device.h: reorganize struct device
struct device is big, around 760 bytes on x86_64. It's not a critical
structure, but it is embedded everywhere, so making it smaller is always
a good thing.
With a recent patch that moved a field from struct device to the private
structure, some benchmarks showed a very odd regression, despite this
structure having nothing to do with those benchmarks. That caused me to
look into the layout of the structure. Using 'pahole', it showed a
number of holes and ways that the structure could be reordered in order
to align some cachelines better, as well as reduce the size of the
overall structure.
Move 'struct kobj' to the start of the structure, to keep that access
in the first cacheline, and try to organize things a bit more compactly
where possible
By doing these few moves, the result removes at least 8 bytes from
'struct device' on a 64bit system. Given we know there are systems with
at least 30k devices in memory at once, every little byte counts, and
this change could be a savings of 240k of kernel memory for them. On
"normal" systems the overall memory savings would be much less.
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 file changed