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Revision d7831a0bdf06b9f722b947bb0c205ff7d77cebd8 authored by Richard Kennedy on 30 June 2009, 18:41:35 UTC, committed by Linus Torvalds on 01 July 2009, 01:56:01 UTC 
mm: prevent balance_dirty_pages() from doing too much work
 
balance_dirty_pages can overreact and move all of the dirty pages to
writeback unnecessarily.

balance_dirty_pages makes its decision to throttle based on the number of
dirty plus writeback pages that are over the calculated limit,so it will
continue to move pages even when there are plenty of pages in writeback
and less than the threshold still dirty.

This allows it to overshoot its limits and move all the dirty pages to
writeback while waiting for the drives to catch up and empty the writeback
list.

A simple fio test easily demonstrates this problem.

fio --name=f1 --directory=/disk1 --size=2G -rw=write --name=f2 --directory=/disk2 --size=1G --rw=write --startdelay=10

This is the simplest fix I could find, but I'm not entirely sure that it
alone will be enough for all cases.  But it certainly is an improvement on
my desktop machine writing to 2 disks.

Do we need something more for machines with large arrays where
bdi_threshold * number_of_drives is greater than the dirty_ratio ?

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent df279ca
Raw File
Kconfig.kgdb 

config HAVE_ARCH_KGDB
	bool

menuconfig KGDB
	bool "KGDB: kernel debugging with remote gdb"
	depends on HAVE_ARCH_KGDB
	depends on DEBUG_KERNEL && EXPERIMENTAL
	help
	  If you say Y here, it will be possible to remotely debug the
	  kernel using gdb.  It is recommended but not required, that
	  you also turn on the kernel config option
	  CONFIG_FRAME_POINTER to aid in producing more reliable stack
	  backtraces in the external debugger.  Documentation of
	  kernel debugger is available at http://kgdb.sourceforge.net
	  as well as in DocBook form in Documentation/DocBook/.  If
	  unsure, say N.

if KGDB

config KGDB_SERIAL_CONSOLE
	tristate "KGDB: use kgdb over the serial console"
	select CONSOLE_POLL
	select MAGIC_SYSRQ
	default y
	help
	  Share a serial console with kgdb. Sysrq-g must be used
	  to break in initially.

config KGDB_TESTS
	bool "KGDB: internal test suite"
	default n
	help
	  This is a kgdb I/O module specifically designed to test
	  kgdb's internal functions.  This kgdb I/O module is
	  intended to for the development of new kgdb stubs
	  as well as regression testing the kgdb internals.
	  See the drivers/misc/kgdbts.c for the details about
	  the tests.  The most basic of this I/O module is to boot
	  a kernel boot arguments "kgdbwait kgdbts=V1F100"

config KGDB_TESTS_ON_BOOT
	bool "KGDB: Run tests on boot"
	depends on KGDB_TESTS
	default n
	help
	  Run the kgdb tests on boot up automatically without the need
	  to pass in a kernel parameter

config KGDB_TESTS_BOOT_STRING
	string "KGDB: which internal kgdb tests to run"
	depends on KGDB_TESTS_ON_BOOT
	default "V1F100"
	help
	  This is the command string to send the kgdb test suite on
	  boot.  See the drivers/misc/kgdbts.c for detailed
	  information about other strings you could use beyond the
	  default of V1F100.

endif # KGDB
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