linux-kernel - Re: [RFC][PATCH 3/8] mem_notify v5: introduce /dev/mem_notify new device (the core of this patch series)

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Date:	Fri, 25 Jan 2008 12:33:14 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	"Daniel Sp蚣g" 
	<daniel.spang@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Marcelo Tosatti" <marcelo@...ck.org>,
	"Rik van Riel" <riel@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Subject: Re: [RFC][PATCH 3/8] mem_notify v5: introduce /dev/mem_notify new device (the core of this patch series)

Hi Daniel

> > +#define PROC_WAKEUP_GUARD  (10*HZ)
> [...]
> > +       timeout = info->last_proc_notify + PROC_WAKEUP_GUARD;
> 
> If only one or a few processes are using the system I think 10 seconds
> is a little long time to wait before they get the notification again.
> Can we decrease this value? Or make it configurable under /proc? Or
> make it lower with fewer users? Something like:

Oh, that is very interesting issue.
tank you good point out.

after deep thinking, I understand my current implementation is fully stupid.
current, worst case is below.

  1. low end
     - many process of used only a bit memory(sh, cp etc..) exist.
     - 1 memory eater process exist(may be, it is fat browser)
       and it watching /dev/mem_notify.

  2. high end
     - many process of used only a bit memory(sh, cp etc..) exist.
     - 1 memory eater process exist(may be, it is DB)
       and it watching /dev/mem_notify.

the point is "only 1 process watch /dev/mem_notify", but not a few processor.
I fix it with pleasure. 

> timeout = info->last_proc_notify + min(mem_notify_users, PROC_WAKEUP_GUARD);

I like this formula.
the rest problem is decide to default value when only 1 process watch /dev/mem_notify.

What do you think it?
and if my low end worst case situation doesn't match yours, 
Could you please explain your usage more?

BTW: 
end up, We will add /proc configuration the future.
but I think it is too early.
sometimes configrable parameter prevent the discussion of nicer default value.
Instead, I hope the default value changed by adjust your usage.

- kosaki

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