Re: [RFC]: unnecessary padding in various kernel structures

On 04/01/2011 19:59, Roman Divacky wrote:
> hi,
>
> clang (svn version) has ability to detect unnecessary padding in structures.
>
> I ran this on kernel build on i386 (stripped GENERIC) and amd64 (full GENERIC),
> preprocessed this and posted on web.
>
> The lists contain the file of the definition, name of the structure, size of
> the unnecessary padding and reason for the alignment. The last field is
> how many times this was hit during the build of the kernel.

It looks like "padding... to alignment boundary" means "because of 
struct {...} __aligned(CACHE_LINE_SIZE)" and such, and we cannot run 
away from those - maybe you should filter those results out?

> The lists are sorted by the size of the padding.
>
> Examples (i386):
>
> dev/usb/controller/ohci.h:64:8:		2 times
> 	padding size of 'struct ohci_hcca' with 120 bytes to alignment boundary
> sys/pcpu.h:156:8:			58 times
> 	padding size of 'struct pcpu' with 108 bytes to alignment boundary
> sys/pcpu.h:199:2:			58 times
> 	padding struct 'struct pcpu' with 84 bytes to align 'pc_monitorbuf'
> dev/usb/controller/ehci.h:170:8:	1 times
> 	padding size of 'struct ehci_qtd' with 58 bytes to alignment boundary
> kern/sched_ule.c:206:8:			1 times
> 	padding size of 'struct tdq' with 41 bytes to alignment boundary
>
> Examples(amd64):
>
> net/flowtable.c:179:11:			1 times
> 	padding struct 'struct flowtable' with 124 bytes to align 'ft_udp_idle'
> dev/usb/controller/ohci.h:64:8:		2 times
> 	padding size of 'struct ohci_hcca' with 120 bytes to alignment boundary
> net/flowtable.c:160:8:			1 times
> 	padding size of 'struct flowtable' with 108 bytes to alignment boundary
> vm/uma_int.h:184:8:			6 times
> 	padding size of 'struct uma_cache' with 96 bytes to alignment boundary
> vm/uma_int.h:338:19:			5 times
> 	padding struct 'struct uma_zone' with 92 bytes to align 'uz_cpu'
> net/flowtable.c:149:8:			1 times
> 	padding size of 'struct flowtable_stats' with 64 bytes to alignment boundary