Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | Deadline IO scheduler tunables |
| 2 | ============================== |
| 3 | |
| 4 | This little file attempts to document how the deadline io scheduler works. |
| 5 | In particular, it will clarify the meaning of the exposed tunables that may be |
| 6 | of interest to power users. |
| 7 | |
| 8 | Each io queue has a set of io scheduler tunables associated with it. These |
| 9 | tunables control how the io scheduler works. You can find these entries |
| 10 | in: |
| 11 | |
| 12 | /sys/block/<device>/queue/iosched |
| 13 | |
| 14 | assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted, |
| 15 | you can do so by typing: |
| 16 | |
| 17 | # mount none /sys -t sysfs |
| 18 | |
| 19 | |
| 20 | ******************************************************************************** |
| 21 | |
| 22 | |
| 23 | read_expire (in ms) |
| 24 | ----------- |
| 25 | |
Matt LaPlante | a2ffd27 | 2006-10-03 22:49:15 +0200 | [diff] [blame] | 26 | The goal of the deadline io scheduler is to attempt to guarantee a start |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 27 | service time for a request. As we focus mainly on read latencies, this is |
| 28 | tunable. When a read request first enters the io scheduler, it is assigned |
| 29 | a deadline that is the current time + the read_expire value in units of |
Matt LaPlante | 2fe0ae7 | 2006-10-03 22:50:39 +0200 | [diff] [blame^] | 30 | milliseconds. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 31 | |
| 32 | |
| 33 | write_expire (in ms) |
| 34 | ----------- |
| 35 | |
| 36 | Similar to read_expire mentioned above, but for writes. |
| 37 | |
| 38 | |
| 39 | fifo_batch |
| 40 | ---------- |
| 41 | |
| 42 | When a read request expires its deadline, we must move some requests from |
| 43 | the sorted io scheduler list to the block device dispatch queue. fifo_batch |
| 44 | controls how many requests we move, based on the cost of each request. A |
| 45 | request is either qualified as a seek or a stream. The io scheduler knows |
| 46 | the last request that was serviced by the drive (or will be serviced right |
| 47 | before this one). See seek_cost and stream_unit. |
| 48 | |
| 49 | |
| 50 | write_starved (number of dispatches) |
| 51 | ------------- |
| 52 | |
| 53 | When we have to move requests from the io scheduler queue to the block |
| 54 | device dispatch queue, we always give a preference to reads. However, we |
| 55 | don't want to starve writes indefinitely either. So writes_starved controls |
| 56 | how many times we give preference to reads over writes. When that has been |
| 57 | done writes_starved number of times, we dispatch some writes based on the |
| 58 | same criteria as reads. |
| 59 | |
| 60 | |
| 61 | front_merges (bool) |
| 62 | ------------ |
| 63 | |
| 64 | Sometimes it happens that a request enters the io scheduler that is contigious |
| 65 | with a request that is already on the queue. Either it fits in the back of that |
| 66 | request, or it fits at the front. That is called either a back merge candidate |
| 67 | or a front merge candidate. Due to the way files are typically laid out, |
| 68 | back merges are much more common than front merges. For some work loads, you |
| 69 | may even know that it is a waste of time to spend any time attempting to |
| 70 | front merge requests. Setting front_merges to 0 disables this functionality. |
| 71 | Front merges may still occur due to the cached last_merge hint, but since |
| 72 | that comes at basically 0 cost we leave that on. We simply disable the |
| 73 | rbtree front sector lookup when the io scheduler merge function is called. |
| 74 | |
| 75 | |
| 76 | Nov 11 2002, Jens Axboe <axboe@suse.de> |
| 77 | |
| 78 | |