5, would this not result in always
returning "reclaim from this memcg" for groups without a
configured softlimit, while groups with a configured
softlimit only get reclaimed from when they are over
their limit?

Is that the desired behaviour when a system has some
cgroups with a configured softlimit, and some without?

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Johannes Weiner]

On Wed, Apr 11, 2012 at 08:45:33PM -0400, Rik van Riel wrote:
> On 04/11/2012 06:00 PM, Ying Han wrote:
> >1. If soft_limit are all set to MAX, it wastes first three periority iterations
> >without scanning anything.
> >
> >2. By default every memcg is eligibal for softlimit reclaim, and we can also
> >set the value to MAX for special memcg which is immune to soft limit reclaim.
> >
> >This idea is based on discussion with Michal and Johannes from LSF.
> 
> Combined with patch 2/5, would this not result in always
> returning "reclaim from this memcg" for groups without a
> configured softlimit, while groups with a configured
> softlimit only get reclaimed from when they are over
> their limit?
> 
> Is that the desired behaviour when a system has some
> cgroups with a configured softlimit, and some without?

Yes, in general I think this new behaviour is welcome.

In the past, soft limits were only used to give excess memory a lower
priority and there was no particular meaning associated with "being
below your soft limit".  This change makes it so that soft limits are
actually a minimum guarantee, too, so you wouldn't get reclaimed if
you behaved (if possible):

		A-unconfigured		B-below-softlimit
old:		reclaim			reclaim
new:		reclaim			no reclaim (if possible)

The much less obvious change here, however, is that we no longer put
extra pressure on groups above their limit compared to unconfigured
groups:

		A-unconfigured		B-above-softlimit
old:		reclaim			reclaim twice
new:		reclaim			reclaim

I still think that it's a reasonable use case to put a soft limit on a
workload to "nice" it memory-wise, without looking at the machine as a
whole and configuring EVERY cgroup based on global knowledge and
static partitioning of the machine.

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Ying Han]

On Wed, Apr 11, 2012 at 5:45 PM, Rik van Riel <riel@redhat.com> wrote:
> On 04/11/2012 06:00 PM, Ying Han wrote:
>>
>> 1. If soft_limit are all set to MAX, it wastes first three periority
>> iterations
>> without scanning anything.
>>
>> 2. By default every memcg is eligibal for softlimit reclaim, and we can
>> also
>> set the value to MAX for special memcg which is immune to soft limit
>> reclaim.
>>
>> This idea is based on discussion with Michal and Johannes from LSF.
>
>
> Combined with patch 2/5, would this not result in always
> returning "reclaim from this memcg" for groups without a
> configured softlimit, while groups with a configured
> softlimit only get reclaimed from when they are over
> their limit?

> Is that the desired behaviour when a system has some
> cgroups with a configured softlimit, and some without?

That is expected behavior. Basically after this change, by default all
memcgs are eligible for reclaim under global memory pressure. Only
those memcgs who sets their softlimit will be skipped if usage less
than softlimit.

Does it answer your question? I might misunderstood.

--Ying
> --
> All rights reversed

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Hillf Danton]

On Thu, Apr 12, 2012 at 6:00 AM, Ying Han <yinghan@google.com> wrote:
> 1. If soft_limit are all set to MAX, it wastes first three periority iterations
s/periority/priority/

> without scanning anything.
>
> 2. By default every memcg is eligibal for softlimit reclaim, and we can also
s/eligibal/eligible/

> set the value to MAX for special memcg which is immune to soft limit reclaim.
>

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Michal Hocko]

On Thu 12-04-12 04:22:33, Johannes Weiner wrote:
> On Wed, Apr 11, 2012 at 08:45:33PM -0400, Rik van Riel wrote:
> > On 04/11/2012 06:00 PM, Ying Han wrote:
> > >1. If soft_limit are all set to MAX, it wastes first three periority iterations
> > >without scanning anything.
> > >
> > >2. By default every memcg is eligibal for softlimit reclaim, and we can also
> > >set the value to MAX for special memcg which is immune to soft limit reclaim.
> > >
> > >This idea is based on discussion with Michal and Johannes from LSF.
> > 
> > Combined with patch 2/5, would this not result in always
> > returning "reclaim from this memcg" for groups without a
> > configured softlimit, while groups with a configured
> > softlimit only get reclaimed from when they are over
> > their limit?
> > 
> > Is that the desired behaviour when a system has some
> > cgroups with a configured softlimit, and some without?
> 
> Yes, in general I think this new behaviour is welcome.
> 
> In the past, soft limits were only used to give excess memory a lower
> priority and there was no particular meaning associated with "being
> below your soft limit".  This change makes it so that soft limits are
> actually a minimum guarantee, too, so you wouldn't get reclaimed if
> you behaved (if possible):
> 
> 		A-unconfigured		B-below-softlimit
> old:		reclaim			reclaim
> new:		reclaim			no reclaim (if possible)
> 
> The much less obvious change here, however, is that we no longer put
> extra pressure on groups above their limit compared to unconfigured
> groups:
> 
> 		A-unconfigured		B-above-softlimit
> old:		reclaim			reclaim twice
> new:		reclaim			reclaim

Agreed and I guess that the above should be a part of the changelog.
This is changing previous behavior and we should rather be explicit
about that.

> I still think that it's a reasonable use case to put a soft limit on a
> workload to "nice" it memory-wise, without looking at the machine as a
> whole and configuring EVERY cgroup based on global knowledge and
> static partitioning of the machine.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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Re: [PATCH V2 3/5] memcg: set soft_limit_in_bytes to 0 by default

[Ying Han]

On Mon, Apr 16, 2012 at 8:15 AM, Michal Hocko <mhocko@suse.cz> wrote:
> On Thu 12-04-12 04:22:33, Johannes Weiner wrote:
>> On Wed, Apr 11, 2012 at 08:45:33PM -0400, Rik van Riel wrote:
>> > On 04/11/2012 06:00 PM, Ying Han wrote:
>> > >1. If soft_limit are all set to MAX, it wastes first three periority iterations
>> > >without scanning anything.
>> > >
>> > >2. By default every memcg is eligibal for softlimit reclaim, and we can also
>> > >set the value to MAX for special memcg which is immune to soft limit reclaim.
>> > >
>> > >This idea is based on discussion with Michal and Johannes from LSF.
>> >
>> > Combined with patch 2/5, would this not result in always
>> > returning "reclaim from this memcg" for groups without a
>> > configured softlimit, while groups with a configured
>> > softlimit only get reclaimed from when they are over
>> > their limit?
>> >
>> > Is that the desired behaviour when a system has some
>> > cgroups with a configured softlimit, and some without?
>>
>> Yes, in general I think this new behaviour is welcome.
>>
>> In the past, soft limits were only used to give excess memory a lower
>> priority and there was no particular meaning associated with "being
>> below your soft limit".  This change makes it so that soft limits are
>> actually a minimum guarantee, too, so you wouldn't get reclaimed if
>> you behaved (if possible):
>>
>>               A-unconfigured          B-below-softlimit
>> old:          reclaim                 reclaim
>> new:          reclaim                 no reclaim (if possible)
>>
>> The much less obvious change here, however, is that we no longer put
>> extra pressure on groups above their limit compared to unconfigured
>> groups:
>>
>>               A-unconfigured          B-above-softlimit
>> old:          reclaim                 reclaim twice
>> new:          reclaim                 reclaim
>
> Agreed and I guess that the above should be a part of the changelog.
> This is changing previous behavior and we should rather be explicit
> about that.

Ok, I will include it on next post.

Thanks !

--Ying

>
>> I still think that it's a reasonable use case to put a soft limit on a
>> workload to "nice" it memory-wise, without looking at the machine as a
>> whole and configuring EVERY cgroup based on global knowledge and
>> static partitioning of the machine.
>
> --
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9
> Czech Republic

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