Cleaning out "look at this" directory, I don't see this applied
upstream but it may already be in Jens' tree. (That's the tree it
should go in through...)
On 03/30/2013 09:55:04 PM, Namjae Jeon wrote:
From: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
Add the documentation text for latency, target_latency & group_idle
tunnable parameters in the block/cfq-iosched.txt.
Also fix few typo(spelling) mistakes.
Signed-off-by: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
Signed-off-by: Amit Sahrawat <a.sahrawat@xxxxxxxxxxx>
---
Documentation/block/cfq-iosched.txt | 47
++++++++++++++++++++++++++++++++---
1 file changed, 44 insertions(+), 3 deletions(-)
diff --git a/Documentation/block/cfq-iosched.txt
b/Documentation/block/cfq-iosched.txt
index a5eb7d1..4d02bca 100644
--- a/Documentation/block/cfq-iosched.txt
+++ b/Documentation/block/cfq-iosched.txt
@@ -5,7 +5,7 @@ The main aim of CFQ scheduler is to provide a fair
allocation of the disk
I/O bandwidth for all the processes which requests an I/O operation.
CFQ maintains the per process queue for the processes which request
I/O
-operation(syncronous requests). In case of asynchronous requests,
all the
+operation(synchronous requests). In case of asynchronous requests,
all the
requests from all the processes are batched together according to
their
process's I/O priority.
@@ -66,6 +66,47 @@ This parameter is used to set the timeout of
synchronous requests. Default
value of this is 124ms. In case to favor synchronous requests over
asynchronous
one, this value should be decreased relative to fifo_expire_async.
+group_idle
+-----------
+This parameter forces idling at the CFQ group level instead of CFQ
You don't need to say "this parameter", you can just start with "Force
idling at..."
+queue level. This is introduced after after a bottleneck was observed
+in higher end storage due to idle on sequential queuee and allow
dispatch
queuee
+from a single queue. The idea with this parameter is that it be be
run with
+slice_idle=0 and group_idle=8, so that idling does not happen on
individual
+queues in the group but happens overall on the group and still keep
the IO
+controller working.
+Not idling on individual queues in the group will dispatch requests
from
+multiple queues in the group at the same time and achieve higher
throughput
+on higher end storage.
+
+Default value for this parameter is 8ms.
+
+latency
+-------
+This parameter is used to enable/disable the latency mode of the CFQ
"Enable/disable the latency mode..."
+scheduler. So if latency mode (called low_latency) is enabled, then
CFQ tries
+to recompute the slice time for each process based on the
target_latency set
+for the system. This favors the fairness over throughput. Disabling
low
+latency (setting it to 0) ignores target latency, allowing each
process in the
+system to get a full time slice.
+
+By default low latency mode is enabled.
Why are latency and target_latency separate parameters? (0 already
disables it... the logical thing to do...?)
I.E. why does this knob even exist separate from target_latency?
+target_latency
+--------------
+This parameter is used to calculate the time slice for a process if
cfq's
+latency mode is enabled. It will ensure that sync requests have an
estimated
+latency. But sometime if sequential workload is more (e.g.
sequential read),
+then to meet the latency constraints, throughput may decrease
because of less
+time for each process to issue I/O request before the cfq queue is
switched.
+
+Though this can be overcome by disabling the latency_mode, but it
may increase
+the read latency for some applications. So, this parameter allows
for changing
+target_latency through sysfs interface which can provide the balanced
+throughput and read latency.
+
+Default value for target_latency is 300ms.
Sorry, I try not to rewrite, but this whole section can just be:
Cap outstanding I/O requests to this many miliseconds (default 300),
ensuring sync requests have an estimated latency. Lowering this may
decrease throughput on sequential workloads by switching queues more
often (interleaving other I/O).
(And it could ahve been called target_latency_ms to be
self-documenting. Oh well.)
Rob--
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