[PATCH] NVMe: Set queue max segments

[PATCH] NVMe: Set queue max segments

[Keith Busch]

This sets the queue's max segment size to match the device's
capabilities. The default of 128 is usable until a device's transfer
capability exceeds 512k, assuming a device page size of 4k. Many nvme
devices exceed that transfer limit, so this lets the block layer know what
kind of commands it to allow to form rather than unnecessarily split them.

One additional segment is added to account for a transfer that may start
in the middle of a page.

Signed-off-by: Keith Busch <keith.busch at intel.com>
---
 drivers/block/nvme-core.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nvme-core.c b/drivers/block/nvme-core.c
index 613c1bc..abe93a5 100644
--- a/drivers/block/nvme-core.c
+++ b/drivers/block/nvme-core.c
@@ -2098,8 +2098,11 @@ static void nvme_alloc_ns(struct nvme_dev *dev, unsigned nsid)
 	list_add_tail(&ns->list, &dev->namespaces);
 
 	blk_queue_logical_block_size(ns->queue, 1 << ns->lba_shift);
-	if (dev->max_hw_sectors)
+	if (dev->max_hw_sectors) {
 		blk_queue_max_hw_sectors(ns->queue, dev->max_hw_sectors);
+		blk_queue_max_segments(ns->queue,
+			((dev->max_hw_sectors << 9) / dev->page_size) + 1);
+	}
 	if (dev->stripe_size)
 		blk_queue_chunk_sectors(ns->queue, dev->stripe_size >> 9);
 	if (dev->vwc & NVME_CTRL_VWC_PRESENT)
-- 
1.7.10.4

[PATCH] NVMe: Set queue max segments

[Christoph Hellwig]

On Wed, Aug 12, 2015@04:17:54PM -0600, Keith Busch wrote:
> This sets the queue's max segment size to match the device's
> capabilities. The default of 128 is usable until a device's transfer
> capability exceeds 512k, assuming a device page size of 4k. Many nvme
> devices exceed that transfer limit, so this lets the block layer know what
> kind of commands it to allow to form rather than unnecessarily split them.
> 
> One additional segment is added to account for a transfer that may start
> in the middle of a page.

But it might not start in the middle of the page, too - in which case
you're overcounting here, aren't you?

[PATCH] NVMe: Set queue max segments

[Keith Busch]

On Wed, 12 Aug 2015, Christoph Hellwig wrote:
> On Wed, Aug 12, 2015@04:17:54PM -0600, Keith Busch wrote:
>> This sets the queue's max segment size to match the device's
>> capabilities. The default of 128 is usable until a device's transfer
>> capability exceeds 512k, assuming a device page size of 4k. Many nvme
>> devices exceed that transfer limit, so this lets the block layer know what
>> kind of commands it to allow to form rather than unnecessarily split them.
>>
>> One additional segment is added to account for a transfer that may start
>> in the middle of a page.
>
> But it might not start in the middle of the page, too - in which case
> you're overcounting here, aren't you?

queue_max_sectors() would exceed before the next segment is considered,
similar to what happens with very large segments.

[PATCH] NVMe: Set queue max segments

[Christoph Hellwig]

Ok, looks fine:

Reviewed-by: Christoph Hellwig <hch at lst.de>

[PATCH] NVMe: Set queue max segments

[Jens Axboe]

On 08/12/2015 04:17 PM, Keith Busch wrote:
> This sets the queue's max segment size to match the device's
> capabilities. The default of 128 is usable until a device's transfer
> capability exceeds 512k, assuming a device page size of 4k. Many nvme
> devices exceed that transfer limit, so this lets the block layer know what
> kind of commands it to allow to form rather than unnecessarily split them.
>
> One additional segment is added to account for a transfer that may start
> in the middle of a page.

Applied for 4.3.

-- 
Jens Axboe