diff options
| author | Marcelo Tosatti <mtosatti@redhat.com> | 2012-08-26 13:58:41 -0300 |
|---|---|---|
| committer | Marcelo Tosatti <mtosatti@redhat.com> | 2012-08-26 13:58:41 -0300 |
| commit | c78aa4c4b94b5b148be576a9f1570e31fe282b46 (patch) | |
| tree | 6d5801eb3e60bcec6a91c7b13a334c6d2ea11b05 /Documentation/block/cfq-iosched.txt | |
| parent | 90993cdd1800dc6ef9587431a0c625b978584e81 (diff) | |
| parent | 9acb172543aecb783e2e1e53e3f447d4c0f5c150 (diff) | |
| download | op-kernel-dev-c78aa4c4b94b5b148be576a9f1570e31fe282b46.zip op-kernel-dev-c78aa4c4b94b5b148be576a9f1570e31fe282b46.tar.gz | |
Merge remote-tracking branch 'upstream/master' into queue
Merging critical fixes from upstream required for development.
* upstream/master: (809 commits)
libata: Add a space to " 2GB ATA Flash Disk" DMA blacklist entry
Revert "powerpc: Update g5_defconfig"
powerpc/perf: Use pmc_overflow() to detect rolled back events
powerpc: Fix VMX in interrupt check in POWER7 copy loops
powerpc: POWER7 copy_to_user/copy_from_user patch applied twice
powerpc: Fix personality handling in ppc64_personality()
powerpc/dma-iommu: Fix IOMMU window check
powerpc: Remove unnecessary ifdefs
powerpc/kgdb: Restore current_thread_info properly
powerpc/kgdb: Bail out of KGDB when we've been triggered
powerpc/kgdb: Do not set kgdb_single_step on ppc
powerpc/mpic_msgr: Add missing includes
powerpc: Fix null pointer deref in perf hardware breakpoints
powerpc: Fixup whitespace in xmon
powerpc: Fix xmon dl command for new printk implementation
xfs: check for possible overflow in xfs_ioc_trim
xfs: unlock the AGI buffer when looping in xfs_dialloc
xfs: fix uninitialised variable in xfs_rtbuf_get()
powerpc/fsl: fix "Failed to mount /dev: No such device" errors
powerpc/fsl: update defconfigs
...
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Diffstat (limited to 'Documentation/block/cfq-iosched.txt')
| -rw-r--r-- | Documentation/block/cfq-iosched.txt | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt index 6d670f5..d89b4fe 100644 --- a/Documentation/block/cfq-iosched.txt +++ b/Documentation/block/cfq-iosched.txt @@ -1,3 +1,14 @@ +CFQ (Complete Fairness Queueing) +=============================== + +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 +requests from all the processes are batched together according to their +process's I/O priority. + CFQ ioscheduler tunables ======================== @@ -25,6 +36,72 @@ there are multiple spindles behind single LUN (Host based hardware RAID controller or for storage arrays), setting slice_idle=0 might end up in better throughput and acceptable latencies. +back_seek_max +------------- +This specifies, given in Kbytes, the maximum "distance" for backward seeking. +The distance is the amount of space from the current head location to the +sectors that are backward in terms of distance. + +This parameter allows the scheduler to anticipate requests in the "backward" +direction and consider them as being the "next" if they are within this +distance from the current head location. + +back_seek_penalty +----------------- +This parameter is used to compute the cost of backward seeking. If the +backward distance of request is just 1/back_seek_penalty from a "front" +request, then the seeking cost of two requests is considered equivalent. + +So scheduler will not bias toward one or the other request (otherwise scheduler +will bias toward front request). Default value of back_seek_penalty is 2. + +fifo_expire_async +----------------- +This parameter is used to set the timeout of asynchronous requests. Default +value of this is 248ms. + +fifo_expire_sync +---------------- +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. + +slice_async +----------- +This parameter is same as of slice_sync but for asynchronous queue. The +default value is 40ms. + +slice_async_rq +-------------- +This parameter is used to limit the dispatching of asynchronous request to +device request queue in queue's slice time. The maximum number of request that +are allowed to be dispatched also depends upon the io priority. Default value +for this is 2. + +slice_sync +---------- +When a queue is selected for execution, the queues IO requests are only +executed for a certain amount of time(time_slice) before switching to another +queue. This parameter is used to calculate the time slice of synchronous +queue. + +time_slice is computed using the below equation:- +time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the +time_slice of synchronous queue, increase the value of slice_sync. Default +value is 100ms. + +quantum +------- +This specifies the number of request dispatched to the device queue. In a +queue's time slice, a request will not be dispatched if the number of request +in the device exceeds this parameter. This parameter is used for synchronous +request. + +In case of storage with several disk, this setting can limit the parallel +processing of request. Therefore, increasing the value can imporve the +performace although this can cause the latency of some I/O to increase due +to more number of requests. + CFQ IOPS Mode for group scheduling =================================== Basic CFQ design is to provide priority based time slices. Higher priority |
