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authorShaohua Li <shli@fb.com>2017-03-27 15:19:42 -0700
committerJens Axboe <axboe@fb.com>2017-03-28 08:02:20 -0600
commitb9147dd1bae2b15d6931ecd42f8606c775fecbc9 (patch)
tree9becbcfbf24e535538680bb53f38962808b4e28e /block/blk-stat.h
parent88eeca495ba7de749ff253376ec6be19bb05368d (diff)
blk-throttle: add a mechanism to estimate IO latency
User configures latency target, but the latency threshold for each request size isn't fixed. For a SSD, the IO latency highly depends on request size. To calculate latency threshold, we sample some data, eg, average latency for request size 4k, 8k, 16k, 32k .. 1M. The latency threshold of each request size will be the sample latency (I'll call it base latency) plus latency target. For example, the base latency for request size 4k is 80us and user configures latency target 60us. The 4k latency threshold will be 80 + 60 = 140us. To sample data, we calculate the order base 2 of rounded up IO sectors. If the IO size is bigger than 1M, it will be accounted as 1M. Since the calculation does round up, the base latency will be slightly smaller than actual value. Also if there isn't any IO dispatched for a specific IO size, we will use the base latency of smaller IO size for this IO size. But we shouldn't sample data at any time. The base latency is supposed to be latency where disk isn't congested, because we use latency threshold to schedule IOs between cgroups. If disk is congested, the latency is higher, using it for scheduling is meaningless. Hence we only do the sampling when block throttling is in the LOW limit, with assumption disk isn't congested in such state. If the assumption isn't true, eg, low limit is too high, calculated latency threshold will be higher. Hard disk is completely different. Latency depends on spindle seek instead of request size. Currently this feature is SSD only, we probably can use a fixed threshold like 4ms for hard disk though. Signed-off-by: Shaohua Li <shli@fb.com> Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'block/blk-stat.h')
-rw-r--r--block/blk-stat.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/block/blk-stat.h b/block/blk-stat.h
index ee47f816d5bd..53f08a63bf15 100644
--- a/block/blk-stat.h
+++ b/block/blk-stat.h
@@ -108,6 +108,9 @@ static inline void blk_stat_set_issue(struct blk_issue_stat *stat,
(((u64)blk_capped_size(size)) << BLK_STAT_SIZE_SHIFT);
}
+/* record time/size info in request but not add a callback */
+void blk_stat_enable_accounting(struct request_queue *q);
+
/*
* blk_stat_rq_ddir() - Bucket callback function for the request data direction.
* @rq: Request.