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Date: Fri, 31 Jul 2009 11:10:29 +0800 From: Shan Wei <shanwei@...fujitsu.com> To: Jens Axboe <jens.axboe@...cle.com> CC: rdunlap@...otime.net, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org Subject: [PATCH]cfq: add the document of the tuning parameters This little file attempts to document how the cfq io scheduler works. In particular, it will clarify the meaning of the tuning parameters. Signed-off-by: Shan Wei <shanwei@...fujitsu.com> Cc: Randy Dunlap <rdunlap@...otime.net> --- Documentation/block/00-INDEX | 2 + Documentation/block/cfq-iosched.txt | 82 +++++++++++++++++++++++++++++++++++ 2 files changed, 84 insertions(+), 0 deletions(-) create mode 100644 Documentation/block/cfq-iosched.txt diff --git a/Documentation/block/00-INDEX b/Documentation/block/00-INDEX index 961a051..2ce5c7a 100644 --- a/Documentation/block/00-INDEX +++ b/Documentation/block/00-INDEX @@ -8,6 +8,8 @@ biodoc.txt - Notes on the Generic Block Layer Rewrite in Linux 2.5 capability.txt - Generic Block Device Capability (/sys/block/<disk>/capability) +cfq-iosched.txt + - CFQ(Complete Fairness Queueing) IO scheduler tunables deadline-iosched.txt - Deadline IO scheduler tunables ioprio.txt diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt new file mode 100644 index 0000000..ccf5c61 --- /dev/null +++ b/Documentation/block/cfq-iosched.txt @@ -0,0 +1,82 @@ +CFQ(Complete Fairness Queueing) IO scheduler tunables +===================================================== +Shan Wei <shanwei@...fujitsu.com> 30 Jun 2009 + + +The main goal of he "Complete Fairness Queueing" elevator is ensuring +a fair allocation of the disk I/O bandwidth among all the processes +that trigger the I/O requests. + +For synchronous requests, CFQ creates per-process queues to manage the requests. +But asynchronous requests are batched together in fewer queues, +one per IO priority. See Documentation/block/ioprio.txt for details +of the IO priority. CFQ schedules queues in a round-robin manner and +dispatches requests in queue's time slice, so that it can ensure that +every process accesses the disk fairly. + +Now, CFQ is the default IO scheduler. + + +Selecting IO schedulers +----------------------- +Refer to Documentation/block/switching-sched.txt for information on +selecting an io scheduler on a per-device basis. + + +Tuning the CFQ IO scheduler +--------------------------- +When using 'cfq', there are 9 parameters under /sys/block/*/queue/iosched/. + +The parameters are: +* back_seek_max (in KByte) + The parameter sets the max distance for backward seeking, + default is 16MBytes. The idea comes from the AS scheduler. + Like forward requests, CFQ may dispatch a back request that accesses sector + at the backward of current magnetic head. + +* back_seek_penalty + The parameter is used to calculate the cost of backward seeking, + default is 2. Relative to front request, if the backward distance of + a request is just 1/2 (1/back_seek_penalty) of it, + the seeking cost of the two requests is considered equivalent. + +* fifo_expire_async (in ms) + The parameter is used to set the timeout of asynchronous requests, + default is 250ms. CFQ maintains a fifo list to manage timeout requests. + CFQ doesn't check the expired requests from the fifo queue, after one + timeout request is dispatched. + +* fifo_expire_sync (in ms) + Same as fifo_expire_async, but the parameter is for synchronous requests, + default is 125ms. + +* slice_sync (in ms) + The parameter is used to calculate time slice of synchronous queue, + default is 100ms. But the actual time slice also depends on + the io priority. The formula is: + time_slice = slice_sync + (slice_sync / 5 * (4 - io_priority)). + +* slice_async (in ms) + Same as slice_sync, but the parameter is for asynchronous queue, + default is 40ms. + +* slice_async_rq + The parameter is used to limit dispatching of asynchronous requests + to the device request-queue in queue's slice time, default is 2. + The max number of requests that are allowed to be dispatched also + depends on the io priority. The formula is: max_nr_requests = + 2 * (slice_async_rq + slice_async_rq * (7 - io_priority)). + +* slice_idle (in ms) + The parameter is the idle time for synchronous queue only, default is 8ms. + In queue's time slice, when there is no request in the synchronous queue, + CFQ will not switch to another queue, but idle to wait for process creating + more requests. If no new requests are submitted, the queue will expire. + +* quantum (number of requests) + The number of dispatched requests to the device request-queue is restricted + by this parameter, default is 4. In queue's time slice, request will not be + dispatched if the number of requests in the device request-queue + exceeds the limit. For asynchronous queue, dispatching is also + restricted by slice_async_rq. Note, the parameter has no effect + on the IDLE queue(IDLE is the class of the IO priority). -- 1.6.0.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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