From 488991e28e55b4fbca8067edf0259f69d1a6f92c Mon Sep 17 00:00:00 2001 From: "Alan D. Brunelle" Date: Fri, 29 Jan 2010 09:04:08 +0100 Subject: 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 Signed-off-by: Jens Axboe --- Documentation/ABI/testing/sysfs-block | 14 ++++++++++++++ Documentation/block/queue-sysfs.txt | 10 +++++----- 2 files changed, 19 insertions(+), 5 deletions(-) (limited to 'Documentation') diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block index d2f90334bb9..4873c759d53 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//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 e164403f60e..f65274081c8 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) ---------------- -- cgit v1.2.3