linux-kernel - Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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Date:	Sun, 23 Sep 2007 08:56:39 +0200
From:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
To:	Andrea Arcangeli <andrea@...e.de>
Cc:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>,
	Mel Gorman <mel@...net.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joern Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Lameter <clameter@....com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Fengguang Wu <fengguang.wu@...il.com>,
	swin wang <wangswin@...il.com>, totty.lu@...il.com,
	hugh@...itas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)

Andrea Arcangeli <andrea@...e.de> writes:

> On Mon, Sep 17, 2007 at 12:56:07AM +0200, Goswin von Brederlow wrote:
>> When has free ever given any usefull "free" number? I can perfectly
>> fine allocate another gigabyte of memory despide free saing 25MB. But
>> that is because I know that the buffer/cached are not locked in.
>
> Well, as you said you know that buffer/cached are not locked in. If
> /proc/meminfo would be rubbish like you seem to imply in the first
> line, why would we ever bother to export that information and even
> waste time writing a binary that parse it for admins?

As a user I know it because I didn't put a kernel source into /tmp. A
programm can't reasonably know that.

>> On the other hand 1GB can instantly vanish when I start a xen domain
>> and anything relying on the free value would loose.
>
> Actually you better check meminfo or free before starting a 1G of Xen!!

Xen has its own memory pool and can quite agressively reclaim memory
from dom0 when needed. I just ment to say that the number in
/proc/meminfo can change in a second so it is not much use knowing
what it said last minute.

>> The only sensible thing for an application concerned with swapping is
>> to whatch the swapping and then reduce itself. Not the amount
>> free. Although I wish there were some kernel interface to get a
>> preasure value of how valuable free pages would be right now. I would
>> like that for fuse so a userspace filesystem can do caching without
>> cripling the kernel.
>
> Repeated drop caches + free can help.

I would kill any programm that does that to find out how much free ram
the system has.

MfG
        Goswin
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