tree f1decfafa5f0d974d2f9c707a514bf68b9e2cddb
parent 4d92a9beb39d80a7d8ff7c04ae12a10290105ae5
author Jens Axboe <axboe@fb.com> 1401378812 -0600
committer Jens Axboe <axboe@fb.com> 1401378812 -0600

block: add queue flag for disabling SG merging

If devices are not SG starved, we waste a lot of time potentially
collapsing SG segments. Enough that 1.5% of the CPU time goes
to this, at only 400K IOPS. Add a queue flag, QUEUE_FLAG_NO_SG_MERGE,
which just returns the number of vectors in a bio instead of looping
over all segments and checking for collapsible ones.

Add a BLK_MQ_F_SG_MERGE flag so that drivers can opt-in on the sg
merging, if they so desire.

Signed-off-by: Jens Axboe <axboe@fb.com>