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block: update documentation for deadline fifo_batch tunable 

Update the description of fifo_batch to match the current implementation,
and include a description of how to tune it.

Signed-off-by: Aaron Carroll <aaronc@gelato.unsw.edu.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Aaron Carroll 2008-08-14 18:17:15 +10:00 committed by Jens Axboe
parent 4fb72f7646
commit 6a421c1dc9
1 changed files with 10 additions and 4 deletions
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14
Documentation/block/deadline-iosched.txt
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@ -30,12 +30,18 @@ write_expire (in ms)
Similar to read_expire mentioned above, but for writes.
fifo_batch
fifo_batch (number of requests)
----------
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.
Requests are grouped into ``batches'' of a particular data direction (read or
write) which are serviced in increasing sector order. To limit extra seeking,
deadline expiries are only checked between batches. fifo_batch controls the
maximum number of requests per batch.
This parameter tunes the balance between per-request latency and aggregate
throughput. When low latency is the primary concern, smaller is better (where
a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
generally improves throughput, at the cost of latency variation.
writes_starved (number of dispatches)