[PATCH 0/5] optimize some data structure in nvme

Christophe JAILLET
2023-05-01T13:56:41Z

This serie is a proposal to slighly optimize the memory needed for some
structures used in nvme.

This follows the discussion in [1].

Honnestly, I'm not convinced that this serie really brings semething.
Because of the way memory alocation works, and its over-allocation to try to
avoid memory fragmentation, some limited gains are most of the time useless.

It could still help:
- many holes in structure can, at some point, have its size reach a threshold
(this is specially true if such structures are allocated with kcalloc() or
kmalloc_array())
- it can save some space in some other structures if embedded in them
- it can save a few cycles if the structure is memcpy()'ed or zeroed, for
example
- can reduce cache usage

With that in mind, patch 3 is a win, patch 4 is likely a win, the other ones are
much more theorical.

The changes are really limited, so even if the gain is marginal, maybe it still
makes sense to merge them.

Each patch gives the layout generated by pahole before and after the patch.

[1]: https://lore.kernel.org/all/67a9e53e-4ac9-7ba8-9713-96c1dfe1e341@nvidia.com/

Christophe JAILLET (5):
nvmet: Reorder fields in 'struct nvmet_sq'
nvmet: Reorder fields in 'struct nvme_ctrl'
nvmet: Reorder fields in 'struct nvmf_ctrl_options'
nvmet: Reorder fields in 'struct nvme_dhchap_queue_context'
nvmet: Reorder fields in 'struct nvmefc_fcp_req'

drivers/nvme/host/auth.c | 6 +++---
drivers/nvme/host/fabrics.h | 8 ++++----
drivers/nvme/host/nvme.h | 6 +++---
drivers/nvme/target/nvmet.h | 4 ++--
include/linux/nvme-fc-driver.h | 10 +++++-----
5 files changed, 17 insertions(+), 17 deletions(-)

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