[RFC PATCH] nvme-pci: Move the sg table allocation/free into init/exit_request

[Keith Busch]

On Sun, Jun 28, 2020 at 06:34:46PM +0800, Baolin Wang wrote:
> Move the sg table allocation and free into the init_request() and
> exit_request(), instead of allocating sg table when queuing requests,
> which can benefit the IO performance.

If you want to pre-allocate something per-request, you can add the size
to the tagset's cmd_size.

But this is adding almost 4k per request. Considering how many requests
we try to allocate, that's a bit too large to count on being available
or sequestor for this driver.