[PATCH] as-iosched.txt update

From: Dave Olien <dmo@osdl.org>

acked by npiggin.

Documentation/as-iosched.txt

@@ -11,6 +11,15 @@ 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
 -----------------------
@@ -19,6 +28,107 @@ To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
 globally at boot time only presently.
 
 
+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 paramter (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 time (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
@@ -26,17 +136,18 @@ When using 'as', the anticipatory IO scheduler there are 5 parameters under
 
 The parameters are:
 * read_expire
-    Controls how long until a request becomes "expired". It also controls the
+    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 it 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.
-    
+    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. Higher value is more efficient. This might be set below read_expire
+    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.
@@ -45,7 +156,8 @@ The parameters are:
 * write_batch_expire are equivalent to the above, for writes.
 
 * antic_expire
-    Controls the maximum amount of time we can anticipate a good read before
+    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