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Subject[RFC PATCH] block: Change default IO scheduler to deadline except SATA
Hi,

I am wondering if CFQ as default scheduler is still the right choice. CFQ
generally works well on slow rotational media (SATA?). But often
underperforms on faster storage (storage arrays, PCIE SSDs, virtualized
disk in linux guests etc). People often put logic in user space to tune their
systems and change IO scheduler to deadline to get better performance on
faster storage.

Though there is not one good answer for all kind of storage and for all
kind of workloads, I am wondering if we can provide a better default and
that is change default IO scheduler to "deadline" except SATA.

One can argue that some SAS disks can be slow too and benefit from CFQ. Yes,
but default IO scheduler choice is not perfect anyway. It just tries to
cater to a wide variety of use cases out of the box.

So I am throwing this patch out see if it flies. Personally, I think it
might turn out to be a more reasonable default.

Thanks
Vivek


Change default IO scheduler to deadline except SATA disks.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
block/Kconfig.iosched | 2 +-
block/elevator.c | 2 +-
drivers/ata/libata-scsi.c | 4 ++++
include/linux/elevator.h | 2 ++
4 files changed, 8 insertions(+), 2 deletions(-)
Index: linux-2.6/block/Kconfig.iosched
===================================================================
--- linux-2.6.orig/block/Kconfig.iosched 2012-04-09 22:18:30.941885325 -0400
+++ linux-2.6/block/Kconfig.iosched 2012-04-09 22:18:51.982885971 -0400
@@ -45,7 +45,7 @@ config CFQ_GROUP_IOSCHED

choice
prompt "Default I/O scheduler"
- default DEFAULT_CFQ
+ default DEFAULT_DEADLINE
help
Select the I/O scheduler which will be used by default for all
block devices.
Index: linux-2.6/drivers/ata/libata-scsi.c
===================================================================
--- linux-2.6.orig/drivers/ata/libata-scsi.c 2012-04-09 22:18:30.946885325 -0400
+++ linux-2.6/drivers/ata/libata-scsi.c 2012-04-10 01:09:10.529292695 -0400
@@ -1146,6 +1146,10 @@ static int ata_scsi_dev_config(struct sc

blk_queue_flush_queueable(q, false);

+ /* Change IO scheduler to CFQ */
+ if (!(*chosen_elevator))
+ elevator_change(q, "cfq");
+
dev->sdev = sdev;
return 0;
}
Index: linux-2.6/block/elevator.c
===================================================================
--- linux-2.6.orig/block/elevator.c 2012-04-09 22:18:30.000000000 -0400
+++ linux-2.6/block/elevator.c 2012-04-10 20:11:10.296866631 -0400
@@ -130,7 +130,7 @@ static int elevator_init_queue(struct re
return -ENOMEM;
}

-static char chosen_elevator[ELV_NAME_MAX];
+char chosen_elevator[ELV_NAME_MAX];

static int __init elevator_setup(char *str)
{
Index: linux-2.6/include/linux/elevator.h
===================================================================
--- linux-2.6.orig/include/linux/elevator.h 2012-03-13 01:07:29.000000000 -0400
+++ linux-2.6/include/linux/elevator.h 2012-04-10 01:07:44.303289797 -0400
@@ -204,5 +204,7 @@ enum {
INIT_LIST_HEAD(&(rq)->csd.list); \
} while (0)

+extern char chosen_elevator[];
+
#endif /* CONFIG_BLOCK */
#endif

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