Re: [PATCH 0/8] v2 De-Couple sysfs memory directories from memorysections

[Robin Holt]

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.

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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