linux-kernel - Re: [PATCH 0/8][for -mm] mem

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Message-Id: <20080501232431.F617.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Fri, 02 May 2008 00:06:05 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	"Tom May" <tom@...may.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	"Daniel Sp蚣g" 
	<daniel.spang@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8][for -mm] mem_notify v6

Hi Tom,

> In my case of a Java virtual machine, where I originally saw the
> problem, most of the code is interpreted byte codes or jit-compiled
> native code, all of which resides not in the text segment but in
> anonymous pages that aren't backed by a file, and there is no swap
> space.  The actual text segment working set can be very small (memory
> allocation, garbage collection, synchronization, other random native
> code).  And, as KOSAKI Motohiro pointed out, it may be wise to mlock
> these areas.  So the text working set doesn't make an adequate
> reserve.

your memnotify check routine is written by native or java?
if native, my suggestion is right.
but if java, it is wrong.

my point is "on swapless system, /dev/mem_notify checked routine should be mlocked".

> However, I can maintain a reserve of cached and/or mapped memory by
> touching pages in the text segment (or any mapped file) as the final
> step of low memory notification handling, if the cached page count is
> getting low.  For my purposes, this is nearly the same as having an
> additional threshold-based notification, since it forces notifications
> to occur while the kernel still has some memory to satisfy allocations
> while userspace code works to free memory.  And it's simple.
> 
> Unfortunately, this is more expensive than it could be since the pages
> need to be read in from some device (mapping /dev/zero doesn't cause
> pages to be allocated). What I'm looking for now is a cheap way to
> populate the cache with pages that the kernel can throw away when it
> needs to reclaim memory.

I hope understand your requirement more.
Can I ask your system more?

I think all java text and data is mapped.
When cached+mapped+free memory is happend?
and at the time, What is used memory?

Please don't think I have objection your proposal.
merely, I don't understand your system yet.

if I make new code before understand your requirement exactly, 
It makes many bug.

IMHO threshold based notification has a problems.
if low memory happend and application has no freeable memory,
mem notification don't stop and increase CPU usage dramatically, but it is perfectly useless.

I don't thin embedded java is not important, but I don't hope
desktop regression...

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