From 80af89ca709d4dfe41178abe29217a0fefa1af12 Mon Sep 17 00:00:00 2001 From: Andrew Morton Date: Fri, 4 Jul 2003 19:37:12 -0700 Subject: [PATCH] block batching fairness From: Nick Piggin This patch fixes the request batching fairness/starvation issue. Its not clear what is going on with 2.4, but it seems that its a problem around this area. Anyway, previously: * request queue fills up * process 1 calls get_request, sleeps * a couple of requests are freed * process 2 calls get_request, proceeds * a couple of requests are freed * process 2 calls get_request... Now as unlikely as it seems, it could be a problem. Its a fairness problem that process 2 can skip ahead of process 1 anyway. With the patch: * request queue fills up * any process calling get_request will sleep * once the queue gets below the batch watermark, processes start being worken, and may allocate. This patch includes Chris Mason's fix to only clear queue_full when all tasks have been woken. Previously I think starvation and unfairness could still occur. With this change to the blk-fair-batches patch, Chris is showing some much improved numbers for 2.4 - 170 ms max wait vs 2700ms without blk-fair-batches for a dbench 90 run. He didn't indicate how much difference his patch alone made, but it is an important fix I think. --- include/linux/blkdev.h | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) (limited to 'include/linux') diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 4295d60bf661..d3a8f6ecd806 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -307,6 +307,8 @@ struct request_queue #define QUEUE_FLAG_CLUSTER 0 /* cluster several segments into 1 */ #define QUEUE_FLAG_QUEUED 1 /* uses generic tag queueing */ #define QUEUE_FLAG_STOPPED 2 /* queue is stopped */ +#define QUEUE_FLAG_READFULL 3 /* write queue has been filled */ +#define QUEUE_FLAG_WRITEFULL 4 /* read queue has been filled */ #define blk_queue_plugged(q) !list_empty(&(q)->plug_list) #define blk_queue_tagged(q) test_bit(QUEUE_FLAG_QUEUED, &(q)->queue_flags) @@ -322,6 +324,30 @@ struct request_queue #define rq_data_dir(rq) ((rq)->flags & 1) +static inline int blk_queue_full(struct request_queue *q, int rw) +{ + if (rw == READ) + return test_bit(QUEUE_FLAG_READFULL, &q->queue_flags); + return test_bit(QUEUE_FLAG_WRITEFULL, &q->queue_flags); +} + +static inline void blk_set_queue_full(struct request_queue *q, int rw) +{ + if (rw == READ) + set_bit(QUEUE_FLAG_READFULL, &q->queue_flags); + else + set_bit(QUEUE_FLAG_WRITEFULL, &q->queue_flags); +} + +static inline void blk_clear_queue_full(struct request_queue *q, int rw) +{ + if (rw == READ) + clear_bit(QUEUE_FLAG_READFULL, &q->queue_flags); + else + clear_bit(QUEUE_FLAG_WRITEFULL, &q->queue_flags); +} + + /* * mergeable request must not have _NOMERGE or _BARRIER bit set, nor may * it already be started by driver. -- cgit v1.2.3