On 09.10.2018 17:20, David Ahern wrote:
> On 10/8/18 2:17 PM, Heiner Kallweit wrote:
>> bool is good as parameter type or function return type, but if used
>> for struct members it consumes more memory than needed.
>> Changing the bool members of struct net_device to bitfield members
>> allows to decrease the memory footprint of this struct.
>
> What does pahole show for the size of the struct before and after? I
> suspect you have not really changed the size and certainly not the
> actual memory allocated.
>
>
Thanks for the hint to use pahole. Indeed we gain nothing,
so there's no justification for this patch.
before:
/* size: 2496, cachelines: 39, members: 116 */
/* sum members: 2396, holes: 8, sum holes: 80 */
/* padding: 20 */
/* paddings: 4, sum paddings: 19 */
/* bit_padding: 31 bits */
after:
/* size: 2496, cachelines: 39, members: 116 */
/* sum members: 2394, holes: 8, sum holes: 82 */
/* bit holes: 1, sum bit holes: 8 bits */
/* padding: 20 */
/* paddings: 4, sum paddings: 19 */
/* bit_padding: 27 bits */
The biggest hole is here, because _tx is annotated to be cacheline-aligned.
struct hlist_node index_hlist; /* 888 16 */
/* XXX 56 bytes hole, try to pack */
/* --- cacheline 15 boundary (960 bytes) --- */
struct netdev_queue * _tx; /* 960 8 */
Reordering the struct members to fill the holes could be a little tricky
and could have side effects because it may make a performance difference
whether certain members are in one cacheline or not.
And whether it's worth to spend this effort (incl. the related risks)
just to save a few bytes (also considering that typically we have quite
few instances of struct net_device)?
