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- This file
- - Anticipatory IO scheduler
- I/O Barriers
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-Anticipatory IO scheduler
-Nick Piggin <piggin@cyberone.com.au> 13 Sep 2003
-Attention! Database servers, especially those using "TCQ" disks should
-investigate performance with the 'deadline' IO scheduler. Any system with high
-disk performance requirements should do so, in fact.
-If you see unusual performance characteristics of your disk systems, or you
-see big performance regressions versus the deadline scheduler, please email
-me. Database users don't bother unless you're willing to test a lot of patches
-from me ;) its a known issue.
-Also, users with hardware RAID controllers, doing striping, may find
-highly variable performance results with using the as-iosched. The
-as-iosched anticipatory implementation is based on the notion that a disk
-device has only one physical seeking head. A striped RAID controller
-actually has a head for each physical device in the logical RAID device.
-However, setting the antic_expire (see tunable parameters below) produces
-very similar behavior to the deadline IO scheduler.
-Selecting IO schedulers
-Refer to Documentation/block/switching-sched.txt for information on
-selecting an io scheduler on a per-device basis.
-Anticipatory IO scheduler Policies
-The as-iosched implementation implements several layers of policies
-to determine when an IO request is dispatched to the disk controller.
-Here are the policies outlined, in order of application.
-1. one-way Elevator algorithm.
-The elevator algorithm is similar to that used in deadline scheduler, with
-the addition that it allows limited backward movement of the elevator
-(i.e. seeks backwards). A seek backwards can occur when choosing between
-two IO requests where one is behind the elevator's current position, and
-the other is in front of the elevator's position. If the seek distance to
-the request in back of the elevator is less than half the seek distance to
-the request in front of the elevator, then the request in back can be chosen.
-Backward seeks are also limited to a maximum of MAXBACK (1024*1024) sectors.
-This favors forward movement of the elevator, while allowing opportunistic
-"short" backward seeks.
-2. FIFO expiration times for reads and for writes.
-This is again very similar to the deadline IO scheduler. The expiration
-times for requests on these lists is tunable using the parameters read_expire
-and write_expire discussed below. When a read or a write expires in this way,
-the IO scheduler will interrupt its current elevator sweep or read anticipation
-to service the expired request.
-3. Read and write request batching
-A batch is a collection of read requests or a collection of write
-requests. The as scheduler alternates dispatching read and write batches
-to the driver. In the case a read batch, the scheduler submits read
-requests to the driver as long as there are read requests to submit, and
-the read batch time limit has not been exceeded (read_batch_expire).
-The read batch time limit begins counting down only when there are
-competing write requests pending.
-In the case of a write batch, the scheduler submits write requests to
-the driver as long as there are write requests available, and the
-write batch time limit has not been exceeded (write_batch_expire).
-However, the length of write batches will be gradually shortened
-when read batches frequently exceed their time limit.
-When changing between batch types, the scheduler waits for all requests
-from the previous batch to complete before scheduling requests for the
-next batch.
-The read and write fifo expiration times described in policy 2 above
-are checked only when in scheduling IO of a batch for the corresponding
-(read/write) type. So for example, the read FIFO timeout values are
-tested only during read batches. Likewise, the write FIFO timeout
-values are tested only during write batches. For this reason,
-it is generally not recommended for the read batch time
-to be longer than the write expiration time, nor for the write batch
-time to exceed the read expiration time (see tunable parameters below).
-When the IO scheduler changes from a read to a write batch,
-it begins the elevator from the request that is on the head of the
-write expiration FIFO. Likewise, when changing from a write batch to
-a read batch, scheduler begins the elevator from the first entry
-on the read expiration FIFO.
-4. Read anticipation.
-Read anticipation occurs only when scheduling a read batch.
-This implementation of read anticipation allows only one read request
-to be dispatched to the disk controller at a time. In
-contrast, many write requests may be dispatched to the disk controller
-at a time during a write batch. It is this characteristic that can make
-the anticipatory scheduler perform anomalously with controllers supporting
-TCQ, or with hardware striped RAID devices. Setting the antic_expire
-queue parameter (see below) to zero disables this behavior, and the
-anticipatory scheduler behaves essentially like the deadline scheduler.
-When read anticipation is enabled (antic_expire is not zero), reads
-are dispatched to the disk controller one at a time.
-At the end of each read request, the IO scheduler examines its next
-candidate read request from its sorted read list. If that next request
-is from the same process as the request that just completed,
-or if the next request in the queue is "very close" to the
-just completed request, it is dispatched immediately. Otherwise,
-statistics (average think time, average seek distance) on the process
-that submitted the just completed request are examined. If it seems
-likely that that process will submit another request soon, and that
-request is likely to be near the just completed request, then the IO
-scheduler will stop dispatching more read requests for up to (antic_expire)
-milliseconds, hoping that process will submit a new request near the one
-that just completed. If such a request is made, then it is dispatched
-immediately. If the antic_expire wait time expires, then the IO scheduler
-will dispatch the next read request from the sorted read queue.
-To decide whether an anticipatory wait is worthwhile, the scheduler
-maintains statistics for each process that can be used to compute
-mean "think time" (the time between read requests), and mean seek
-distance for that process. One observation is that these statistics
-are associated with each process, but those statistics are not associated
-with a specific IO device. So for example, if a process is doing IO
-on several file systems on separate devices, the statistics will be
-a combination of IO behavior from all those devices.
-Tuning the anticipatory IO scheduler
-When using 'as', the anticipatory IO scheduler there are 5 parameters under
-/sys/block/*/queue/iosched/. All are units of milliseconds.
-The parameters are:
-* read_expire
- Controls how long until a read request becomes "expired". It also controls the
- interval between which expired requests are served, so set to 50, a request
- might take anywhere < 100ms to be serviced _if_ it is the next on the
- expired list. Obviously request expiration strategies won't make the disk
- go faster. The result basically equates to the timeslice a single reader
- gets in the presence of other IO. 100*((seek time / read_expire) + 1) is
- very roughly the % streaming read efficiency your disk should get with
- multiple readers.
-* read_batch_expire
- Controls how much time a batch of reads is given before pending writes are
- served. A higher value is more efficient. This might be set below read_expire
- if writes are to be given higher priority than reads, but reads are to be
- as efficient as possible when there are no writes. Generally though, it
- should be some multiple of read_expire.
-* write_expire, and
-* write_batch_expire are equivalent to the above, for writes.
-* antic_expire
- Controls the maximum amount of time we can anticipate a good read (one
- with a short seek distance from the most recently completed request) before
- giving up. Many other factors may cause anticipation to be stopped early,
- or some processes will not be "anticipated" at all. Should be a bit higher
- for big seek time devices though not a linear correspondence - most
- processes have only a few ms thinktime.
-In addition to the tunables above there is a read-only file named est_time
-which, when read, will show:
- - The probability of a task exiting without a cooperating task
- submitting an anticipated IO.
- - The current mean think time.
- - The seek distance used to determine if an incoming IO is better.

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