Commit 959f5f5b authored by Bart Van Assche's avatar Bart Van Assche Committed by Jens Axboe

blk-mq: Use all available hardware queues

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
queues:

  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: default avatarBart Van Assche <bvanassche@acm.org>
Reviewed-by: default avatarSagi Grimberg <sagig@mellanox.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent 52f7eb94
......@@ -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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