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    SubjectRe: [PATCH 0/8] v2 De-Couple sysfs memory directories from memory sections
    On Tue, Sep 28, 2010 at 10:12:18AM -0500, Robin Holt wrote:
    > On Tue, Sep 28, 2010 at 02:44:40PM +0200, Avi Kivity wrote:
    > > On 09/27/2010 09:09 PM, Nathan Fontenot wrote:
    > > >This set of patches decouples the concept that a single memory
    > > >section corresponds to a single directory in
    > > >/sys/devices/system/memory/. On systems
    > > >with large amounts of memory (1+ TB) there are perfomance issues
    > > >related to creating the large number of sysfs directories. For
    > > >a powerpc machine with 1 TB of memory we are creating 63,000+
    > > >directories. This is resulting in boot times of around 45-50
    > > >minutes for systems with 1 TB of memory and 8 hours for systems
    > > >with 2 TB of memory. With this patch set applied I am now seeing
    > > >boot times of 5 minutes or less.
    > > >
    > > >The root of this issue is in sysfs directory creation. Every time
    > > >a directory is created a string compare is done against all sibling
    > > >directories to ensure we do not create duplicates. The list of
    > > >directory nodes in sysfs is kept as an unsorted list which results
    > > >in this being an exponentially longer operation as the number of
    > > >directories are created.
    > > >
    > > >The solution solved by this patch set is to allow a single
    > > >directory in sysfs to span multiple memory sections. This is
    > > >controlled by an optional architecturally defined function
    > > >memory_block_size_bytes(). The default definition of this
    > > >routine returns a memory block size equal to the memory section
    > > >size. This maintains the current layout of sysfs memory
    > > >directories as it appears to userspace to remain the same as it
    > > >is today.
    > > >
    > >
    > > Why not update sysfs directory creation to be fast, for example by
    > > using an rbtree instead of a linked list. This fixes an
    > > implementation problem in the kernel instead of working around it
    > > and creating a new ABI.
    >
    > Because the old ABI creates 129,000+ entries inside
    > /sys/devices/system/memory with their associated links from
    > /sys/devices/system/node/node*/ back to those directory entries.
    >
    > Thankfully things like rpm, hald, and other miscellaneous commands scan
    > that information.

    Really? Why? Why would rpm care about this? hald is dead now so we
    don't need to worry about that anymore, but what other commands/programs
    read this information?

    thanks,

    greg k-h


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