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authorAlan D. Brunelle <Alan.Brunelle@hp.com>2010-01-29 09:04:08 +0100
committerJens Axboe <jens.axboe@oracle.com>2010-01-29 09:04:08 +0100
commit488991e28e55b4fbca8067edf0259f69d1a6f92c (patch)
treefea5e0aca42e338137cc050e66aaeb5f539e3d21 /Documentation
parent47483e25205f1f8d79784f0f7c733941bc080ec0 (diff)
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block: Added in stricter no merge semantics for block I/O
Updated 'nomerges' tunable to accept a value of '2' - indicating that _no_ merges at all are to be attempted (not even the simple one-hit cache). The following table illustrates the additional benefit - 5 minute runs of a random I/O load were applied to a dozen devices on a 16-way x86_64 system. nomerges Throughput %System Improvement (tput / %sys) -------- ------------ ----------- ------------------------- 0 12.45 MB/sec 0.669365609 1 12.50 MB/sec 0.641519199 0.40% / 2.71% 2 12.52 MB/sec 0.639849750 0.56% / 2.96% Signed-off-by: Alan D. Brunelle <alan.brunelle@hp.com> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/ABI/testing/sysfs-block14
-rw-r--r--Documentation/block/queue-sysfs.txt10
2 files changed, 19 insertions, 5 deletions
diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index d2f90334bb93..4873c759d535 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -128,3 +128,17 @@ Description:
preferred request size for workloads where sustained
throughput is desired. If no optimal I/O size is
reported this file contains 0.
+
+What: /sys/block/<disk>/queue/nomerges
+Date: January 2010
+Contact:
+Description:
+ Standard I/O elevator operations include attempts to
+ merge contiguous I/Os. For known random I/O loads these
+ attempts will always fail and result in extra cycles
+ being spent in the kernel. This allows one to turn off
+ this behavior on one of two ways: When set to 1, complex
+ merge checks are disabled, but the simple one-shot merges
+ with the previous I/O request are enabled. When set to 2,
+ all merge tries are disabled. The default value is 0 -
+ which enables all types of merge tries.
diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index e164403f60e1..f65274081c8d 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -25,11 +25,11 @@ size allowed by the hardware.
nomerges (RW)
-------------
-This enables the user to disable the lookup logic involved with IO merging
-requests in the block layer. Merging may still occur through a direct
-1-hit cache, since that comes for (almost) free. The IO scheduler will not
-waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults
-to 0, enabling all merges.
+This enables the user to disable the lookup logic involved with IO
+merging requests in the block layer. By default (0) all merges are
+enabled. When set to 1 only simple one-hit merges will be tried. When
+set to 2 no merge algorithms will be tried (including one-hit or more
+complex tree/hash lookups).
nr_requests (RW)
----------------