[PATCH] blk-mq: Use all available hardware queues

From: Bart Van Assche
Date: Wed Oct 08 2014 - 09:17:04 EST

Suppose that a system has two CPU sockets, three cores per socket,
that it does not support hyperthreading and that four hardware
queues are provided by a block driver. With the current algorithm
this will lead to the following assignment of CPU cores to hardware

HWQ 0: 0 1
HWQ 1: 2 3
HWQ 2: 4 5
HWQ 3: (none)

This patch changes the queue assignment into:

HWQ 0: 0 1
HWQ 1: 2
HWQ 2: 3 4
HWQ 3: 5

In other words, this patch has the following three effects:
- All four hardware queues are used instead of only three.
- CPU cores are spread more evenly over hardware queues. For the
above example the range of the number of CPU cores associated
with a single HWQ is reduced from [0..2] to [1..2].
- If the number of HWQ's is a multiple of the number of CPU sockets
it is now guaranteed that all CPU cores associated with a single
HWQ reside on the same CPU socket.

Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
Reviewed-by: Sagi Grimberg <sagig@xxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxxxxx>
block/blk-mq-cpumap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 1065d7c..8e56455 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -17,7 +17,7 @@
static int cpu_to_queue_index(unsigned int nr_cpus, unsigned int nr_queues,
const int cpu)
- return cpu / ((nr_cpus + nr_queues - 1) / nr_queues);
+ return cpu * nr_queues / nr_cpus;
static int get_first_sibling(unsigned int cpu)

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