mirror of
https://github.com/torvalds/linux.git
synced 2024-11-16 00:52:01 +00:00
1da177e4c3
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!
166 lines
8.3 KiB
Plaintext
166 lines
8.3 KiB
Plaintext
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
|
|
-----------------------
|
|
To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
|
|
'noop' and 'as' (the default) are also available. IO schedulers are assigned
|
|
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
|
|
/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.
|
|
|