On 05/08/2012 04:11 PM, KOSAKI Motohiro wrote:
On Tue, May 8, 2012 at 1:53 AM, Pekka Enberg penberg@kernel.org wrote:
On Tue, May 8, 2012 at 8:42 AM, KOSAKI Motohiro kosaki.motohiro@gmail.com 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@gmail.com 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 repositry)
I think following thing should be fixed.
- 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
I would like to add a concern.
11) understand storage speed.
As I mentioned, system can have various storage type(SSD, disk, eMMC, ramfs) In some system, user can tolerate ramfs and SSD write or swapout. We should consdier that to make it really useful.
The problem is user can't know it in advance so it should be detected by kernel. Unfortunately, it's not easy now.
The idea is that we can make some levels in advane and explain it to user
Level 1: It a immediate response to user when kernel decide there are not fast-reclaimable pages any more. Level 2: It's rather slower response than level 1 but kernel will consider it as reclaimable target Level 3: It's slowest response because kernel will consider page needed long time to reclaim as reclaimable target.
It doesn't expose any internal of kernel and can implment it in internal. For simple example,
Level 1: non-mapped clean page Level 2: Level 1 + mapped clean-page Level 3: Level 2 + dirty pages
So users of vmevent_fd can select his level. Of course, latency sensitive application with slow stoarge would select Level 1. Some application might use Level 4(Level 3 + half of swap) because it has very fast storage.
And application receives event can make strategy folloing as.
When it receives level 1 notification, it could request to others if it can release their own buffers. When it receives level 2 notification, it could request to suicide if it's not critical application. When it receives level 3 notification, it could kill others.
It's a just example and my point is we should storage speed to make it general.