linux-kernel - Re: [PATCH 0/8] De-couple sysfs memory directories from memory sections

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Message-ID: <20100923184002.GM3952@balbir.in.ibm.com>
Date:	Fri, 24 Sep 2010 00:10:02 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Nathan Fontenot <nfont@...tin.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linuxppc-dev@...abs.org, Greg KH <greg@...ah.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/8] De-couple sysfs memory directories from memory
 sections

* Nathan Fontenot <nfont@...tin.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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