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linux-yocto/Documentation/sysctl
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Mel Gorman e5f06bf8f4 mm: do not boost watermarks to avoid fragmentation for the DISCONTIG memory model 
commit 24512228b7 upstream.

Mikulas Patocka reported that commit 1c30844d2d ("mm: reclaim small
amounts of memory when an external fragmentation event occurs") "broke"
memory management on parisc.

The machine is not NUMA but the DISCONTIG model creates three pgdats
even though it's a UMA machine for the following ranges

        0) Start 0x0000000000000000 End 0x000000003fffffff Size   1024 MB
        1) Start 0x0000000100000000 End 0x00000001bfdfffff Size   3070 MB
        2) Start 0x0000004040000000 End 0x00000040ffffffff Size   3072 MB

Mikulas reported:

	With the patch 1c30844d2, the kernel will incorrectly reclaim the
	first zone when it fills up, ignoring the fact that there are two
	completely free zones. Basiscally, it limits cache size to 1GiB.

	For example, if I run:
	# dd if=/dev/sda of=/dev/null bs=1M count=2048

	- with the proper kernel, there should be "Buffers - 2GiB"
	when this command finishes. With the patch 1c30844d2, buffers
	will consume just 1GiB or slightly more, because the kernel was
	incorrectly reclaiming them.

The page allocator and reclaim makes assumptions that pgdats really
represent NUMA nodes and zones represent ranges and makes decisions on
that basis.  Watermark boosting for small pgdats leads to unexpected
results even though this would have behaved reasonably on SPARSEMEM.

DISCONTIG is essentially deprecated and even parisc plans to move to
SPARSEMEM so there is no need to be fancy, this patch simply disables
watermark boosting by default on DISCONTIGMEM.

Link: http://lkml.kernel.org/r/20190419094335.GJ18914@techsingularity.net
Fixes: 1c30844d2d ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2019-05-02 10:02:45 +02:00
..
abi.txt
fs.txt Documentation: Fix grammatical error in sysctl/fs.txt & clarify negative dentry 2019-02-11 09:10:22 -08:00
kernel.txt kernel/sysctl: add panic_print into sysctl 2019-01-04 13:13:47 -08:00
net.txt bpf: add bpf_jit_limit knob to restrict unpriv allocations 2018-10-25 17:11:42 -07:00
README docs: Update binfmt_misc links 2017-10-03 15:23:38 -06:00
sunrpc.txt
user.txt Documentation/sysctl/user.txt: fix typo 2018-02-06 18:32:48 -08:00
vm.txt mm: do not boost watermarks to avoid fragmentation for the DISCONTIG memory model 2019-05-02 10:02:45 +02:00

README

Documentation for /proc/sys/ kernel version 2.2.10 (c) 1998, 1999, Rik van Riel riel@nl.linux.org

'Why', I hear you ask, 'would anyone even want documentation for them sysctl files? If anybody really needs it, it's all in the source...'

Well, this documentation is written because some people either don't know they need to tweak something, or because they don't have the time or knowledge to read the source code.

Furthermore, the programmers who built sysctl have built it to be actually used, not just for the fun of programming it :-)

==============================================================

Legal blurb:

As usual, there are two main things to consider:

  1. you get what you pay for
  2. it's free

The consequences are that I won't guarantee the correctness of this document, and if you come to me complaining about how you screwed up your system because of wrong documentation, I won't feel sorry for you. I might even laugh at you...

But of course, if you do manage to screw up your system using only the sysctl options used in this file, I'd like to hear of it. Not only to have a great laugh, but also to make sure that you're the last RTFMing person to screw up.

In short, e-mail your suggestions, corrections and / or horror stories to: riel@nl.linux.org

Rik van Riel.

==============================================================

Introduction:

Sysctl is a means of configuring certain aspects of the kernel at run-time, and the /proc/sys/ directory is there so that you don't even need special tools to do it! In fact, there are only four things needed to use these config facilities:

  • a running Linux system
  • root access
  • common sense (this is especially hard to come by these days)
  • knowledge of what all those values mean

As a quick 'ls /proc/sys' will show, the directory consists of several (arch-dependent?) subdirs. Each subdir is mainly about one part of the kernel, so you can do configuration on a piece by piece basis, or just some 'thematic frobbing'.

The subdirs are about: abi/ execution domains & personalities debug/ dev/ device specific information (eg dev/cdrom/info) fs/ specific filesystems filehandle, inode, dentry and quota tuning binfmt_misc <Documentation/admin-guide/binfmt-misc.rst> kernel/ global kernel info / tuning miscellaneous stuff net/ networking stuff, for documentation look in: <Documentation/networking/> proc/ sunrpc/ SUN Remote Procedure Call (NFS) vm/ memory management tuning buffer and cache management user/ Per user per user namespace limits

These are the subdirs I have on my system. There might be more or other subdirs in another setup. If you see another dir, I'd really like to hear about it :-)