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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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