Re: [PATCH 3/3] vmevent: Implement special low-memory attribute

From: KOSAKI Motohiro
Date: Tue May 08 2012 - 03:12:06 EST

On Tue, May 8, 2012 at 1:53 AM, Pekka Enberg <penberg@xxxxxxxxxx> wrote:
> On Tue, May 8, 2012 at 8:42 AM, KOSAKI Motohiro
> <kosaki.motohiro@xxxxxxxxx> wrote:
>>> That said, I think you are being unfair to Anton who's one of the few
>>> that's actually taking the time to implement this properly instead of
>>> settling for an out-of-tree hack.
>> Unfair? But only I can talk about technical comment. To be honest, I
>> really dislike
>> I need say the same explanation again and again. A lot of people don't read
>> past discussion. And as far as the patches take the same mistake, I must say
>> the same thing. It is just PITA.
> Unfair because you are trying to make it look as if Anton is only
> concerned with his specific use case. That's simply not true.

However current proposal certainly don't refer past discuss and don't work
many environment.

> On Tue, May 8, 2012 at 8:42 AM, KOSAKI Motohiro
> <kosaki.motohiro@xxxxxxxxx> wrote:
>> I don't disagree vmevent notification itself, but I must disagree lie
>> notification.
>> And also, To make just idea statistics doesn't make sense at all. How do an
>> application choose the right events? If that depend on hardware configuration,
>> userland developers can't write proper applications.
> That's exactly the problem we're trying to tackle here! We _want_ the
> ABI to provide sane, well-defined events that solve real world
> problems.

Ok, sane. Then I take my time a little and review current vmevent code briefly.
(I read vmevent/core branch in pekka's tree. please let me know if
there is newer

I think following thing should be fixed.

1) sample_period is brain damaged idea. If people ONLY need to
sampling stastics, they
only need to read /proc/vmstat periodically. just remove it and
implement push notification.
_IF_ someone need unfrequent level trigger, just use
"usleep(timeout); read(vmevent_fd)"
on userland code.
2) VMEVENT_ATTR_STATE_ONE_SHOT is misleading name. That is effect as
edge trigger
shot. not only once.
3) vmevent_fd() seems sane interface. but it has name space unaware.
maybe we discuss how
to harmonize name space feature. No hurry. but we have to think
that issue since at beginning.
4) Currently, vmstat have per-cpu batch and vmstat updating makes 3
second delay at maximum.
This is fine for usual case because almost userland watcher only
read /proc/vmstat per second.
But, for vmevent_fd() case, 3 seconds may be unacceptable delay. At
worst, 128 batch x 4096
x 4k pagesize = 2G bytes inaccurate is there.
5) __VMEVENT_ATTR_STATE_VALUE_WAS_LT should be removed from userland
exporting files.
When exporing kenrel internal, always silly gus used them and made unhappy.
6) Also vmevent_event must hide from userland.
7) vmevent_config::size must be removed. In 20th century, M$ API
prefer to use this technique. But
They dropped the way because a lot of application don't initialize
size member and they can't use
it for keeping upper compitibility.
8) memcg unaware
9) numa unaware
10) zone unaware

And, we may need vm internal change if we really need lowmem
notification. current kernel don't
have such info. _And_ there is one more big problem. Currently the
kernel maintain memory per
zone. But almost all userland application aren't aware zone nor node.
Thus raw notification aren't
useful for userland. In the other hands, total memory and total free
memory is useful? Definitely No!
Even though total free memory are lots, system may start swap out and
oom invokation. If we can't
oom invocation, this feature has serious raison d'etre issue. (i.e.
(4), (8), (9) and (19) are not ignorable
issue. I think)
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