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    SubjectRe: [PATCH 0/8] De-couple sysfs memory directories from memory sections
    * Nathan Fontenot <nfont@austin.ibm.com> [2010-09-22 09:15:43]:

    > 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 performance 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.
    >
    > For architectures that define their own version of this routine,
    > as is done for powerpc in this patchset, the view in userspace
    > would change such that each memoryXXX directory would span
    > multiple memory sections. The number of sections spanned would
    > depend on the value reported by memory_block_size_bytes.
    >
    > In both cases a new file 'end_phys_index' is created in each
    > memoryXXX directory. This file will contain the physical id
    > of the last memory section covered by the sysfs directory. For
    > the default case, the value in 'end_phys_index' will be the same
    > as in the existing 'phys_index' file.
    >

    What does this mean for memory hotplug or hotunplug?

    --
    Three Cheers,
    Balbir


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