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Date: Thu, 07 Jun 2012 23:35:20 -0400
From: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
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To: leonid.moiseichuk@nokia.com
CC: penberg@kernel.org, minchan@kernel.org, cbouatmailru@gmail.com,
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Subject: Re: [PATCH 0/5] Some vmevent fixes...
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(6/5/12 4:16 AM), leonid.moiseichuk@nokia.com wrote:
>> -----Original Message-----
>> From: penberg@gmail.com [mailto:penberg@gmail.com] On Behalf Of ext
>> Pekka Enberg
>> Sent: 05 June, 2012 11:02
>> To: Minchan Kim
> ...
>>> Next concern is that periodic timer of implementation.
>>> I think it would add direct hook in vmscan.c rather than peeking raw
>>> vmstat periodically by timer so we can control more fine-grained way
>> without unnecessary overhead.
>>
>> If the hooks are clean and it doesn't hurt the  !CONFIG_VMEVENT case, I'm
>> completely OK with that.
>
> On the previous iteration hooking vm was pointed as very bad idea, so in my version I installed shrinker to handle cases when we have memory pressure.
> Using deferred timer with adequate timeout (0.250 ms or larger) fully suitable for userspace and produce adequate overhead
> ->  by nature such API should not be 100% accurate, anyhow applications cannot handle situation as good as kernel can provide, 0.5MB space accuracy, 100ms is maximum user-space require for 64-1024MB devices.

I believe that's bad idea. In fact, An "adequate" timeout depend on hardware, not application performance tendency. Thus, applications can't know "adequate" value.