linux-kernel - Re: controlling mmap()'d vs read/write() pages

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Date:	Fri, 23 Mar 2007 19:16:26 +0100
From:	Herbert Poetzl <herbert@...hfloor.at>
To:	Dave Hansen <hansendc@...ibm.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.osdl.org, menage@...gle.com, xemul@...ru,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: controlling mmap()'d vs read/write() pages

On Fri, Mar 23, 2007 at 09:41:12AM -0700, Dave Hansen wrote:
> On Fri, 2007-03-23 at 04:12 -0600, Eric W. Biederman wrote:
> > Would any of them work on a system on which every filesystem was on
> > ramfs, and there was no swap?  If not then they are not memory attacks
> > but I/O attacks.
> 
> I truly understand your point here. But, I don't think this thought
> exercise is really helpful here. In a pure sense, nothing is keeping
> an unmapped page cache file in memory, other than the user's prayers.
> But, please don't discount their prayers, it's what they want!
>
> I seem to remember a quote attributed to Alan Cox around OLS time last
> year, something about any memory controller being able to be fair,
> fast, and accurate. Please pick any two, but only two. Alan, did I get
> close?

so we would pick fair and fast then :)

> To me, one of the keys of Linux's "global optimizations" is being able
> to use any memory globally for its most effective purpose, globally
> (please ignore highmem :).  Let's say I have a 1GB container on a
> machine that is at least 100% committed.  I mmap() a 1GB file and touch
> the entire thing (I never touch it again).  I then go open another 1GB
> file and r/w to it until the end of time.  I'm at or below my RSS limit,
> but that 1GB of RAM could surely be better used for the second file.
> How do we do this if we only account for a user's RSS?  Does this fit
> into Alan's unfair bucket? ;)

what's the difference to a normal Linux system here?
when low on memory, the system will reclaim pages, and
guess what pages will be reclaimed first ...

> Also, in a practical sense, it is also a *LOT* easier to describe to a
> customer that they're getting 1GB of RAM than >=20GB/hr of bandwidth
> from the disk.  

if you want something which is easy to describe for the
'customer', then a VM is what you are looking for, it has
a perfectly well defined amount of resources which will
not be shared or used by other machines ...

> -- Dave
> 
> P.S. Do we have an quotas on ramfs?  If we have an ramfs filesystems,
> what keeps the containerized users from just filling up RAM?

tmpfs has hard limits, you simply specify it on mount

 none	/tmp		tmpfs	size=16m,mode=1777	0 0

best,
Herbert

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