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Konrad Rzeszutek Wilk 013c3ca184 xen/blkback: Stick REQ_SYNC on WRITEs to deal with CFQ I/O scheduler. 
If one runs a simple fio request with random read/write with a
20%/80% ratio, the numbers are incredibly bad when using the CFQ scheduler.

IOmeter       |       |      |          |
64K, randrw   |  NOOP | CFQ  | deadline |
randrwmix=80  |       |      |          |
--------------+-------+------+----------+
blkback       |103/27 |32/10 | 102/27   |
--------------+-------+------+----------+
QEMU qdisk    |103/27 |102/27| 102/27   |

The problem as explained by Vivek Goyal was:

".. that difference is that sync vs async requests. In the case of
a kernel thread submitting IO, [..] all the WRITES might be being
considered as async and will go in a different queue. If you mix those
with some READS, they are always sync and will go in differnet queue.
In presence of sync queue, CFQ will idle and choke up WRITES in
an attempt to improve latencies of READs.

In case of AIO [note: this is what QEMU qdisk is doing] , [..]
it is direct IO and both READS and WRITES will be considered SYNC
and will go in a single queue and no choking of WRITES will take place."

The solution is quite simple, tack on REQ_SYNC (which is
what the WRITE_ODIRECT macro points to) and the numbers go
back up.

Suggested-by: Vivek Goyal <vgoyal@redhat.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
2011-04-26 16:24:18 -04:00
..
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acpi Fix common misspellings 2011-03-31 11:26:23 -03:00
amba Fix common misspellings 2011-03-31 11:26:23 -03:00
ata Fix common misspellings 2011-03-31 11:26:23 -03:00
atm Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
auxdisplay Fix common misspellings 2011-03-31 11:26:23 -03:00
base Fix common misspellings 2011-03-31 11:26:23 -03:00
block xen/blkback: Stick REQ_SYNC on WRITEs to deal with CFQ I/O scheduler. 2011-04-26 16:24:18 -04:00
bluetooth Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
cdrom Fix common misspellings 2011-03-31 11:26:23 -03:00
char Fix common misspellings 2011-03-31 11:26:23 -03:00
clk
clocksource
connector
cpufreq Fix common misspellings 2011-03-31 11:26:23 -03:00
cpuidle
crypto Fix common misspellings 2011-03-31 11:26:23 -03:00
dca
dio
dma Merge branch 'spi/merge' of git://git.secretlab.ca/git/linux-2.6 2011-04-11 15:44:38 -07:00
edac Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
eisa
firewire Fix common misspellings 2011-03-31 11:26:23 -03:00
firmware
gpio Merge branch 'spi/merge' of git://git.secretlab.ca/git/linux-2.6 2011-04-11 15:44:38 -07:00
gpu Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
hid Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
hwmon Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
hwspinlock
i2c Fix common misspellings 2011-03-31 11:26:23 -03:00
ide Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
idle
ieee802154
infiniband Fix common misspellings 2011-03-31 11:26:23 -03:00
input Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
isdn Fix common misspellings 2011-03-31 11:26:23 -03:00
leds Fix common misspellings 2011-03-31 11:26:23 -03:00
lguest Fix common misspellings 2011-03-31 11:26:23 -03:00
macintosh Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
mca
md Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
media Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
memstick Fix common misspellings 2011-03-31 11:26:23 -03:00
message Fix common misspellings 2011-03-31 11:26:23 -03:00
mfd Fix common misspellings 2011-03-31 11:26:23 -03:00
misc Fix common misspellings 2011-03-31 11:26:23 -03:00
mmc Fix common misspellings 2011-03-31 11:26:23 -03:00
mtd Merge git://git.infradead.org/mtd-2.6 2011-04-09 13:23:50 -07:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-2.6 2011-04-11 07:27:24 -07:00
nfc
nubus
of Fix common misspellings 2011-03-31 11:26:23 -03:00
oprofile
parisc Fix common misspellings 2011-03-31 11:26:23 -03:00
parport Fix common misspellings 2011-03-31 11:26:23 -03:00
pci pci: fix PCI bus allocation alignment handling 2011-04-11 10:53:11 -07:00
pcmcia Fix common misspellings 2011-03-31 11:26:23 -03:00
platform Fix common misspellings 2011-03-31 11:26:23 -03:00
pnp Fix common misspellings 2011-03-31 11:26:23 -03:00
power
pps Fix common misspellings 2011-03-31 11:26:23 -03:00
ps3 Fix common misspellings 2011-03-31 11:26:23 -03:00
rapidio Fix common misspellings 2011-03-31 11:26:23 -03:00
regulator Fix common misspellings 2011-03-31 11:26:23 -03:00
rtc Merge branches 'x86-fixes-for-linus', 'sched-fixes-for-linus', 'timers-fixes-for-linus', 'irq-fixes-for-linus' and 'perf-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2011-04-07 12:12:58 -07:00
s390 Merge branch 'for-linus' of git://git390.marist.edu/pub/scm/linux-2.6 2011-04-08 07:36:14 -07:00
sbus Fix common misspellings 2011-03-31 11:26:23 -03:00
scsi Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
sfi Fix common misspellings 2011-03-31 11:26:23 -03:00
sh
sn
spi Merge branch 'spi/merge' of git://git.secretlab.ca/git/linux-2.6 2011-04-11 15:44:38 -07:00
ssb Fix common misspellings 2011-03-31 11:26:23 -03:00
staging Merge branch 'staging-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-04-07 11:36:44 -07:00
target Fix common misspellings 2011-03-31 11:26:23 -03:00
tc
telephony Fix common misspellings 2011-03-31 11:26:23 -03:00
thermal
tty Merge branch 'sh-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/lethal/sh-2.6 2011-04-07 12:48:45 -07:00
uio Fix common misspellings 2011-03-31 11:26:23 -03:00
usb Merge branch 'for-linus2' of git://git.profusion.mobi/users/lucas/linux-2.6 2011-04-07 11:14:49 -07:00
uwb Fix common misspellings 2011-03-31 11:26:23 -03:00
vhost
video Merge branch 'fbdev-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/lethal/fbdev-2.6 2011-04-07 12:49:17 -07:00
virtio
vlynq
w1 Fix common misspellings 2011-03-31 11:26:23 -03:00
watchdog watchdog: mpc8xxx_wdt: fix build 2011-04-07 20:20:24 +00:00
xen xen/blkback: Move it from drivers/xen to drivers/block 2011-04-18 14:30:26 -04:00
zorro
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