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    SubjectRe: [PATCH 0/8] v2 De-Couple sysfs memory directories from memory sections
    On 09/28/2010 07:38 AM, Robin Holt wrote:
    > I was tasked with looking at a slowdown in similar sized SGI machines
    > booting x86_64. Jack Steiner had already looked into the memory_dev_init.
    > I was looking at link_mem_sections().
    > I made a dramatic improvement on a 16TB machine in that function by
    > merely caching the most recent memory section and checking to see if
    > the next memory section happens to be the subsequent in the linked list
    > of kobjects.
    > That simple cache reduced the time for link_mem_sections from 1 hour 27
    > minutes down to 46 seconds.


    > I would like to propose we implement something along those lines also,
    > but I am currently swamped. I can probably get you a patch tomorrow
    > afternoon that applies at the end of this set.

    Should this be done as a separate patch? This patch set concentrates on
    updates to the memory code with the node updates only being done due to the
    memory changes.

    I think its a good idea to do the caching and have no problem adding on to
    this patchset if no one else has any objections.


    > Thanks,
    > Robin
    > On Mon, Sep 27, 2010 at 02:09:31PM -0500, 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.
    >> 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.
    >> This version of the patch set includes an update to to properly
    >> report block_size_bytes, phys_index, and end_phys_index. Additionally,
    >> the patch that adds the end_phys_index sysfs file is now patch 5/8
    >> instead of being patch 2/8 as in the previous version of the patches.
    >> -Nathan Fontenot
    >> --
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