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    SubjectRe: [PATCH 0/8] v2 De-Couple sysfs memory directories from memory sections
    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. On our 8 TB test machine, hald runs continuously
    following boot for nearly an hour mostly scanning useless information
    from /sys/


    > New ABIs mean old tools won't work, and new tools need to understand
    > both ABIs.
    > --
    > error compiling committee.c: too many arguments to function
    > --
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      Last update: 2010-09-28 17:15    [W:0.024 / U:12.728 seconds]
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