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    SubjectRe: 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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