[PATCH 0/3] nvme: Avoid preallocating big SGL for data

[Israel Rukshin <israelr@mellanox.com>]

When nvme allocates a tagset it preallocates a big buffer for the IO SGL
based on SG_CHUNK_SIZE.

Modern DMA engines are often capable of dealing with very big segments so
the SG_CHUNK_SIZE is often too big. SG_CHUNK_SIZE results in a static 4KB
SGL allocation per command.

If a controller has lots of deep queues, preallocation for the sg list can
consume substantial amounts of memory. Switch to runtime allocation for SGL
for lists longer than 2 entries. This is the approach used by NVMe PCI so
it should be reasonable for NVMeOF as well. Runtime SGL allocation has
always been the case for the legacy I/O path so this is nothing new.

The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
support SG_CHAIN, use only runtime allocation for the SGL.

Israel Rukshin (3):
  nvme-rdma: Avoid preallocating big SGL for data
  nvme-fc: Avoid preallocating big SGL for data
  nvmet-loop: Avoid preallocating big SGL for data

 drivers/nvme/host/fc.c     |  8 ++++----
 drivers/nvme/host/nvme.h   |  6 ++++++
 drivers/nvme/host/rdma.c   | 10 +++++-----
 drivers/nvme/target/loop.c |  8 ++++----
 4 files changed, 19 insertions(+), 13 deletions(-)

-- 
1.8.3.1


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