From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 16 Apr 2005 15:20:36 -0700 Subject: Linux-2.6.12-rc2 Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip! --- Documentation/block/deadline-iosched.txt | 78 ++++++++++++++++++++++++++++++++ 1 file changed, 78 insertions(+) create mode 100644 Documentation/block/deadline-iosched.txt (limited to 'Documentation/block/deadline-iosched.txt') diff --git a/Documentation/block/deadline-iosched.txt b/Documentation/block/deadline-iosched.txt new file mode 100644 index 00000000000..c918b3a6022 --- /dev/null +++ b/Documentation/block/deadline-iosched.txt @@ -0,0 +1,78 @@ +Deadline IO scheduler tunables +============================== + +This little file attempts to document how the deadline io scheduler works. +In particular, it will clarify the meaning of the exposed tunables that may be +of interest to power users. + +Each io queue has a set of io scheduler tunables associated with it. These +tunables control how the io scheduler works. You can find these entries +in: + +/sys/block//queue/iosched + +assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted, +you can do so by typing: + +# mount none /sys -t sysfs + + +******************************************************************************** + + +read_expire (in ms) +----------- + +The goal of the deadline io scheduler is to attempt to guarentee a start +service time for a request. As we focus mainly on read latencies, this is +tunable. When a read request first enters the io scheduler, it is assigned +a deadline that is the current time + the read_expire value in units of +miliseconds. + + +write_expire (in ms) +----------- + +Similar to read_expire mentioned above, but for writes. + + +fifo_batch +---------- + +When a read request expires its deadline, we must move some requests from +the sorted io scheduler list to the block device dispatch queue. fifo_batch +controls how many requests we move, based on the cost of each request. A +request is either qualified as a seek or a stream. The io scheduler knows +the last request that was serviced by the drive (or will be serviced right +before this one). See seek_cost and stream_unit. + + +write_starved (number of dispatches) +------------- + +When we have to move requests from the io scheduler queue to the block +device dispatch queue, we always give a preference to reads. However, we +don't want to starve writes indefinitely either. So writes_starved controls +how many times we give preference to reads over writes. When that has been +done writes_starved number of times, we dispatch some writes based on the +same criteria as reads. + + +front_merges (bool) +------------ + +Sometimes it happens that a request enters the io scheduler that is contigious +with a request that is already on the queue. Either it fits in the back of that +request, or it fits at the front. That is called either a back merge candidate +or a front merge candidate. Due to the way files are typically laid out, +back merges are much more common than front merges. For some work loads, you +may even know that it is a waste of time to spend any time attempting to +front merge requests. Setting front_merges to 0 disables this functionality. +Front merges may still occur due to the cached last_merge hint, but since +that comes at basically 0 cost we leave that on. We simply disable the +rbtree front sector lookup when the io scheduler merge function is called. + + +Nov 11 2002, Jens Axboe + + -- cgit v1.2.3